Javascript or and assignment operators

Question

I was looking through some compiled coffee-script code, and I noticed something like the following, which I thought was really strange:

var current, x = 8;
current = this._head || (this._head = x);

after running this, current has a value of 8. Judging by the way that the || logical operator works, I would have expected it to evaluate the left side first. After getting an 'undefined' on the left hand side, it moves onto the right, where it assigns this._head to 8. Afterwards it returns a true, but this part isn't that important? I don't see how it could go back and affect the "current" variable? Any help would be appreciated, thanks!

OH! I just realized I was getting hungup on operator precedence. The || operator happens before any value has been assigned to current. I was essentially imagining current = this._head having parentheses around it. thanks everyone! — elju
– elju, Commented Mar 5, 2012 at 11:44

James M · Accepted Answer · 2012-03-05 08:35:58Z

1

The || operator returns the value, not true. Maybe it helps to say that

current = this._head || (this._head = x)

could also be written as

current = this._head ? this._head : (this._head = x);

or

current = this._head;

if(!current)
    current = this._head = x;

answered Mar 5, 2012 at 8:35

James M

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Salman Arshad · Accepted Answer · 2012-03-05 09:26:25Z

1

The || operator returns the left hand side if it is "truthy", otherwise, the right hand side -- regardless of its truthiness. It does not cast the expression to the boolean true/false!
- undefined || (this._head = x) returns the right hand side
The assignment operator also returns a value!
- this._head = x returns 8 in the above example
The first assignment operator assigns the value 8 to the variable current

edited Mar 5, 2012 at 9:26

answered Mar 5, 2012 at 9:14

Salman Arshad

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Comments

Kunal Vashist · Accepted Answer · 2012-03-05 08:37:51Z

0

You can use expresion

var current=this._head ? this._head : (this._head = x);

answered Mar 5, 2012 at 8:37

Kunal Vashist

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