Iterate through object properties

Question

 var obj = {
		name: "Simon",
		age: "20",
		clothing: {
			style: "simple",
			hipster: false
		}
	}

	for(var propt in obj){
		alert(propt + ': ' + obj[propt]);
	}

How does the variable propt represent the properties of the object? It's not a built-in method, or property. Then why does it come up with every property in the object?

Answer 1

As of JavaScript 1.8.5 you can use Object.keys(obj) to get an Array of properties defined on the object itself (the ones that return true for obj.hasOwnProperty(key)).

Object.keys(obj).forEach(function(key,index) {
    // key: the name of the object key
    // index: the ordinal position of the key within the object 
});

This is better (and more readable) than using a for-in loop.

Its supported on these browsers:

• Firefox (Gecko): 4 (2.0)
• Chrome: 5
• Internet Explorer: 9

See the Mozilla Developer Network Object.keys()'s reference for futher information.

Answer 2

You can use Lodash. The documentation

var obj = {a: 1, b: 2, c: 3};
_.keys(obj).forEach(function (key) {
    ...
});

Answer 3

The above answers are a bit annoying because they don't explain what you do inside the for loop after you ensure it's an object: YOU DON'T ACCESS IT DIRECTLY! You are actually only delivered the KEY that you need to apply to the OBJ:

var obj = {
  a: "foo",
  b: "bar",
  c: "foobar"
};

// We need to iterate the string keys (not the objects)
for(var someKey in obj)
{
  // We check if this key exists in the obj
  if (obj.hasOwnProperty(someKey))
  {
    // someKey is only the KEY (string)! Use it to get the obj:
    var myActualPropFromObj = obj[someKey]; // Since dynamic, use [] since the key isn't literally named "someKey"

    // NOW you can treat it like an obj
    var shouldBeBar = myActualPropFromObj.b;
  }
}

This is all ECMA5 safe. Even works in the lame JS versions like Rhino ;)

Answer 4

You basically want to loop through each property in the object.

JSFiddle

var Dictionary = {
  If: {
    you: {
      can: '',
      make: ''
    },
    sense: ''
  },
  of: {
    the: {
      sentence: {
        it: '',
        worked: ''
      }
    }
  }
};

function Iterate(obj) {
  for (prop in obj) {
    if (obj.hasOwnProperty(prop) && isNaN(prop)) {
      console.log(prop + ': ' + obj[prop]);
      Iterate(obj[prop]);
    }
  }
}
Iterate(Dictionary);

Answer 5

If your environment supports ES2017 then I would recommend Object.entries:

Object.entries(obj).forEach(([key, value]) => {
  console.log(`${key} ${value}`);
});

As shown in Mozillas Object.entries() documentation:

The Object.entries() method returns an array of a given object's own enumerable property [key, value] pairs, in the same order as that provided by a for...in loop (the difference being that a for-in loop enumerates properties in the prototype chain as well).

Basically with Object.entries we can forgo the following extra step that is required with the older for...in loop:

// This step is not necessary with Object.entries
if (object.hasOwnProperty(property)) {
  // do stuff
}

Answer 6

Nowadays you can convert a standard JS object into an iterable object just by adding a Symbol.iterator method. Then you can use a for of loop and acceess its values directly or even can use a spread operator on the object too. Cool. Let's see how we can make it:

var o = {a:1,b:2,c:3},
    a = [];
o[Symbol.iterator] = function*(){
                       var ok = Object.keys(this);
                            i = 0;
                       while (i < ok.length) yield this[ok[i++]];
                     };
for (var value of o) console.log(value);
// or you can even do like
a = [...o];
console.log(a);