Initialize a JavaScript object using JSON

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'

By the Title of the question, maybe a question stackoverflow.com/questions/5873624/… will be useful. — Michael Freidgeim
– Michael Freidgeim, Commented Apr 18, 2020 at 2:27

ThiefMaster · Accepted Answer · 2011-04-14 12:06:13Z

7

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).

answered Apr 14, 2011 at 12:06

ThiefMaster

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2 Comments

Dan Over a year ago

could you please explain your last sentence better?

ThiefMaster Over a year ago

If you extend Object.prototype, iterating over objects will include the properties you added to the prototype which is most likely not what you want when iterating over an object coming from JSON. If a property comes from the prototype, the hasOwnProperty check will fail so it will prevent breakage as only the properties which are actually in the passed objects will be copied to your object.

Alex K. · Accepted Answer · 2011-04-14 12:13:27Z

2

If you want defaults how about;

function obj(json){
  var defaults = {
    a: 'apple',
    b: 0,
    c: 'other default'
  }

  for (var k in json)
    if (json.hasOwnProperty(k))
      defaults[k] = json[k];

  return defaults
}

answered Apr 14, 2011 at 12:13

Alex K.

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