Javascript逻辑运算符：？

Question

I was examining the src of underscore.js and discovered this:

_.isRegExp = function(obj) {
    return !!(obj && obj.test && obj.exec && (obj.ignoreCase || obj.ignoreCase === false));
};

Why was "!!" used? Should it be read as NOT-NOT or is there some esoteric JS nuance going on here?

Answer 1

将结果转换为布尔值只是一种迟钝的方法。

Answer 2

Yes, it's NOT-NOT. It is commonly used idiom to convert a value to a boolean of equivalent truthiness.

JavaScript understands 0.0 , '' , null , undefined and false as falsy, and any other value (including, obviously, true ) as truthy. This idiom converts all the former ones into boolean false , and all the latter ones into boolean true .

In this particular case,

a && b

will return b if both a and b are truthy;

!!(a && b)

will return true if both a and b are truthy.

Answer 3

The && operator returns either false or the last value in the expression:

("a" && "b") == "b"

The || operator returns the first value that evaluates to true

("a" || "b") == "a"

The ! operator returns a boolean

!"a" == false

So if you want to convert a variable to a boolean you can use !!

var myVar = "a"
!!myVar == true

myVar = undefined
!!myVar == false

etc.

Answer 4

It is just two ! operators next to each other. But a double-negation is pointless unless you are using !! like an operator to convert to Boolean type.

It will convert anything to true or false...