How to iterate (keys, values) in javascript?

Question

I have a dictionary that has the format of

dictionary = {0: {object}, 1:{object}, 2:{object}}

How can I iterate through this dictionary by

Answer 1

tl;dr

1. In ECMAScript 5, it is not possible.
2. In ECMAScript 2015, it is possible with Maps.
3. In ECMAScript 2017, it would be readily available.

ECMAScript 5:

No, its not possible with objects.

You should either iterate with for..in, or Object.keys, like this

for (var key in dictionary) {
    // check if the property/key is defined in the object itself, not in parent
    if (dictionary.hasOwnProperty(key)) {           
        console.log(key, dictionary[key]);
    }
}

Note: The if condition above is necessary, only if you want to iterate the properties which are dictionary object's very own. Because for..in will iterate through all the inherited enumerable properties.

Or

Object.keys(dictionary).forEach(function(key) {
    console.log(key, dictionary[key]);
});

---

ECMAScript 2015

In ECMAScript 2015, you can use Map objects and iterate them with Map.prototype.entries. Quoting example from that page,

var myMap = new Map();
myMap.set("0", "foo");
myMap.set(1, "bar");
myMap.set({}, "baz");

var mapIter = myMap.entries();

console.log(mapIter.next().value); // ["0", "foo"]
console.log(mapIter.next().value); // [1, "bar"]
console.log(mapIter.next().value); // [Object, "baz"]

Or iterate with for..of, like this

'use strict';

var myMap = new Map();
myMap.set("0", "foo");
myMap.set(1, "bar");
myMap.set({}, "baz");

for (const entry of myMap.entries()) {
  console.log(entry);
}

Output

[ '0', 'foo' ]
[ 1, 'bar' ]
[ {}, 'baz' ]

Or

for (const [key, value] of myMap.entries()) {
  console.log(key, value);
}

Output

0 foo
1 bar
{} baz

---

ECMAScript 2017

ECMAScript 2017 would introduce a new function Object.entries. You can use this to iterate the object as you wanted.

'use strict';

const object = {'a': 1, 'b': 2, 'c' : 3};
for (const [key, value] of Object.entries(object)) {
  console.log(key, value);
}

Output

a 1
b 2
c 3