Js in the article to find out if (in) Usage

 

Consider the following example:

 

        names = the let [ 'Lily', 'Barry', 'Dendi', 'Boogie', 'Lily' ]; 

        the let nameNum = names.reduce ((pre, CUR) => {
             IF (CUR in pre) { // pre if there are attributes cur 
                pre [cur] ++ ; 
            } the else { 
                pre [cur] =. 1 ; // add pre cur object attributes and assigned. 1 
            } 
            return pre; 
        }, {}) // the reduce () , for ES6 merge array method, where the initial value is set to a null object 

        the console.log (nameNum); // {Barry:. 1, Boogie:. 1, Dendi:. 1, Lily: 2}

 

As can be seen, IF (key in obj) means obj whether there are key attributes, there is return to true , it does not return to false .