Using Objects in For Of Loops

Why isn't is possible to use objects in for of loops? Or is this a browser bug? This code doesn't work in Chrome 42, saying undefined is not a function:

test = { first: "one"}

for(var item of test) {
  console.log(item)
}

Solution 1:

The for..of loop only supports iterable objects like arrays, not objects.

To iterate over the values of an object, use:

for (var key in test) {
    var item = test[key];
}

Solution 2:

You can use this syntax:

const myObject = {
  first: "one",
  second: "two",
};

for (const [key, value] of Object.entries(myObject)) {
  console.log(key, value);  // first one, second two
}

However, Object.entries has poor support right now does not work in IE or iOS Safari. You'll probably might need a polyfill. See https://caniuse.com/mdn-javascript_builtins_object_entries for the latest scoop.

See also Object.keys to iterate just the keys, or Object.values for just the values.

Solution 3:

If you are storing data in a key-value store, please use Map which is explicitly designed for this purpose.

If you have to use an object though, ES2017 (ES8) allows you to use Object.values:

const foo = { a: 'foo', z: 'bar', m: 'baz' };
for (let value of Object.values(foo)) {
    console.log(value);
}

If that isn't supported yet, use a polyfill: Alternative version for Object.values()

And finally if you're supporting an older environment that don't support this syntax, you'll have to resort to using forEach and Object.keys:

var obj = { a: 'foo', z: 'bar', m: 'baz' };
Object.keys(obj).forEach(function (prop) {
    var value = obj[prop];
    console.log(value);
});

Solution 4:

Iterator, Iterable and for..of loop in ECMAScript 2015/ ES6

let tempArray = [1,2,3,4,5];

for(element of tempArray) {
  console.log(element);
}

// 1
// 2
// 3
// 4
// 5

But if we do

let tempObj = {a:1, b:2, c:3};

for(element of tempObj) {
   console.log(element);
}
// error

We get error because for..of loop works only on Iterables, that is, the object which has an @@iterator that adheres to Iterator protocol, meaning it must have an object with a next method. The next method takes no arguments and it should return an object with these two properties.

done: signals that the sequence has ended when true, and false means there may be more values value: this is the current item in the sequence

So, to make an object Iterable that is to make it work with for..of we can:

1 .Make an object an Iterable by assigning to it’s mystical @@iterator property through the Symbol.iterator property.Here is how:

let tempObj = {a:1, b:2, c:3};

tempObj[Symbol.iterator]= () => ({
next: function next () {
return {
    done: Object.keys(this).length === 0,
    value: Object.keys(this).shift()
     }
    }
  })

for(key in tempObj){
 console.log(key)
}
// a
// b
// c

2.Use Object.entries, which returns an Iterable:

let tempObj = {a:1, b:2, c:3};

for(let [key, value] of Object.entries(tempObj)) {
    console.log(key, value);
}
// a 1
// b 2
// c 3

3.Use Object.keys, here is how:

let tempObj = {a:1, b:2, c:3};
for (let key of Object.keys(tempObj)) {
    console.log(key);
}

// a
// b
// c

Hope this helps!!!!!!

Solution 5:

I made objects iterable with this code:

Object.prototype[Symbol.iterator] = function*() {
 for(let key of Object.keys(this)) {
  yield([ key, this[key] ])
} }

Usage:

for(let [ key, value ] of {}) { }

Alternativly:

for(let [ key, value ] of Object.entries({})) { }