Images are reused AI-generated editorial illustrations, not documentary photographs or verified product screenshots.
JavaScript indexOf returns the first matching index or -1 when no match exists. Since zero is a valid index, treating the result as a simple true-or-false value can reverse the intended behavior.
Inspect the numeric result explicitly
For ['A', 'B'].indexOf('A'), the result is 0. A missing value returns -1, which is truthy in JavaScript even though it indicates failure. Use an explicit comparison such as index !== -1 before using the position.
Understand the comparison rule
IndexOf uses strict equality for matching and skips empty slots. It is not a field-based search for object records: two separately created objects with the same properties are different references. For more complex criteria, use an appropriate predicate-based search.
Guard any operation that uses the index
Do not pass an unverified result into splice or another indexing operation. A -1 argument can refer to the last position in some APIs. When only membership matters, consider includes and its own documented comparison behavior, rather than inventing a manual truthiness shortcut.
A practical checklist
- Accept zero as a valid found index.
- Compare the result explicitly with -1.
- Check strict-equality expectations.
- Use predicates for record-field searches.
- Guard mutations that consume the index.
Worked example
Illustrative example: a synthetic array contains A at index 0. An if (index) test incorrectly treats that match as false. Changing the condition to index !== -1 correctly distinguishes the first item from a missing value without altering the array.
Common questions
Does zero mean not found? No. Is -1 false in JavaScript? No. Does indexOf match independent objects by their field values? No; object identity matters. Can NaN be found with indexOf? Its strict-equality rule means that case needs different handling.
What to do next
Keep tests for a match at zero and a missing value. Those two cases catch common search-result bugs before the index drives a removal or update.