ES6 Classes Default Value

Object.assign works for me as well

class RenderProperties {
   constructor(options = {}){
        Object.assign(this, {
            fill : false, 
            fillStyle : 'rgba(0, 255, 0, 0.5)',
            lineWidth : 1,
            strokeStyle : '#00FF00'
        }, options);
    }
}

If you're going to use ES6, why not use all of ES6, i.e. default values for parameters and destructuring assignment

class myClass {
  constructor({a = 'default a value', b = 'default b value', c = 'default c value'} = {a:'default option a', b:'default option b', c:'default option c'}) {
    this.a = a;
    this.b = b;
    this.c = c;
  }
}
var v = new myClass({a:'a value', b: 'b value'});
console.log(v.toSource());
var w = new myClass();
console.log(w.toSource());

http://www.es6fiddle.net/ibxq6qcx/

edit: also tested and confirmed to run on https://babeljs.io/repl/

If you would like to have some properties default but some not you can use also use spread operator (work as same as Object.assign answer)

const defaults = {someDefault: true};

class SomeClass {
    constructor(config) {
        this.config = {...defaults, ...config};
    }
}