Loop through Object in Javascript

One of my colleague asked me today on how to loop through objects in javascript, and I thought it will be a good opportunity to put in on a short article. 🤓

There are several of methods of looping through object in Javascript:

• Objects.keys()

• Objects.values()

• Objects.entries()

• for ... in loop

Let's look an object below

const superHeroNicknames = {
  'IronMan': 'Mecha Man 🚀',
  'Thor': 'Thunder Bro 🌩',
  'Hulk': 'Green Guy 🟩'
}

We will use this data for the rest of this article. Let's jump to the explanation!

1. Objects.keys()

Objects.keys() was introduced within the release of ES6. As the name of the method says "keys", this methods will returns an array of object's keys.

const keyRes = Objects.keys(superHeroNicknames);
for (let key of keyRes) {
    console.log(key + " ==> " + p[key]);
};

// Result:
// IronMan ==> Mecha Man 🚀
// Thor ==> Thunder Bro 🌩
// Hulk ==> Green Guy 🟩

This method is useful to get a list of the object properties quickly.

2. Objects.values()

Objects.values() was the opposite from Objects.keys(). instead returning a keys, it will returning an array of object's values. This method is very useful to get a list of all property values quickly.

const keyVals = Objects.value(superHeroNicknames);
for (let val of keyVals) {
    console.log(val);
};

// Result:
// Mecha Man 🚀
// Thunder Bro 🌩
// Green Guy 🟩

3. Object.entries()

Object.entries() was introduced in ES8. This method will returns a list of an array of keys and values. This method is very useful to convert an object into an array. ✨

const res = Objects.keys(superHeroNicknames);
for (let [key, val] of res) {
    console.log(`${key} ==> ${val}`);
};

// Result:
// IronMan ==> Mecha Man 🚀
// Thor ==> Thunder Bro 🌩
// Hulk ==> Green Guy 🟩

Please note that, this method is still not supported natively by Internet Explorer, if you are targeting those browsers polyfill may be required to do.

4. for ... in loop

for ... in loop method is an another way to loop through an object. This method is already supported for all browser, including Internet Explorer.

for (let key in superHeroNicknames) {
      if (!superHeroNicknames.hasOwnProperty(key)) {
        //The current property is not a direct property of p
        continue;
    }
    console.log(`${key}: ${superHeroNicknames[key]}`);
}

// Result:
// IronMan ==> Mecha Man 🚀
// Thor ==> Thunder Bro 🌩
// Hulk ==> Green Guy 🟩

One caveat when using for ... in loop is it will loop all the properties along the prototype chain. Therefore, always make use of the Object.hasOwnProperty() to determine if the current property in iteration is really a property of the object. ❗️

That's it for this article. One final note, if you are only needed to check whether an object has a specific property, you can use Object.hasOwnProperty() rather than to do a loop through object and check it's properties one by one.