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I normally use while loop as:

while (i<some_value)

I saw while(i--) syntax and thought it is shorter and cooler and tried the following in google-chrome.

var num_arr= [4,8,7,1,3];


var length_of_num_arr=num_arr.length;


while(length_of_num_arr--) console.log(num_arr);
 [4, 8, 7, 1, 3]
 [4, 8, 7, 1, 3]
 [4, 8, 7, 1, 3]
 [4, 8, 7, 1, 3]
 [4, 8, 7, 1, 3] **// THIS IS EXPECTED RESULT**

But When I try...

while((num_arr.length)--) console.log(num_arr);
[4, 8, 7, 1]
[4, 8, 7]
[4, 8]
[4]
[] // WHY IS THIS HAPPENING??

Is there some hidden things you need to understand to use this syntax?

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4
  • num_arr.length-- is the same as num_arr.length = num_arr.length - 1. Is that clearer now? Commented Jun 19, 2015 at 2:28
  • Never thought array length was writable! Thanks Commented Jun 19, 2015 at 2:29
  • @Jack_of_All_Trades: Although, if it wasn’t, this still wouldn’t work – the trick depends on length_of_num_arr being decremented, after all. Commented Jun 19, 2015 at 2:31
  • @minitech: yes you are right....my brain working bad.... Thanks to all of you guys... Commented Jun 19, 2015 at 2:33

3 Answers 3

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8

Arrays’ length property is writable, and will cut off their elements or add empty slots as appropriate when you set it.

var items = [1, 2, 3, 4, 5];
items.length = 3; // items is now [1, 2, 3]
items.length = 6; // items is now a sparse array: [1, 2, 3, undefined × 3]

So, don’t do that.

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7 Comments

Wow!....One thing I knew today at 10:28 PM ..Javascript you never fail to surprise me..
Is it one of the thing that only happens in javascript? I don't think you can change the length of array this way in any other languages....I don't remember at least in python....
@Jack_of_All_Trades: There’s probably something else out there, but I’m not aware of a writable length property in any other common languages, no. VB.NET has ReDim, Python has del arr[-5:], etc. as far as unusual slightly unusual possible resizing methods go.
Note setting length to a higher value does not append undefineds.
That should be: items.length = 6; // items is now a sparse array: [1, 2, 3,,,,];. The additional elements aren't undefined, they don't exist (i.e. they are not defined, rather than exist and have the value undefined).
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2

When you do array.length-- you're potentially shortening the array by one element each time.

See the reference under the section Shortening the array from: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/length

Array.prototype.length can be re-written programmatically and it potentially shorten your array by the new length you assign.

For example

a = [1,2,3,4,5,6,7,8,9,10];

// Shorten the array by one element
a.length--; // a <-- [1,2,3,4,5,6,7,8,9]

// In case you want to shorten your array to 3 elements, you can:
a.length = 3; // a <-- [1,2,3]
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1

When you set the length property of an array to a lower value, the items at the end are removed:

var arr = [1,2,3,4,5];
arr.length; // 5
arr.length = 3;
arr; // [1,2,3]

This is described in the spec:

While newLen < oldLen repeat,

  1. Set oldLen to oldLen – 1.
  2. Let deleteSucceeded be the result of calling the [[Delete]] internal method of A passing ToString(oldLen) and false as arguments.

In your code you use the postfix decrement operator (--) which reduces the length of the array.

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