CODEWITHSUNDEEP
javascriptnode.jsobject
Tamil
Tamil

Reputation: 5358

Default values for objects in Javascript

Javascript Code

var a = {};
a.test += 1; //NaN
++a.test; //NaN

Instead 

var a = {};
a.test = 0;
++a.test; //1
a.test += 1; //2

I wonder if there could be anyway that can make first code sample work the same as second, i.e without an explicit assignment to 0. As in assigning default value for any property of an object to 0 instead undefined. I'm trying to do this in node.js. So, no problem of cross browser things and old ECMA Specs. 

var i;
for(i = 0; i<10; i++) {
   if(a.test) {
     ++a.test;
   } else {
     a.test = 0;
     ++a.test;
   }
   //a.test = a.test || 0; (1)
   //++a.test;
}

If it is possible then the inner if/else or the assignment statement(1) in the above code can be eliminated.

Upvotes: 1

Views: 3935

Answers (6)

Alnitak
Alnitak

Reputation: 339816

There's no way to do this for any arbitrary undefined property.

For a known property name, there is a way, but DO NOT USE IT !! 1

Object.prototype.test = 0;

This will give every object an implicit .test property with that value in it, and the first time you attempt to modify it the result will be stored in your own object:

> Object.prototype.test = 0
> a = {}
> a.test++
> a.test
1

1 Adding stuff to Object.prototype will break stuff, including for (key in obj)

Upvotes: 3

Shahbaz Chishty
Shahbaz Chishty

Reputation: 502

I think this would do:

var a = {"test":0};
a.test += 1;

Upvotes: 0

Joseph
Joseph

Reputation: 119847

In the first code, you can't. Any number added to a non-number or NaN value will always result in NaN

var a = {};
a.test += 1; // undefined + 1 = NaN
++a.test;    // ++(undefined) = NaN

as for the inner if

for(i = 0; i<10; i++) {
   a.test = a.test || 0; //use existing value, or if undefined, 0
   ++a.test;             //increment
}

Upvotes: 0

ninjagecko
ninjagecko

Reputation: 91092

This is where prototypes come in useful in javascript:

function Demo() {
}
Demo.prototype.x = 0;

 

> a = new Demo();
> a.x += 1
> a.x
1

 

> b = new Demo()
> b.x
0

Upvotes: 1

Koenyn
Koenyn

Reputation: 704

Using standard JS it's not possible to do what you're asking. Unless you prototype I guess? But I'm not experienced with Prototype.And I don't think prototyping works with Node.js

The reason for this is because js is not a typed language (i.e. we only use var to declare a variable, not int a or string b), so in order to use the ++ operator, the variable needs to be given a type through assignment.

hope that helps

Upvotes: -1

Ivan Drinchev
Ivan Drinchev

Reputation: 19581

Javascript by default defines all new variables as undefined ( if not explicitly defined ) , which is different from Number object, which you are trying to define. So you should use smth like :

for (i=0; i<10; i++) {
    a.test = a.test || 0;
    ++a.test
}

Upvotes: 4

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