The indexOf() method returns the first index at which a given element can be found in the array, or -1 if it is not present.

Note: For the String method, see String.prototype.indexOf().

var a = [2, 9, 9]; 
a.indexOf(2); // 0 
a.indexOf(7); // -1

if (a.indexOf(7) === -1) {
  // element doesn't exist in array
}

Syntax

arr.indexOf(searchElement[, fromIndex])

Parameters

searchElement
Element to locate in the array.
fromIndex Optional
The index to start the search at. If the index is greater than or equal to the array's length, -1 is returned, which means the array will not be searched. If the provided index value is a negative number, it is taken as the offset from the end of the array. Note: if the provided index is negative, the array is still searched from front to back. If the calculated index is less than 0, then the whole array will be searched. Default: 0 (entire array is searched).

Return value

The first index of the element in the array; -1 if not found.

Description

indexOf() compares searchElement to elements of the Array using strict equality (the same method used by the === or triple-equals operator).

Examples

Using indexOf()

The following example uses indexOf() to locate values in an array.

var array = [2, 9, 9];
array.indexOf(2);     // 0
array.indexOf(7);     // -1
array.indexOf(9, 2);  // 2
array.indexOf(2, -1); // -1
array.indexOf(2, -3); // 0

Finding all the occurrences of an element

var indices = [];
var array = ['a', 'b', 'a', 'c', 'a', 'd'];
var element = 'a';
var idx = array.indexOf(element);
while (idx != -1) {
  indices.push(idx);
  idx = array.indexOf(element, idx + 1);
}
console.log(indices);
// [0, 2, 4]

Finding if an element exists in the array or not and updating the array

function updateVegetablesCollection (veggies, veggie) {
    if (veggies.indexOf(veggie) === -1) {
        veggies.push(veggie);
        console.log('New veggies collection is : ' + veggies);
    } else if (veggies.indexOf(veggie) > -1) {
        console.log(veggie + ' already exists in the veggies collection.');
    }
}

var veggies = ['potato', 'tomato', 'chillies', 'green-pepper'];

updateVegetablesCollection(veggies, 'spinach'); 
// New veggies collection is : potato,tomato,chillies,green-pepper,spinach
updateVegetablesCollection(veggies, 'spinach'); 
// spinach already exists in the veggies collection.

Polyfill

indexOf() was added to the ECMA-262 standard in the 5th edition; as such it may not be present in all browsers. You can work around this by utilizing the following code at the beginning of your scripts. This will allow you to use indexOf() when there is still no native support. This algorithm matches the one specified in ECMA-262, 5th edition, assuming TypeError and Math.abs() have their original values.

if (!Array.prototype.indexOf) {

  Array.prototype.indexOf = function (vMember, nStartFrom) {

    /*
    In non-strict mode, if the `this` variable is null or undefined, then it is
    set the the window object. Otherwise, `this` is automaticly converted to an
    object. In strict mode if the `this` variable is null or undefined a
    `TypeError` is thrown.
    */

    if (this == null) {

      throw new TypeError("Array.prototype.indexOf() - can't convert `" + this + "` to object");

    }

    var
      nIdx = isFinite(nStartFrom) ? Math.floor(nStartFrom) : 0,
      oThis = this instanceof Object ? this : new Object(this),
      nLen = isFinite(oThis.length) ? Math.floor(oThis.length) : 0;

    if (nIdx >= nLen) {

      return -1;

    }

    if (nIdx < 0) {

      nIdx = Math.max(nLen + nIdx, 0);

    } 
    
    if (vMember === undefined) {

      /*
      Since `vMember` is undefined, keys that don't exist will have the same
      value as `vMember`, and thus do need to be checked.
      */

      do {

        if (nIdx in oThis && oThis[nIdx] === undefined) {

          return nIdx;

        }

      } while (++nIdx < nLen);

    } else {

      do {

        if (oThis[nIdx] === vMember) {

          return nIdx;

        }

      } while (++nIdx < nLen);

    }

    return -1;

  };

}

However, if you are more interested in all the little technical bits defined by the ECMA standard, and are less concerned about performance or conciseness, then you may find this more descriptive polyfill to be more usefull.

// Production steps of ECMA-262, Edition 5, 15.4.4.14
// Reference: http://es5.github.io/#x15.4.4.14
if (!Array.prototype.indexOf) {
  Array.prototype.indexOf = function(searchElement, fromIndex) {

    var k;

    // 1. Let o be the result of calling ToObject passing
    //    the this value as the argument.
    if (this == null) {
      throw new TypeError('"this" is null or not defined');
    }

    var o = Object(this);

    // 2. Let lenValue be the result of calling the Get
    //    internal method of o with the argument "length".
    // 3. Let len be ToUint32(lenValue).
    var len = o.length >>> 0;

    // 4. If len is 0, return -1.
    if (len === 0) {
      return -1;
    }

    // 5. If argument fromIndex was passed let n be
    //    ToInteger(fromIndex); else let n be 0.
    var n = fromIndex | 0;

    // 6. If n >= len, return -1.
    if (n >= len) {
      return -1;
    }

    // 7. If n >= 0, then Let k be n.
    // 8. Else, n<0, Let k be len - abs(n).
    //    If k is less than 0, then let k be 0.
    k = Math.max(n >= 0 ? n : len - Math.abs(n), 0);

    // 9. Repeat, while k < len
    while (k < len) {
      // a. Let Pk be ToString(k).
      //   This is implicit for LHS operands of the in operator
      // b. Let kPresent be the result of calling the
      //    HasProperty internal method of o with argument Pk.
      //   This step can be combined with c
      // c. If kPresent is true, then
      //    i.  Let elementK be the result of calling the Get
      //        internal method of o with the argument ToString(k).
      //   ii.  Let same be the result of applying the
      //        Strict Equality Comparison Algorithm to
      //        searchElement and elementK.
      //  iii.  If same is true, return k.
      if (k in o && o[k] === searchElement) {
        return k;
      }
      k++;
    }
    return -1;
  };
}

Specifications

Specification Status Comment
ECMAScript 5.1 (ECMA-262)
The definition of 'Array.prototype.indexOf' in that specification.
Standard Initial definition. Implemented in JavaScript 1.6.
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Array.prototype.indexOf' in that specification.
Standard  
ECMAScript Latest Draft (ECMA-262)
The definition of 'Array.prototype.indexOf' in that specification.
Living Standard  

Browser compatibility

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic Support (Yes) (Yes)1.59 (Yes) (Yes)
FeatureAndroidChrome for AndroidEdge mobileFirefox for AndroidIE mobileOpera AndroidiOS Safari
Basic Support (Yes) (Yes) (Yes)4 (Yes) (Yes) (Yes)

Compatibility notes

  • Starting with Firefox 47 (Firefox 47 / Thunderbird 47 / SeaMonkey 2.44),  this method will no longer return -0. For example, [0].indexOf(0, -0) will now always return +0 (bug 1242043).

See also