what does "object || {} " means in javascript?

Question

I found below line of code in javascript application.

var auth = parent.auth = parent.auth || {};

I know there is existing Object parent which is extended with auth Object but i don't understand what is parent.auth || {} doing here.

`var newVar = false || null || true` will assign `true` to `newVar`. the logical or operator is used to prevent _falsy_ values from being assigned. If `parent.auth` is falsy, or doesn't exist, then a new object literal is assigned — Elias Van Ootegem, Jun 18 '14 at 09:41
Related: [What does the construct x = x || y mean?](http://stackoverflow.com/questions/2802055/what-does-the-construct-x-x-y-mean/) — Michał Perłakowski, Jan 11 '16 at 01:59

score 6 · Accepted Answer · answered Jun 18 '14 at 09:39

6

parent.auth || {} means if parent.auth is undefined, null or false in boolean case then new empty object will be initialized and assigned.

or you can understand it like:

var auth;
if(parent.auth){       
    auth=parent.auth;   
} else {
    auth={};   
}

answered Jun 18 '14 at 09:39

Zaheer Ahmed

1

@sp00m the second approach explains the same. CLEARLY!! – Mr_Green Jun 18 '14 at 09:46
@Mr_Green What to you mean? I just wanted to add that 0, "" or NaN will be evaluated to false (i.e. falsy) as well as null, false or undefined. – sp00m Jun 18 '14 at 09:49
@sp00m the caps letter in my above comment means it is very clear that this six types falsies will be evaluated to false but not as I am shouting at you. :) – Mr_Green Jun 18 '14 at 09:55

score 3 · Answer 2 · answered Jun 18 '14 at 09:41

3

it means if the value of parent.auth is falsy(false, 0, null, undefied etc) then assign the value {}(empty object) to the variable auth

answered Jun 18 '14 at 09:41

Arun P Johny

but why parent.auth is assigned two times – Sandeep vashisth Jun 18 '14 at 09:47
@Sandeepvashisth it is because the `parent.auth`'s value itself is updated with the empty object if it is a falsy value... so once the statement is executed `parent.auth` will have a non falsy value – Arun P Johny Jun 18 '14 at 09:49
@Sandeepvashisth Maybe reading about [operator precedence](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Operator_Precedence) helps to understand this better? – Teemu Jun 18 '14 at 09:53

score 0 · Answer 3 · answered Jun 18 '14 at 09:39

0

|| is or, the code then returns an empty object, if parent.auth is undefined.

Like checking for null, then creating a new object if null (from java/c#).

answered Jun 18 '14 at 09:39

NimChimpsky

3 Answers3