Best way to find if an item is in a JavaScript array? [duplicate]

As of ECMAScript 2016 you can use includes()

arr.includes(obj);

If you want to support IE or other older browsers:

function include(arr,obj) {
    return (arr.indexOf(obj) != -1);
}

EDIT:
This will not work on IE6, 7 or 8 though. The best workaround is to define it yourself if it’s not present:

1. Mozilla’s (ECMA-262) version:

      if (!Array.prototype.indexOf)
      {
   
           Array.prototype.indexOf = function(searchElement /*, fromIndex */)
   
        {
   
   
        "use strict";
   
        if (this === void 0 || this === null)
          throw new TypeError();
   
        var t = Object(this);
        var len = t.length >>> 0;
        if (len === 0)
          return -1;
   
        var n = 0;
        if (arguments.length > 0)
        {
          n = Number(arguments[1]);
          if (n !== n)
            n = 0;
          else if (n !== 0 && n !== (1 / 0) && n !== -(1 / 0))
            n = (n > 0 || -1) * Math.floor(Math.abs(n));
        }
   
        if (n >= len)
          return -1;
   
        var k = n >= 0
              ? n
              : Math.max(len - Math.abs(n), 0);
   
        for (; k < len; k++)
        {
          if (k in t && t[k] === searchElement)
            return k;
        }
        return -1;
      };
   
    }
   

2. Daniel James’s version:

    if (!Array.prototype.indexOf) {
      Array.prototype.indexOf = function (obj, fromIndex) {
        if (fromIndex == null) {
            fromIndex = 0;
        } else if (fromIndex < 0) {
            fromIndex = Math.max(0, this.length + fromIndex);
        }
        for (var i = fromIndex, j = this.length; i < j; i++) {
            if (this[i] === obj)
                return i;
        }
        return -1;
      };
    }
   

3. roosteronacid’s version:

    Array.prototype.hasObject = (
      !Array.indexOf ? function (o)
      {
        var l = this.length + 1;
        while (l -= 1)
        {
            if (this[l - 1] === o)
            {
                return true;
            }
        }
        return false;
      } : function (o)
      {
        return (this.indexOf(o) !== -1);
      }
    );