Array.prototype.indexOf()

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

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]

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.

if (!Array.prototype.indexOf) {
  Array.prototype.indexOf = function indexOf(member, startFrom) {
    /*
    In non-strict mode, if the `this` variable is null or undefined, then it is
    set to the window object. Otherwise, `this` is automatically 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 index = isFinite(startFrom) ? Math.floor(startFrom) : 0,
      that = this instanceof Object ? this : new Object(this),
      length = isFinite(that.length) ? Math.floor(that.length) : 0;

    if (index >= length) {
      return -1;
    }

    if (index < 0) {
      index = Math.max(length + index, 0);
    }

    if (member === undefined) {
      /*
        Since `member` is undefined, keys that don't exist will have the same
        value as `member`, and thus do need to be checked.
      */
      do {
        if (index in that && that[index] === undefined) {
          return index;
        }
      } while (++index < length);
    } else {
      do {
        if (that[index] === member) {
          return index;
        }
      } while (++index < length);
    }

    return -1;
  };
}

// 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;
  };
}