array.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
}

arr.indexOf(searchElement[, fromIndex])

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(searchValue, index) {
    // In non-strict-mode, if the `this` variable is null
    // or undefined, then it is set the the window object.
    // Else, `this` is automaticly converted to an object.
    var len = this.length >>> 0; // convert ot number or 0
    
    index |= 0;              // rounds and NaN-checks
    if (index < 0)             // check if negative start
      index = Math.max(len - index, 0
    else if (index >= len) // else, check if too big
      return -1;
    
    if (searchValue === undefined)
      // Because searchValue is undefined, keys that
      // don't exist will have the same value as the
      // searchValue, and thus do need to be checked.
      do {
        if (index in this && this[index] === undefined)
          return index;
      } while (++index !== len)
    else
      // Because searchValue is not undefined, there's no
      // need to check if the current key is in the array
      // because if the key isn't in the array, then it's
      // undefined which is not equal to the searchValue.
      do {
        if (this[index] === searchValue)
          return index;
      } while (++index !== len)
    
    // if nothing was found, then simply return -1
    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;
  };
}