Array.prototype.keys()

The keys() method returns a new Array Iterator object that contains the keys for each index in the array.

arr.keys()

A new Array iterator object.

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()
• Iteration protocols
• A polyfill