brad37.html

<script>
let f = function(){
    this.a = 1;
    this.b = 2;
}

let obj = new f();  // {a:1, b:2}
console.log(obj.a + ":" + obj.b);

f.prototype.b = 3;
f.prototype.c = 4;

console.log(obj.a);
console.log(obj.b);
console.log(obj.c);



</script>