CODEWITHSUNDEEP
javascript
user1207289
user1207289

Reputation: 3253

Why do these two dynamic property assignments collapse into a single value?

I may be asking a basic question but I don't know how to clear such a confusion through google search

this below code works fine and count words, so I was trying to understand further

var words = chunk.trim().split(' ');
       var counts = {};

   // Count words
   words.forEach(function (word) {
       word = word.trim();

       if (word.length) {
           if (!counts[word]) {
               counts[word] = 0;
           }

           counts[word]++;
       }
   });

then why not this below gives me 1,2 . I am expecting 1,2 and its giving me 2,2

var a = function(){
  var a, b;
  var  obj = {}
   obj[a]=1;
   obj[b]=2;

  console.log(obj[a]);
  console.log(obj[b]);
}

var res = a();

Upvotes: 2

Views: 110

Answers (3)

Karim
Karim

Reputation: 8632

Because you're only declaring (not assigning) both a and b, and in javascript a not assigned variable is implicitely undefined.

you're essentially referring to obj[undefined] twice

var a,b; //here both the variables are undefined

obj[a]=1; 
obj[b]=2; //you're overriding the variable


console.log(obj[a]); //2
console.log(obj[b]); //2

Upvotes: 6

gyre
gyre

Reputation: 16787

Why doesn't this give me 1,2? I am expecting 1,2 and it's giving me 2,2

This is happening because a == b; they are both undefined because you never assigned them values on the line var a, b. When you omit the = assignment in var declarations, the default value of undefined is used instead, meaning that your line becomes var a = undefined, b = undefined.

Thus, the second property assignment (obj[b] = 2) is actually overwriting the first (obj[a] = 1). Your obj only contains a single key, and after running your code its overall structure looks like:

{
  'undefined': 2
}


var a = function () {
  var a, b
  var obj = {}
  obj[a] = 1
  obj[b] = 2

  console.log(obj)
}

var res = a()

If you want the results you expected, you need to use distinct keys, like the strings 'a' and 'b':

var a = function() {
  var a = 'a', b = 'b'
  var obj = {}
  obj[a] = 1
  obj[b] = 2
  
  console.log(obj)
  console.log(obj[a], obj[b])
}

var res = a()

Upvotes: 1

Pankaj Shukla
Pankaj Shukla

Reputation: 2672

When you say obj[a], javascript tries to create a property on obj with the value in a. Since value in a is undefined, the string representation is "undefined" and hence a property undefined is created on obj. Since b also happens to have undefined value, you are essentially writing 2 onto the same property undefined.

Hence you get 2 both the times. Actually if you log obj and see the value in console, you would see:

Object {undefined: 2}
undefined:2

Upvotes: 1

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