Can someone explain this ( !! ) operator

Question

Angular 6 is here already, so in short tutorial provided by Medium i founded this lines of code.

else if (!!this.day4Name && !this.day5Name && days[date] !== this.day4Name) {
        this.day5Name = days[date];
        this.day5State = data[i].weather[0].main;
        this.day5Temp = Math.round(data[i].main.temp);

      }

I'v try to google it, but with no results for reasonable explanation. Can someone explain its behavior. Thanks :)

[What is the !! (not not) operator in JavaScript?](https://stackoverflow.com/questions/784929/what-is-the-not-not-operator-in-javascript) — Lars Beck, Jun 11 '18 at 10:35

Karim · Accepted Answer · 2019-01-09T14:54:02.950

4

!! represents a double negation, you're basically calling the not operator twice.

It's useful if you want to force a cast from any type to a boolean

e.g.

var somethingTruthy = {};
somethingTruthy = !!somethingTruthy //force cast to boolean
console.log(somethingTruthy); //print true

or

var somethingFalsy = "";
somethingFalsy = !!somethingFalsy //force cast to boolean
console.log(somethingFalsy); //print false

edited Jan 09 '19 at 14:54

answered Jun 11 '18 at 10:33

Karim

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if day4Name is defined !!day4Name will be evaluated to true. – Karim Jun 11 '18 at 10:35
To make it easy to understand, you can replace `!!` with `Boolean()` – Fez Vrasta Jun 19 '18 at 18:01

Can someone explain this ( !! ) operator

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