indexOf

IndexOf is a method name commonly used in JavaScript to locate the position of a value or substring within a sequence. It is defined for both strings and arrays. The method returns the index of the first match, or -1 if no match is found. An optional fromIndex parameter specifies the starting position for the search.

For arrays, indexOf(searchElement[, fromIndex]) searches from fromIndex (default 0) and returns the first index where searchElement is strictly equal (===) to an element of the array. If not found, it returns -1. A negative fromIndex counts from the end of the array. The comparison uses strict equality, so values like NaN are not considered equal to NaN.

For strings, indexOf(searchValue[, fromIndex]) searches for the first occurrence of the substring searchValue starting at fromIndex (default 0). If the substring is found, the function returns its starting index; otherwise it returns -1. For strings, a negative fromIndex is treated as 0.

Common uses include checking presence (for example, indexOf(...) !== -1) and locating positions for substring or element extraction. The operation is linear in the length of the sequence, making it a simple but potentially costly search for large arrays or long strings. In modern JavaScript, a related method called includes returns a boolean indicating presence, and lastIndexOf finds the last occurrence. IndexOf remains widely supported and frequently used in code that predates includes.