Fill array in two dimensional array at a specific index using JavaScript

Question

I want to fill inner array of two dimensional array with items at specific index. The problem in this case is that every inner array is filled with that items.

Try: link_to_code_img

Result: link_to_result_img

the code:

Array.prototype.repeat= function(what, L){
  while(L) this[--L]= what;
  return this;
};

var yearsdays = [].repeat([], 365);

for(var i = 0; i<= yearsdays.length; i++){
   if(i === 99) {
      yearsdays[i].push(99)
   }
}

the result:

Array[365]
  [0 … 99]
     0: Array[1]
       0: 99
       length: 1
       __proto__: Array[0]

     1: Array[1]
       0: 99
       length: 1
       __proto__: Array[0]

     2: Array[1]
       0: 99
       length: 1
       __proto__: Array[0]

     and etc.......

The problem is that as a result every array in yearsdays was filled with number 99, but not only array with index 99 as I expect. What am I doing wrong?

@jackjop I should correct that to say "Please don't extend built-in prototypes, except from a centralised point and never as part of a library for other people to use." — gcampbell
– gcampbell, Commented Jun 4, 2016 at 9:55
@jackjop What if the next version of the browser has its own method with the same name? Your extension will override it, which could break things. — Barmar
– Barmar, Commented Jun 4, 2016 at 10:02

sertsedat · Accepted Answer · 2016-06-04 09:56:19Z

2

As Redu stated in the comments

Your inner arrays are all references of each other

You can get away from this problem like this. It somehow clones the array so the references are different.

Array.prototype.repeat= function(what, L){
  while(L) this[--L]= what.slice(0); // <--- Here
  return this;
};

answered Jun 4, 2016 at 9:56

sertsedat

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Comments

Charlie · Accepted Answer · 2016-06-04 09:59:26Z

0

You pass the same array to the repeat function. So it is going to be 365 references to a single array.

A correct way to do this is:

Array.prototype.repeat= function(L){
      while(L) this[--L]= new Array();
      return this;
};

answered Jun 4, 2016 at 9:59

Charlie

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Comments

Redu · Accepted Answer · 2016-06-04 10:03:22Z

0

In order to generate a proper 2D array all nested array items should be unique. Due to objects being reference types in JS you have to be ceraful when generating multi-dimensional arrays in JS. One possible and generic way of creating a multi-dimensional array is;

Array.prototype.clone = function(){
  return this.reduce((p,c,i) => (p[i] = Array.isArray(c) ? c.clone() : c, p),[])
}

function arrayND(...n){
  return n.reduceRight((p,c) => c = (new Array(c)).fill(true).map(e => Array.isArray(p) ? p.clone() : p ));
}

yearsdays = arrayND(365,1,0);
yearsdays[6][0]="Sunday";
console.log(yearsdays);

answered Jun 4, 2016 at 10:03

Redu

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