Iterator.find returns a produced source value, not the truthy value returned by its predicate. That distinction matters when predicates return scores or diagnostic objects, and when a matching source value is itself undefined. This guide follows the first match, normal exhaustion and cleanup with explicit event ledgers.
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Separate the source value returned by Iterator.find from the truthy predicate result that selects it, including undefined matches, indices and closing.
ROUTE 1 · CHAPTERS 1–3
Build the first model
Name the source, consuming method and first complete event trace.
ROUTE 2 · CHAPTERS 4–6
Follow the decisive mechanism
Work through pulls, counters, callback results and completion in order.
ROUTE 3 · CHAPTERS 7–9
Change the context
Change one condition and rebuild the trace from that premise.
ROUTE 4 · CHAPTERS 10–12
Handle limits and evidence
Test cleanup, errors, shared state and evidence that separates rival explanations.
ROUTE 5 · CHAPTERS 13–15
Use and explain the result
Turn the mechanism into dependable tests, reviews and learner explanations.
Chapter contents
Build the first model · Chapters 1–3
1. Return the source value, not the predicate result
Follow the decisive mechanism · Chapters 4–6
4. Expose matched undefined ambiguity
Handle limits and evidence · Chapters 10–12
Use and explain the result · Chapters 13–15
CHAPTER 1 OF 15 · Build the first model
1. Return the source value, not the predicate result
Iterator.find tests each source value and returns the first value whose predicate result is truthy. The raw predicate result chooses; it is not itself returned. Begin with the exact receiver, the underlying iterator and the operation that advances it. Similar array methods can help with vocabulary, but the decisive event order must come from the iterator method being studied.
Worked case. Source records A,B,C; predicate returns false for A and the string match for B. The method returns record B, not the string match. Write a ledger with one row for each source pull, callback or procedure call when present, Boolean conversion, stored or returned value, completion and return. The ledger makes the stopping point visible instead of hiding it inside one final result.
Trace the case from left to right. Validate the receiver and callable arguments, obtain the direct iterator, request the next value, perform the method-specific step, and interpret the result only after the stopping rule is known. Include the final done step when exhaustion matters.
Changed-context check. Return object {ok:true} for B. The object is truthy and B is still the returned value. Rebuild the trace from the changed premise rather than editing the first answer by intuition. Compare the first event that differs, the values already consumed and the state left in the source; those facts show which parts of the explanation transfer.
Limit and misconception check. A truthy diagnostic or score can select a record without becoming the find result. State the narrow claim supported by the fixture. A finite generator does not establish behaviour for an infinite source, a throwing callback, a shared iterator or a source whose next and return methods have visible side effects.
For a practical evidence check, log next, return and the callback when one exists. Assert the ordered log as well as the final result. A snapshot of the Boolean, found value, array or undefined result can hide extra pulls, early closing and counter changes.
A strong review separates three questions: what did the source produce, what did the method do with that event, and what did the caller observe? Keeping those layers separate prevents an output position or final value from being mistaken for the source protocol.
Transfer the chapter into a short Websites explanation with four parts: the starting state, the decisive protocol step, the changed-context result and the remaining caveat. This structure lets a learner reuse the reasoning without memorising one fixture as a universal rule.
The predicate receives each produced value and a zero-based counter. Falsey results advance the counter until a match, exhaustion or error. Begin with the exact receiver, the underlying iterator and the operation that advances it. Similar array methods can help with vocabulary, but the decisive event order must come from the iterator method being studied.
Worked case. A fails at zero, B fails at one and C succeeds at two; C is returned. Write a ledger with one row for each source pull, callback or procedure call when present, Boolean conversion, stored or returned value, completion and return. The ledger makes the stopping point visible instead of hiding it inside one final result.
Trace the case from left to right. Validate the receiver and callable arguments, obtain the direct iterator, request the next value, perform the method-specific step, and interpret the result only after the stopping rule is known. Include the final done step when exhaustion matters.
Changed-context check. Insert X before C. C remains the found value but its callback index becomes three. Rebuild the trace from the changed premise rather than editing the first answer by intuition. Compare the first event that differs, the values already consumed and the state left in the source; those facts show which parts of the explanation transfer.
Limit and misconception check. The found values identity does not encode its traversal index. State the narrow claim supported by the fixture. A finite generator does not establish behaviour for an infinite source, a throwing callback, a shared iterator or a source whose next and return methods have visible side effects.
For a practical evidence check, log next, return and the callback when one exists. Assert the ordered log as well as the final result. A snapshot of the Boolean, found value, array or undefined result can hide extra pulls, early closing and counter changes.
A strong review separates three questions: what did the source produce, what did the method do with that event, and what did the caller observe? Keeping those layers separate prevents an output position or final value from being mistaken for the source protocol.
Transfer the chapter into a short Websites explanation with four parts: the starting state, the decisive protocol step, the changed-context result and the remaining caveat. This structure lets a learner reuse the reasoning without memorising one fixture as a universal rule.
When the source reports done before any truthy predicate result, find returns undefined. Begin with the exact receiver, the underlying iterator and the operation that advances it. Similar array methods can help with vocabulary, but the decisive event order must come from the iterator method being studied.
Worked case. Values 1,3,5 under an even predicate exhaust and return undefined. Write a ledger with one row for each source pull, callback or procedure call when present, Boolean conversion, stored or returned value, completion and return. The ledger makes the stopping point visible instead of hiding it inside one final result.
Trace the case from left to right. Validate the receiver and callable arguments, obtain the direct iterator, request the next value, perform the method-specific step, and interpret the result only after the stopping rule is known. Include the final done step when exhaustion matters.
Changed-context check. Append 6. The predicate succeeds and find returns 6 before the next done step. Rebuild the trace from the changed premise rather than editing the first answer by intuition. Compare the first event that differs, the values already consumed and the state left in the source; those facts show which parts of the explanation transfer.
Limit and misconception check. Undefined can mean no match, but it can also be a matched source value in a different fixture. State the narrow claim supported by the fixture. A finite generator does not establish behaviour for an infinite source, a throwing callback, a shared iterator or a source whose next and return methods have visible side effects.
For a practical evidence check, log next, return and the callback when one exists. Assert the ordered log as well as the final result. A snapshot of the Boolean, found value, array or undefined result can hide extra pulls, early closing and counter changes.
A strong review separates three questions: what did the source produce, what did the method do with that event, and what did the caller observe? Keeping those layers separate prevents an output position or final value from being mistaken for the source protocol.
Transfer the chapter into a short Websites explanation with four parts: the starting state, the decisive protocol step, the changed-context result and the remaining caveat. This structure lets a learner reuse the reasoning without memorising one fixture as a universal rule.
CHAPTER 4 OF 15 · Follow the decisive mechanism
4. Expose matched undefined ambiguity
Because find returns the source value, a produced undefined that satisfies the predicate gives the same final value as no match. The callback trace separates the cases. Begin with the exact receiver, the underlying iterator and the operation that advances it. Similar array methods can help with vocabulary, but the decisive event order must come from the iterator method being studied.
Worked case. Source yields undefined; predicate returns true at index zero; find closes and returns undefined. Write a ledger with one row for each source pull, callback or procedure call when present, Boolean conversion, stored or returned value, completion and return. The ledger makes the stopping point visible instead of hiding it inside one final result.
Trace the case from left to right. Validate the receiver and callable arguments, obtain the direct iterator, request the next value, perform the method-specific step, and interpret the result only after the stopping rule is known. Include the final done step when exhaustion matters.
Changed-context check. Make the predicate false. The source exhausts and also returns undefined, but the event ledger differs. Rebuild the trace from the changed premise rather than editing the first answer by intuition. Compare the first event that differs, the values already consumed and the state left in the source; those facts show which parts of the explanation transfer.
Limit and misconception check. Do not use the bare final value alone when matched undefined must be distinguished from no match. State the narrow claim supported by the fixture. A finite generator does not establish behaviour for an infinite source, a throwing callback, a shared iterator or a source whose next and return methods have visible side effects.
For a practical evidence check, log next, return and the callback when one exists. Assert the ordered log as well as the final result. A snapshot of the Boolean, found value, array or undefined result can hide extra pulls, early closing and counter changes.
A strong review separates three questions: what did the source produce, what did the method do with that event, and what did the caller observe? Keeping those layers separate prevents an output position or final value from being mistaken for the source protocol.
Transfer the chapter into a short Websites explanation with four parts: the starting state, the decisive protocol step, the changed-context result and the remaining caveat. This structure lets a learner reuse the reasoning without memorising one fixture as a universal rule.
CHAPTER 5 OF 15 · Follow the decisive mechanism
5. Apply Boolean conversion to predicate output
Find converts the predicate result to Boolean. Zero, empty string and null reject; objects select. Begin with the exact receiver, the underlying iterator and the operation that advances it. Similar array methods can help with vocabulary, but the decisive event order must come from the iterator method being studied.
Worked case. Return a numeric score: A gets zero and B gets 0.2, so B is returned at the first nonzero score. Write a ledger with one row for each source pull, callback or procedure call when present, Boolean conversion, stored or returned value, completion and return. The ledger makes the stopping point visible instead of hiding it inside one final result.
Trace the case from left to right. Validate the receiver and callable arguments, obtain the direct iterator, request the next value, perform the method-specific step, and interpret the result only after the stopping rule is known. Include the final done step when exhaustion matters.
Changed-context check. Return {score:0} for A. The object is truthy, so A is returned unexpectedly unless the numeric field is returned. Rebuild the trace from the changed premise rather than editing the first answer by intuition. Compare the first event that differs, the values already consumed and the state left in the source; those facts show which parts of the explanation transfer.
Limit and misconception check. A structured result is not interpreted by its internal fields. State the narrow claim supported by the fixture. A finite generator does not establish behaviour for an infinite source, a throwing callback, a shared iterator or a source whose next and return methods have visible side effects.
For a practical evidence check, log next, return and the callback when one exists. Assert the ordered log as well as the final result. A snapshot of the Boolean, found value, array or undefined result can hide extra pulls, early closing and counter changes.
A strong review separates three questions: what did the source produce, what did the method do with that event, and what did the caller observe? Keeping those layers separate prevents an output position or final value from being mistaken for the source protocol.
Transfer the chapter into a short Websites explanation with four parts: the starting state, the decisive protocol step, the changed-context result and the remaining caveat. This structure lets a learner reuse the reasoning without memorising one fixture as a universal rule.
Iterator.find consumes immediately and returns a value or undefined; it does not return a lazy helper. Begin with the exact receiver, the underlying iterator and the operation that advances it. Similar array methods can help with vocabulary, but the decisive event order must come from the iterator method being studied.
Worked case. Call find on a logging generator and observe source pulls during the method call. Write a ledger with one row for each source pull, callback or procedure call when present, Boolean conversion, stored or returned value, completion and return. The ledger makes the stopping point visible instead of hiding it inside one final result.
Trace the case from left to right. Validate the receiver and callable arguments, obtain the direct iterator, request the next value, perform the method-specific step, and interpret the result only after the stopping rule is known. Include the final done step when exhaustion matters.
Changed-context check. Create a filter helper with the same predicate and observe no pulls until the helper is advanced. Rebuild the trace from the changed premise rather than editing the first answer by intuition. Compare the first event that differs, the values already consumed and the state left in the source; those facts show which parts of the explanation transfer.
Limit and misconception check. Find is a terminal search operation rather than a reusable filtered view. State the narrow claim supported by the fixture. A finite generator does not establish behaviour for an infinite source, a throwing callback, a shared iterator or a source whose next and return methods have visible side effects.
For a practical evidence check, log next, return and the callback when one exists. Assert the ordered log as well as the final result. A snapshot of the Boolean, found value, array or undefined result can hide extra pulls, early closing and counter changes.
A strong review separates three questions: what did the source produce, what did the method do with that event, and what did the caller observe? Keeping those layers separate prevents an output position or final value from being mistaken for the source protocol.
Transfer the chapter into a short Websites explanation with four parts: the starting state, the decisive protocol step, the changed-context result and the remaining caveat. This structure lets a learner reuse the reasoning without memorising one fixture as a universal rule.
The first truthy result triggers IteratorClose with the found source value as the normal completion. Begin with the exact receiver, the underlying iterator and the operation that advances it. Similar array methods can help with vocabulary, but the decisive event order must come from the iterator method being studied.
Worked case. A generator with finally yields A,B,C; B matches, cleanup runs and B is returned without pulling C. Write a ledger with one row for each source pull, callback or procedure call when present, Boolean conversion, stored or returned value, completion and return. The ledger makes the stopping point visible instead of hiding it inside one final result.
Trace the case from left to right. Validate the receiver and callable arguments, obtain the direct iterator, request the next value, perform the method-specific step, and interpret the result only after the stopping rule is known. Include the final done step when exhaustion matters.
Changed-context check. Make no value match. The source completes normally and undefined is returned after exhaustion. Rebuild the trace from the changed premise rather than editing the first answer by intuition. Compare the first event that differs, the values already consumed and the state left in the source; those facts show which parts of the explanation transfer.
Limit and misconception check. Early matching and normal exhaustion have different cleanup paths. State the narrow claim supported by the fixture. A finite generator does not establish behaviour for an infinite source, a throwing callback, a shared iterator or a source whose next and return methods have visible side effects.
For a practical evidence check, log next, return and the callback when one exists. Assert the ordered log as well as the final result. A snapshot of the Boolean, found value, array or undefined result can hide extra pulls, early closing and counter changes.
A strong review separates three questions: what did the source produce, what did the method do with that event, and what did the caller observe? Keeping those layers separate prevents an output position or final value from being mistaken for the source protocol.
Transfer the chapter into a short Websites explanation with four parts: the starting state, the decisive protocol step, the changed-context result and the remaining caveat. This structure lets a learner reuse the reasoning without memorising one fixture as a universal rule.
Once a match is found, later source values and predicate side effects do not occur. Begin with the exact receiver, the underlying iterator and the operation that advances it. Similar array methods can help with vocabulary, but the decisive event order must come from the iterator method being studied.
Worked case. The second record matches; a fourth record would also match but is never read. Write a ledger with one row for each source pull, callback or procedure call when present, Boolean conversion, stored or returned value, completion and return. The ledger makes the stopping point visible instead of hiding it inside one final result.
Trace the case from left to right. Validate the receiver and callable arguments, obtain the direct iterator, request the next value, perform the method-specific step, and interpret the result only after the stopping rule is known. Include the final done step when exhaustion matters.
Changed-context check. Make the second fail so the fourth becomes the first match and observe the added work. Rebuild the trace from the changed premise rather than editing the first answer by intuition. Compare the first event that differs, the values already consumed and the state left in the source; those facts show which parts of the explanation transfer.
Limit and misconception check. Find answers first match in source order, not best match across the whole source. State the narrow claim supported by the fixture. A finite generator does not establish behaviour for an infinite source, a throwing callback, a shared iterator or a source whose next and return methods have visible side effects.
For a practical evidence check, log next, return and the callback when one exists. Assert the ordered log as well as the final result. A snapshot of the Boolean, found value, array or undefined result can hide extra pulls, early closing and counter changes.
A strong review separates three questions: what did the source produce, what did the method do with that event, and what did the caller observe? Keeping those layers separate prevents an output position or final value from being mistaken for the source protocol.
Transfer the chapter into a short Websites explanation with four parts: the starting state, the decisive protocol step, the changed-context result and the remaining caveat. This structure lets a learner reuse the reasoning without memorising one fixture as a universal rule.
Find returns one source value and closes early; filter creates a lazy helper that can yield every selected value as consumers advance it. Begin with the exact receiver, the underlying iterator and the operation that advances it. Similar array methods can help with vocabulary, but the decisive event order must come from the iterator method being studied.
Worked case. For matching B and D, find returns B and stops, while a fully consumed filter yields B then D. Write a ledger with one row for each source pull, callback or procedure call when present, Boolean conversion, stored or returned value, completion and return. The ledger makes the stopping point visible instead of hiding it inside one final result.
Trace the case from left to right. Validate the receiver and callable arguments, obtain the direct iterator, request the next value, perform the method-specific step, and interpret the result only after the stopping rule is known. Include the final done step when exhaustion matters.
Changed-context check. Consume only one filter result and then return; the traces can look similar but the APIs and result types differ. Rebuild the trace from the changed premise rather than editing the first answer by intuition. Compare the first event that differs, the values already consumed and the state left in the source; those facts show which parts of the explanation transfer.
Limit and misconception check. Do not substitute find when the reader job is to preserve all matches. State the narrow claim supported by the fixture. A finite generator does not establish behaviour for an infinite source, a throwing callback, a shared iterator or a source whose next and return methods have visible side effects.
For a practical evidence check, log next, return and the callback when one exists. Assert the ordered log as well as the final result. A snapshot of the Boolean, found value, array or undefined result can hide extra pulls, early closing and counter changes.
A strong review separates three questions: what did the source produce, what did the method do with that event, and what did the caller observe? Keeping those layers separate prevents an output position or final value from being mistaken for the source protocol.
Transfer the chapter into a short Websites explanation with four parts: the starting state, the decisive protocol step, the changed-context result and the remaining caveat. This structure lets a learner reuse the reasoning without memorising one fixture as a universal rule.
When find consumes a map helper, the returned value is the mapped value produced by that helper, not the original upstream value. Begin with the exact receiver, the underlying iterator and the operation that advances it. Similar array methods can help with vocabulary, but the decisive event order must come from the iterator method being studied.
Worked case. Map records to labels A0,B1,C2, then find label ending1; the result is B1. Write a ledger with one row for each source pull, callback or procedure call when present, Boolean conversion, stored or returned value, completion and return. The ledger makes the stopping point visible instead of hiding it inside one final result.
Trace the case from left to right. Validate the receiver and callable arguments, obtain the direct iterator, request the next value, perform the method-specific step, and interpret the result only after the stopping rule is known. Include the final done step when exhaustion matters.
Changed-context check. Run find before map. It returns the original record B, which can then be transformed separately. Rebuild the trace from the changed premise rather than editing the first answer by intuition. Compare the first event that differs, the values already consumed and the state left in the source; those facts show which parts of the explanation transfer.
Limit and misconception check. Pipeline order changes both predicate inputs and the identity of the returned value. State the narrow claim supported by the fixture. A finite generator does not establish behaviour for an infinite source, a throwing callback, a shared iterator or a source whose next and return methods have visible side effects.
For a practical evidence check, log next, return and the callback when one exists. Assert the ordered log as well as the final result. A snapshot of the Boolean, found value, array or undefined result can hide extra pulls, early closing and counter changes.
A strong review separates three questions: what did the source produce, what did the method do with that event, and what did the caller observe? Keeping those layers separate prevents an output position or final value from being mistaken for the source protocol.
Transfer the chapter into a short Websites explanation with four parts: the starting state, the decisive protocol step, the changed-context result and the remaining caveat. This structure lets a learner reuse the reasoning without memorising one fixture as a universal rule.
A predicate exception closes the iterator and propagates the error; it does not become undefined or a non-match. Begin with the exact receiver, the underlying iterator and the operation that advances it. Similar array methods can help with vocabulary, but the decisive event order must come from the iterator method being studied.
Worked case. A fails normally and B throws at index one; record cleanup and the propagated error. Write a ledger with one row for each source pull, callback or procedure call when present, Boolean conversion, stored or returned value, completion and return. The ledger makes the stopping point visible instead of hiding it inside one final result.
Trace the case from left to right. Validate the receiver and callable arguments, obtain the direct iterator, request the next value, perform the method-specific step, and interpret the result only after the stopping rule is known. Include the final done step when exhaustion matters.
Changed-context check. Return false for B instead. The method continues to later values. Rebuild the trace from the changed premise rather than editing the first answer by intuition. Compare the first event that differs, the values already consumed and the state left in the source; those facts show which parts of the explanation transfer.
Limit and misconception check. No match and callback failure are different outcomes even if application code later catches errors. State the narrow claim supported by the fixture. A finite generator does not establish behaviour for an infinite source, a throwing callback, a shared iterator or a source whose next and return methods have visible side effects.
For a practical evidence check, log next, return and the callback when one exists. Assert the ordered log as well as the final result. A snapshot of the Boolean, found value, array or undefined result can hide extra pulls, early closing and counter changes.
A strong review separates three questions: what did the source produce, what did the method do with that event, and what did the caller observe? Keeping those layers separate prevents an output position or final value from being mistaken for the source protocol.
Transfer the chapter into a short Websites explanation with four parts: the starting state, the decisive protocol step, the changed-context result and the remaining caveat. This structure lets a learner reuse the reasoning without memorising one fixture as a universal rule.
CHAPTER 12 OF 15 · Handle limits and evidence
12. Separate source failure from predicate failure
A next failure happens before the predicate for that step, so the predicate log identifies how far valid traversal progressed. Begin with the exact receiver, the underlying iterator and the operation that advances it. Similar array methods can help with vocabulary, but the decisive event order must come from the iterator method being studied.
Worked case. Source yields A with false predicate, then next throws; no index-one predicate call exists. Write a ledger with one row for each source pull, callback or procedure call when present, Boolean conversion, stored or returned value, completion and return. The ledger makes the stopping point visible instead of hiding it inside one final result.
Trace the case from left to right. Validate the receiver and callable arguments, obtain the direct iterator, request the next value, perform the method-specific step, and interpret the result only after the stopping rule is known. Include the final done step when exhaustion matters.
Changed-context check. Yield B and throw inside its predicate to observe B and index one before cleanup. Rebuild the trace from the changed premise rather than editing the first answer by intuition. Compare the first event that differs, the values already consumed and the state left in the source; those facts show which parts of the explanation transfer.
Limit and misconception check. Do not report a source transport failure as a clean no-match result. State the narrow claim supported by the fixture. A finite generator does not establish behaviour for an infinite source, a throwing callback, a shared iterator or a source whose next and return methods have visible side effects.
For a practical evidence check, log next, return and the callback when one exists. Assert the ordered log as well as the final result. A snapshot of the Boolean, found value, array or undefined result can hide extra pulls, early closing and counter changes.
A strong review separates three questions: what did the source produce, what did the method do with that event, and what did the caller observe? Keeping those layers separate prevents an output position or final value from being mistaken for the source protocol.
Transfer the chapter into a short Websites explanation with four parts: the starting state, the decisive protocol step, the changed-context result and the remaining caveat. This structure lets a learner reuse the reasoning without memorising one fixture as a universal rule.
Find returns the exact produced value. For objects, the result preserves reference identity rather than cloning the record. Begin with the exact receiver, the underlying iterator and the operation that advances it. Similar array methods can help with vocabulary, but the decisive event order must come from the iterator method being studied.
Worked case. Source yields object a then object b; b matches and the returned value is strictly identical to b. Write a ledger with one row for each source pull, callback or procedure call when present, Boolean conversion, stored or returned value, completion and return. The ledger makes the stopping point visible instead of hiding it inside one final result.
Trace the case from left to right. Validate the receiver and callable arguments, obtain the direct iterator, request the next value, perform the method-specific step, and interpret the result only after the stopping rule is known. Include the final done step when exhaustion matters.
Changed-context check. Yield a lookalike object instead. Matching fields can be equal while reference identity differs. Rebuild the trace from the changed premise rather than editing the first answer by intuition. Compare the first event that differs, the values already consumed and the state left in the source; those facts show which parts of the explanation transfer.
Limit and misconception check. The method guarantees source-value return, not structural copying. State the narrow claim supported by the fixture. A finite generator does not establish behaviour for an infinite source, a throwing callback, a shared iterator or a source whose next and return methods have visible side effects.
For a practical evidence check, log next, return and the callback when one exists. Assert the ordered log as well as the final result. A snapshot of the Boolean, found value, array or undefined result can hide extra pulls, early closing and counter changes.
A strong review separates three questions: what did the source produce, what did the method do with that event, and what did the caller observe? Keeping those layers separate prevents an output position or final value from being mistaken for the source protocol.
Transfer the chapter into a short Websites explanation with four parts: the starting state, the decisive protocol step, the changed-context result and the remaining caveat. This structure lets a learner reuse the reasoning without memorising one fixture as a universal rule.
A strong fixture includes false candidates, a truthy non-Boolean predicate result, a later candidate, a return logger, matched undefined and no-match cases. Begin with the exact receiver, the underlying iterator and the operation that advances it. Similar array methods can help with vocabulary, but the decisive event order must come from the iterator method being studied.
Worked case. Assert that B is returned while the string match is not; assert return runs and C is unvisited. Write a ledger with one row for each source pull, callback or procedure call when present, Boolean conversion, stored or returned value, completion and return. The ledger makes the stopping point visible instead of hiding it inside one final result.
Trace the case from left to right. Validate the receiver and callable arguments, obtain the direct iterator, request the next value, perform the method-specific step, and interpret the result only after the stopping rule is known. Include the final done step when exhaustion matters.
Changed-context check. Change B to undefined and test the callback log to separate match from exhaustion. Rebuild the trace from the changed premise rather than editing the first answer by intuition. Compare the first event that differs, the values already consumed and the state left in the source; those facts show which parts of the explanation transfer.
Limit and misconception check. A single primitive match cannot reveal the value-versus-predicate-result distinction. State the narrow claim supported by the fixture. A finite generator does not establish behaviour for an infinite source, a throwing callback, a shared iterator or a source whose next and return methods have visible side effects.
For a practical evidence check, log next, return and the callback when one exists. Assert the ordered log as well as the final result. A snapshot of the Boolean, found value, array or undefined result can hide extra pulls, early closing and counter changes.
A strong review separates three questions: what did the source produce, what did the method do with that event, and what did the caller observe? Keeping those layers separate prevents an output position or final value from being mistaken for the source protocol.
Transfer the chapter into a short Websites explanation with four parts: the starting state, the decisive protocol step, the changed-context result and the remaining caveat. This structure lets a learner reuse the reasoning without memorising one fixture as a universal rule.
A complete account names source order, predicate value-index pairs, conversions, first match or exhaustion, returned source value and closing evidence. Begin with the exact receiver, the underlying iterator and the operation that advances it. Similar array methods can help with vocabulary, but the decisive event order must come from the iterator method being studied.
Worked case. Report A false at zero, B truthy at one, source return and result B, with C unpulled. Write a ledger with one row for each source pull, callback or procedure call when present, Boolean conversion, stored or returned value, completion and return. The ledger makes the stopping point visible instead of hiding it inside one final result.
Trace the case from left to right. Validate the receiver and callable arguments, obtain the direct iterator, request the next value, perform the method-specific step, and interpret the result only after the stopping rule is known. Include the final done step when exhaustion matters.
Changed-context check. Changed report: all values reject, the source exhausts and result is undefined. Rebuild the trace from the changed premise rather than editing the first answer by intuition. Compare the first event that differs, the values already consumed and the state left in the source; those facts show which parts of the explanation transfer.
Limit and misconception check. Do not describe find as returning true, a score or the predicate object unless that object was itself the source value. State the narrow claim supported by the fixture. A finite generator does not establish behaviour for an infinite source, a throwing callback, a shared iterator or a source whose next and return methods have visible side effects.
For a practical evidence check, log next, return and the callback when one exists. Assert the ordered log as well as the final result. A snapshot of the Boolean, found value, array or undefined result can hide extra pulls, early closing and counter changes.
A strong review separates three questions: what did the source produce, what did the method do with that event, and what did the caller observe? Keeping those layers separate prevents an output position or final value from being mistaken for the source protocol.
Transfer the chapter into a short Websites explanation with four parts: the starting state, the decisive protocol step, the changed-context result and the remaining caveat. This structure lets a learner reuse the reasoning without memorising one fixture as a universal rule.
Continue with connected lessons
Trace Iterator.filter indices through rejected values
Trace Iterator.filter indices through rejected values. Compare a lazy all-matches view with finds single consuming result.
Trace Iterator.some early success
Trace Iterator.some early success. Use the existing quantifier owner to contrast a Boolean terminal result with a returned source value.
Separate Iterator.from identity and wrapping
Separate Iterator.from identity and wrapping. Confirm which direct iterator supplies the values and identity returned by find.
Frequently asked questions
What does Iterator.find return?
It returns the first source value whose predicate result is truthy.
Does it return the predicate result?
No. The predicate result only selects; the produced source value is returned.
What happens when there is no match?
The source is exhausted and undefined is returned.
Can a matched value also be undefined?
Yes. The callback trace is needed to distinguish that from no match.
What happens after the first match?
The iterator closes and later values remain unvisited.
What happens if the predicate throws?
The source closes and the error propagates.
Authoritative sources and further reading
ECMAScript: Iterator.prototype.find. Normative predicate, returned source value, exhaustion and closing.
ECMAScript: IteratorClose. Normal early-match and abrupt cleanup rules.
ECMAScript: ToBoolean. Conversion of predicate results used to decide a match.
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