ES6 - How to declare implicit class member?

Question

If class members created from some options, how to declare them?

class cls {

    constructor(options) {

        Object.assign(this, {
            arr: [],
        }, options);

        this.arr[0] = 1; // <- IDE thinks that arr is an unresolved variable
    }
}

Answer 1

In order for class property that wasn't explicitly defined to be recognized by IDE (Jetbrains PhpStorm/WebStorm), it's possible to specify a member with JSDoc:

class cls {
    /**
     @name cls#arr
     @type Array
     */

    constructor(options) {...}
}

Since property assignment with Object.assign doesn't provide additional benefits, a more natural way is to explicitly define a property explicitly on this (as other answers mention).

In ES6:

class cls {
   constructor(options) {
        this.arr = [];

        Object.assign(this, options);

        this.arr[0] = 1;
    }
}

In ES.next with class field proposal, requires stage 3 (or lower) preset in Babel:

class cls {
   arr = [];

   constructor(options) {
        Object.assign(this, options);

        this.arr[0] = 1;
    }
}

Class fields are evaluated before constructor body, ES.next option is syntactic sugar for ES6 option. These two options are identical and recognized by IDE.