initArray.js

/**
 * Create an array with `len` elements.
 * 
 * @param [initFunc] Optional: a function that returns
 * the elements with which to fill the array.
 * If the function is omitted, all elements are `undefined`.
 * `initFunc` receives a single parameter: the index of an element.
 */
function initArray(len, initFunc) {
    if (typeof initFunc !== 'function') {
        // Yes, the return is uncessary, but more descriptive
        initFunc = function () { return undefined };
    }
    var result = new Array(len);
    for (var i=0; i < len; i++) {
        result[i] = initFunc(i);
    }
    return result;
}

/* Interaction:

> initArray(3)
[ undefined, undefined, undefined ]
> initArray(3, function (x) { return x })
[ 0, 1, 2 ]

*/

In order to init an Array "with n elements" , I usually use (to ensure no hole inside) :

Array.apply(null, Array(3)); // => Array(undefined, undefined, undefined)
Array.apply(null, Array(3)).map(function() {return 'x'}); // => ["x", "x", "x"]
Array.apply(null, Array(3)).map(function(x, i) {return i}); // => [0, 1, 2]

Btw, as far as I remember, I saw this trick on 2ality :)

So, what about this solution ?

function initArray(len, initFunc) {
  if (typeof initFunc !== 'function') {
    // Yes, the return is uncessary, but more descriptive
    initFunc = function () { return undefined };
  }
  return Array.apply(null, Array(len)).map(initFunc);
}

console.log(initArray(3)); // [undefined, undefined, undefined]
console.log(initArray(3, function(x,i) {return i;})); // [0, 1, 2] 
console.log(initArray(3, function() { return 'whatever';})); // ["whatever", "whatever", "whatever"]