What does it mean "return !! userId"

 

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  • What is the !! (not not) operator in JavaScript? 31 answers

    • 它返回userId作为boolean

    It's a boolean cast, from a fasly or truethy value to false or true respectively.

    You might see: 

    var string = ""; // empty string is falsy
var bool = !!string; // false

    falsy by the way means that it follows: myvar == false as opposed to false which follows: myvar === false (triple comparison).

    Similarly truethy is myvar == true .

    !!userId与Boolean(userId)相同。

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