Array.prototype.keys()

El método keys() devuelve un nuevo objeto  Array Iterator que contiene las claves de índice con las que acceder a cada elemento en el array.

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arr.keys()

Un nuevo objeto iterador Array.

var arr = ['a', 'b', 'c'];
var iterator = arr.keys();

console.log(iterator.next()); // { value: 0, done: false }
console.log(iterator.next()); // { value: 1, done: false }
console.log(iterator.next()); // { value: 2, done: false }
console.log(iterator.next()); // { value: undefined, done: true }

var arr = ['a', , 'c'];
var sparseKeys = Object.keys(arr);
var denseKeys = [...arr.keys()];
console.log(sparseKeys); // ['0', '2']
console.log(denseKeys);  // [0, 1, 2]

• Array.prototype.values()
• Array.prototype.entries()
• Protocolos de iteración