Membership with a position
Check whether a value occurs while retaining its position when present.
JavaScript · Function
indexOf
Array.prototype.indexOf() returns the first index where a value occurs in an array, or -1 when it is absent. It optionally accepts a starting index and uses strict equality for matching.
How it works
Call this method with a value to search an array from the beginning, or provide a starting index. It checks elements in ascending order with strict equality, returns the first matching position, and returns -1 when no match exists; sparse-array holes are skipped.
Syntax
indexOf(searchElement)
indexOf(searchElement, fromIndex)Parameters
| Name | Type | Description |
|---|---|---|
searchElement | any | The value to locate in the array. |
fromIndex | integer | Optional zero-based starting index, converted to an integer. Negative values count from the end. |
Returns
The first index of searchElement in the array; -1 if not found.
Key points
- The result identifies the earliest matching position, or signals that the value is absent with -1.
- The method accepts the value to locate and an optional starting position.
- fromIndex is zero-based and converted to an integer before searching begins.
- Negative starting positions are interpreted relative to the array's end.
- Values are matched with strict equality rather than coercive equality.
- The operation can be borrowed for suitable array-like objects.
Common mistakes
- A search for NaN always fails because strict equality does not consider NaN equal to itself.
- It cannot locate a sparse-array hole, because empty slots are not visited.
- A starting index at or beyond the array length immediately produces -1.
Example: Find an array element's index
const fruits = ["Apple", "Banana", "Apple"];
const position = fruits.indexOf("Banana");
console.log(position); // 1
console.log(fruits.indexOf("Cherry")); // -1
console.log(fruits.indexOf("Apple", 1)); // 2This finds the first matching value, reports absence with -1, and can begin searching at a specified index.
When to use it
Search from an offset
Find a later occurrence by beginning the search after an earlier match.
Handle array-like data
Apply the method to objects that expose length and integer-keyed properties.
Interview questions
QWhat does Array.prototype.indexOf return?
It returns the first index where the searched value occurs, or -1 when no element matches. Matching uses strict equality, equivalent to the === comparison algorithm.
Follow-up: How would you find NaN? Use Number.isNaN while scanning, or use includes when only a presence check is needed, because indexOf(NaN) always returns -1.
QHow does fromIndex affect the search?
The optional fromIndex is converted to an integer and determines where the search starts. A negative value is counted from the end, while a value beyond the array length prevents searching.
Follow-up: What happens with a negative fromIndex smaller than -array.length? It is treated as 0, so the complete array is searched.
QIs indexOf limited to Array instances?
The method is generic: it only requires a this value with a length property and integer-keyed properties. Therefore, it can operate on array-like objects as well as actual arrays.
Follow-up: Does generic mean it works on every object? No; the object still needs a usable length and indexed properties for meaningful results.
Related
Array.prototype.includes()Array.prototype.lastIndexOf()Array.prototype.findIndex()TypedArray.prototype.indexOf()