Object.keys() vs Array.prototype.keys()

Question

let arr = new Array(3)
console.log(Object.keys(arr)) // []
console.log([...arr.keys()]) // [0,1,2]

Why is there a difference? It seems that an array in javascript can simultaneously have a length of 3 and be empty. How do you know the behavior of the different functions that operate on sparse lists?

Answer 1

Object.keys() does expect the parameter to be of type object and will cast anything else implicitly to one . Passing an empty array will be casted to an empty object I guess: Object.assign({}, new Array(3)) will give {}