JavaScript - 'Normalize' array indexes

Question

I am looking for a method to 'Normalize' array indexes in Javascript. I have a large array ( _spm ) containing values with numeric indexes like ( _spm[201023] ).

( For You, to understand: Re-indexing the array starting from zero. )

I would need a method to make all the records have a more human-friendly index ( and may also be sorted ).

I mean:

function normalize( arrayWithMessedUpIndexes ){ return reIndexedAndOrderedArray }

A possible input is the array _spm considering that:

 _spm[201023] = "s";
 _spm[376615] = "m";

A possible output of the method I am looking for ( using the input array above ):

 _spm[0] = "s"
 _spm[1] = "m"

Can't you just loop through and push the entries to a new array? Or is there some mystery to this that you are leaving out? — epascarello
– epascarello, Commented Nov 18, 2013 at 14:27

Moritz Roessler · Accepted Answer · 2013-11-18 15:03:26Z

2

You could use Array.prototype.filter

var arr = [];
arr [42] = "foo";
arr [1337] = "bar";

arr.filter (function () {return true}); //["foo", "bar"]

Now, how does this work?. Let's take a look at ES5 §15.4.4.20, which describes the process.

... some initialization steps
9. Repeat, while k < len
- a. Let Pk be ToString(k).
- b. Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
- c. If kPresent is true, then
  - do the magic

Since the first initialized index of our example array is 42, for every n < 42, calling n in arr or arr.hasOwnProperty (n) evaluates to false.

Given that, the condition, described in step 9c, is not met, hence, the index is skipped.

Note that [].filter is ES5 and may not be compatible with older browsers.

edited Nov 18, 2013 at 15:03

answered Nov 18, 2013 at 14:29

Moritz Roessler

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3 Comments

István Pálinkás Over a year ago

As far as I know this will not reindex the array's records, will it?

Moritz Roessler Over a year ago

@StevenPalinkas Yes, it will, I added a little explanation. Looking for a nice Answer on SO that describes those implicit undefined. I'll add it when i found one

Moritz Roessler Over a year ago

@StevenPalinkas didn't found a good one, added an explanation based on the ES5 specification

shyam · Accepted Answer · 2013-11-18 14:29:30Z

1

Array indexes are always sorted so, all you need to do is remove the undesirable elements

_spm = _spm.filter(function(v) { return v != undefined });

answered Nov 18, 2013 at 14:29

shyam

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2 Comments

Moritz Roessler Over a year ago

What a coincidence ^^. No need for the undefined check though, unless you want to filter explicit undefineds. As the others are implicit and won't get iterated over

shyam Over a year ago

@C5H8NNaO4 True. Never had to rely on that behavior before :)

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