Is there a shortcut to create padded array in JavaScript?

Question

I have this javascript:

function padded_array(k, value){
    var a = [];
    a[k] = value;
    return a;
}

padded_array(3, "hello"); //=> [undefined, undefined, undefined, 'hello']

Is it possible to shorten the code in the function body?

Answer 1

for all the googlers coming here - you're probably looking for this:

var pad_array = function(arr,len,fill) {
  return arr.concat(Array(len).fill(fill)).slice(0,len);
}

Answer 2

From 2020 & 2021 : straight forward options

Let assume that is your Array

const yourArray = [1,2]

If you just want to loop 4 times (maybe for react jsx )

Array.from({length:4}) //[undefined,undefined,undefined,undefined]
Array(4).fill()//[undefined,undefined,undefined,undefined]

If you want to loop yourArray 4 times, but to start with values you already have

// unmutation option
Array.from({...yourArray, length:4}) //[1,2,undefined,undefined]

// unmutation option, but need some calcualtion
[...yourArray , ...Array(2)  ] //[1,2,undefined,undefined]
[...Array(2), ...yourArray   ] //[undefined,undefined,1,2]

// loop on your array several times 
Array(3).fill(yourArray).flat() // [1, 2, 1, 2, 1, 2]

// mutation the original array.
yourArray.length = 4;
Array.from(yourArray) //[1,2,undefined,undefined]

If You actually want an Array with full of values. ex. with increment numbers.
Remap it

// unmutation option

Array.from({...yourArray,length:4}, (v,i) => v ?? i+1 ) 
// [1,'2',3, 4]

// Or, mutation the original array. and fill with "x"
array.yourArray.length = 4;

Array.from(yourArray, (v) => v ?? 'x') 
// [1,'2','x','x']