# Array.prototype.includes()

## Sumário

O método includes`()` determina se um array contém um determinado elemento, retornando `true` ou `false` apropriadamente.

Sintaxe

``array.includes(searchElement[, fromIndex])``

### Parâmetros

`searchElement`
O elemento a buscar
`fromIndex`
Opcional. A posição no array de onde a busca pelo `searchElement `se iniciará. Por padrão, 0.

## Exemplos

``````[1, 2, 3].includes(2);     // true
[1, 2, 3].includes(4);     // false
[1, 2, 3].includes(3, 3);  // false
[1, 2, 3].includes(3, -1); // true
[1, 2, NaN].includes(NaN); // true
``````

## Polyfill

``````// https://tc39.github.io/ecma262/#sec-array.prototype.includes
if (!Array.prototype.includes) {
Object.defineProperty(Array.prototype, 'includes', {
value: function(searchElement, fromIndex) {

// 1. Let O be ? ToObject(this value).
if (this == null) {
throw new TypeError('"this" is null or not defined');
}

var o = Object(this);

// 2. Let len be ? ToLength(? Get(O, "length")).
var len = o.length >>> 0;

// 3. If len is 0, return false.
if (len === 0) {
return false;
}

// 4. Let n be ? ToInteger(fromIndex).
//    (If fromIndex is undefined, this step produces the value 0.)
var n = fromIndex | 0;

// 5. If n ≥ 0, then
//  a. Let k be n.
// 6. Else n < 0,
//  a. Let k be len + n.
//  b. If k < 0, let k be 0.
var k = Math.max(n >= 0 ? n : len - Math.abs(n), 0);

// 7. Repeat, while k < len
while (k < len) {
// a. Let elementK be the result of ? Get(O, ! ToString(k)).
// b. If SameValueZero(searchElement, elementK) is true, return true.
// c. Increase k by 1.
// NOTE: === provides the correct "SameValueZero" comparison needed here.
if (o[k] === searchElement) {
return true;
}
k++;
}

// 8. Return false
return false;
}
});
}
``````