Question

I want to do the following

var my_json = {
    a : 'lemon',
    b : 1
}

function obj(json){
    this.a = 'apple';
    this.b = 0;
    this.c = 'other default';
}

after assigning

var instance = obj(my_json)

I want to get

instance.a == 'lemon'

Answer 1

for(var key in json) {
    if(json.hasOwnProperty(key)) {
        this[key] = json[key];
    }
}

The if block is optional if you know for sure that nothing is every going to extend Object.prototype (which is a bad thing anyway).