Is the !! operator even necessary in Javascript?

Question

I can understand cases when you will want to convert an object value to a boolean and save it in a variable. However, I came across the following code in a jQuery template and was wondering if the !! (double exclamation operators) is even necessary.

{{if !!sectionId}} // do something... {{/if}}

I am assuming that it is not since Javascript will automatically evaluate the expression following the if as boolean. Therefore, you could just write:

{{if sectionId}} // do something... {{/if}}

Am I right in my assumption?

possible duplicate of stackoverflow.com/questions/784929/… — jbabey
– jbabey, Commented Oct 26, 2011 at 18:09
This is the most close duplicate IMO: How to use the double not (!!) operator in javascript — Christian C. Salvadó
– Christian C. Salvadó, Commented Oct 26, 2011 at 18:17

Pointy · Accepted Answer · 2011-10-26 18:15:16Z

8

There is no !! operator in JavaScript. There's just !. What you're seeing is a doubled application of that single operator.

A single application of ! will return a boolean by evaluating the "truthiness" of its argument, giving the boolean inverse of that. The second ! therefore gives the boolean inverse of that value, which is thus the boolean "truthiness" of the original value.

Personally I wouldn't use it in a simple if statement as in your example, but it's handy for APIs that might explicitly check for a boolean-typed parameter:

someAPI( !! someExpression );

edited Oct 26, 2011 at 18:15

answered Oct 26, 2011 at 18:04

Pointy

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5 Comments

Vincent Catalano Over a year ago

I realize that. I had looked at this post initially stackoverflow.com/questions/784929/…. I am still curious if using this implementation is even necessary for this situation.

Pointy Over a year ago

It's necessary when the code, for whatever reason, really needs a boolean value. Some APIs explicitly check for a boolean and not a truthy value in function argument lists etc.

zzzzBov Over a year ago

type security can be an issue, if you want to make sure a function returns a boolean, it can be important to cast: return !!(this.foo || this.bar) would be unnecessary in other languages, but || and && don't actually return booleans, they just returns the first truthy and falsey values they come across (respectively).

Matt Over a year ago

To add to Pointy's answer and comment, JavaScript has many truthy values, aside from 'true'. See stackoverflow.com/questions/1995113/strangest-language-feature/…

kybernetikos Over a year ago

If you're casting to a boolean, it's more clear to use the boolean cast function Boolean(x);

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