javascript array - what is the quickest most efficient way to create an array with a starting index and a default value

Question

I want to create an array in javascript to have 20 elements with a starting index of 15, assign every element with a value of 00000. What is the most efficient way, and can it be done without looping through the index?

TIA.

Answer 1

Array indexes in javascript are always in the interval [0, num_elements) . If you do not want this, you need to use an object. Then you have arbitraty indexes but no fixed order.

Answer 2

Without loop you can only use instant initialization:

var arr = [null, null, null, ... 15 times ...., '000000', '000000', '000000', '000000', '000000'];

But probably it's better to use loop :)

Answer 3

It's better to use an object , if you want to use an offset:

var iOffset = 15;
var iCount = 20;
var oValues = {};
for (var i = iOffset ; i < iOffset + iCount; i++) {
    oValues[i] = 0;
}

Answer 4

You can't have an array starting with an index of 15. The index of an array always start with index 0, as is demonstrated with:

var a = [];
a[15] = '000';
alert(a.length); //=> 16

You could use a recursive function to create an array of 20 elements, something like:

function arr(len){
    return len ? arr(len-1).concat('0000') : [];
}
var myArr = arr(20);

Or create an array of small objects, containing your custom index:

function arr(len){
    return len ? arr(len-1).concat({index:len+14,value:'0000'}) : [];
}
var myArr = arr(20);
alert(myArr[0].index); //=> 15

Alternatively, using a loop, you can do something like:

var nwArr = function(len,start){
  var l = len+start, a = Array(l);
  while ((l-=1)>=start){ a[l] = '0000'; }
  return a;
}(20,15);
alert(nwArr[15]); //=> '0000'
alert(nwArr[0]);  //=> undefined

Answer 5

For the specific case where each item in the array is a string, you can do it without a loop with the new Array(length) constructor and a sneaky join :

new Array(15).concat(('000000'+new Array(20).join(',000000')).split(','))

Unlikely to be faster than the loop though... certainly less clear. I'd also question whether an Array with missing properties 0–14 is something it's generally sensible to have.