indexOf Method (Array) (JavaScript)

The index of the first occurrence of searchElement in the array, or -1 if searchElement is not found.

The indexOf method searches an array for a specified value. The method returns the index of the first occurrence, or -1 if the specified value is not found.

The search occurs in ascending index order.

The array elements are compared to the searchElement value by strict equality, similar to the === operator. For more information, see Comparison Operators (JavaScript).

The optional fromIndex argument specifies the array index at which to begin the search. If fromIndex is greater than or equal to the array length, -1 is returned. If fromIndex is negative, the search starts at the array length plus fromIndex.

The following examples illustrate the use of the indexOf method.

// Create an array. (The elements start at index 0.)
var ar = ["ab", "cd", "ef", "ab", "cd"];

// Determine the first location of "cd".
document.write(ar.indexOf("cd") + "<br/>");

// Output: 1

// Find "cd" starting at index 2.
document.write(ar.indexOf("cd", 2) + "<br/>");

// Output: 4

// Find "gh" (which is not found).
document.write (ar.indexOf("gh")+ "<br/>");

// Output: -1

// Find "ab" with a fromIndex argument of -2.
// The search starts at index 3, which is the array length plus -2.
document.write (ar.indexOf("ab", -2) + "<br/>");
// Output: 3

Supported in the following document modes: Internet Explorer 9 standards and Internet Explorer 10 standards. Also supported in Windows Store apps. See Version Information.

Not supported in the following document modes: Quirks, Internet Explorer 6 standards, Internet Explorer 7 standards, Internet Explorer 8 standards.

Reference

Concepts