How to load hash in a loop using objects as keys in JavaScript?

Question

I get unexpected behavior when trying to load a hash using objects as keys; ie, when retrieving my data later the hash always refers to the last key used. I would expect this to be due to the behavior of closures, however, I thought that I have done what would be necessary to prevent this:

var hash = {};
var arry = [];
var list = [{val:"a"},{val:"b"},{val:"c"}];

var len  = list.length;

dump("load : \n");

for (var i=0;i<len;i++) {

  let pos = i;
  let obj = list[pos];
  hash[obj] = obj.val;
  arry.push(obj);
  dump("    "+obj.val+"    "+hash[obj]+"\n");
}

dump("retrieve : \n");

for (var i=0;i<len;i++) {

  let pos = i;
  let obj = list[pos];
  dump("    "+obj.val+"    "+arry[pos].val+"    "+hash[obj]+"\n");
}

output is:

load :
    a    a
    b    b
    c    c
retrieve :
    a    a    c
    b    b    c
    c    c    c

I have purposely gone overboard in trying to prevent this by raising the scope of the iteration objects using let, still I am apparently missing something. I would like to understand the reason behind this, and how to prevent it using Javascript.

Community · Accepted Answer · 2017-05-23 12:11:14Z

6

Object keys in JavaScript can only be strings. This means that if you pass something that is not a string, it gets converted to a string. Since you're using objects that don't "override" Object.toString, they'll all have the same string representation (it's "[object Object]"), which is why the hash always refers to the last key used.

More here: Strange equality issue with Javascript object properties

edited May 23, 2017 at 12:11

CommunityBot

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answered Jun 22, 2013 at 18:05

Matt Ball

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Community · Accepted Answer · 2017-05-23 12:11:14Z

0

To extend @Matt Ball answer:

You can use JSON.stringify to string objects.

var a = {};
a.b = 1;

JSON.stringify(a);
// '{"b": 1}'

Note: You can't stringify circular objects:

var a = {};
a.a = a;

JSON.stringify(a);
// TypeError: Converting circular structure to JSON

edited May 23, 2017 at 12:11

CommunityBot

11 silver badge

answered Jun 22, 2013 at 18:09

Andreas Louv

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