How do I loop through or enumerate a JavaScript object?

Question 1

I have a JavaScript object like the following:

var p = {
 "p1": "value1",
 "p2": "value2",
 "p3": "value3"
};

How do I loop through all of p's elements (p1, p2, p3...) and get their keys and values?

Question 2

You can use the for-in loop as shown by others. However, you also have to make sure that the key you get is an actual property of an object, and doesn't come from the prototype.

Here is the snippet:

var p = {
 "p1": "value1",
 "p2": "value2",
 "p3": "value3"
};
for (var key in p) {
 if (p.hasOwnProperty(key)) {
 console.log(key + " -> " + p[key]);
 }
}

For-of with Object.keys() alternative:

var p = {
 0: "value1",
 "b": "value2",
 key: "value3"
};
for (var key of Object.keys(p)) {
 console.log(key + " -> " + p[key])
}

Notice the use of for-of instead of for-in, if not used it will return undefined on named properties, and Object.keys() ensures the use of only the object's own properties without the whole prototype-chain properties

Using the new Object.entries() method:

Note: This method is not supported natively by Internet Explorer. You may consider using a Polyfill for older browsers.

const p = {
 "p1": "value1",
 "p2": "value2",
 "p3": "value3"
};
for (const [key, value] of Object.entries(p)) {
 console.log(`${key}: ${value}`);
}

Question 3

In javascript, every object has a bunch of built-in key-value pairs that have meta-information. When you loop through all the key-value pairs for an object you're looping through them too. hasOwnPropery() filters these out.

Question 4

Actually, For...in is not deprecated. For each...in is. But I really like the term archaeologists...I'm going to have to start using that.

Question 5

How can I add the values that was looped? Thanks

Question 6

(for..in) for objects, (for.. of) for arrays

Question 7

for anyone concerned about performance, for..in + hasOwnProperty is marginally faster in normal use (jsben.ch/pCDAk), with Object.keys being significantly faster beyond a few thousand properties (jsben.ch/ptzsL).

Question 8

Under ECMAScript 5, you can combine Object.keys() and Array.prototype.forEach():

var obj = { first: "John", last: "Doe" };
Object.keys(obj).forEach(function(key) {
 console.log(key, obj[key]);
});

ECMAScript 6 adds for...of:

for (const key of Object.keys(obj)) {
 console.log(key, obj[key]);
}

ECMAScript 8 adds Object.entries() which avoids having to look up each value in the original object:

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

You can combine for...of, destructuring, and Object.entries:

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

Both Object.keys() and Object.entries() iterate properties in the same order as a for...in loop but ignore the prototype chain. Only the object's own enumerable properties are iterated.

Question 9

Why didn't the standard provide Object.forEach(obj, function (value, key) {...})? :( Certainly obj.forEach(function...) would be shorter and complement Array.prototype.forEach, but that would risk having objects define their own forEach property. I suppose Object.keys guards against the callback modifying the object's keys.

Question 10

Python is so easy, javascript I have to look up basics every time.

Question 11

@DavidHarkness Why make it easy when it can be hard? Programmers LOVE to over-engineer things.

Question 12

You have to use the for-in loop

But be very careful when using this kind of loop, because this will loop all the properties along the prototype chain.

Therefore, when using for-in loops, always make use of the hasOwnProperty method to determine if the current property in iteration is really a property of the object you're checking on:

for (var prop in p) {
 if (!p.hasOwnProperty(prop)) {
 //The current property is not a direct property of p
 continue;
 }
 //Do your logic with the property here
}

Question 13

This is better than levik's solution because it allows the main logic to be only one nested loop in rather than two; making for easier to read code. Although I'd loose the the brackets around the continue; they are superfluous.

Question 14

I would not remove the { } personally because an if without them makes it a little unclear what is part of the if and what is not. But I guess that's just a matter of opinion :)

Question 15

Yes, I prefer keeping the { } mainly to avoid confusion if one later on needs to add something to the if scope.

Question 16

Reading my previous comment, I realized that I didn't use the correct terms, because I said "if scope"; but keep in mind that JavaScript only has function scope. So what I actually meant was "if block".

Question 17

The question won't be complete if we don't mention about alternative methods for looping through objects.

Nowadays many well known JavaScript libraries provide their own methods for iterating over collections, i.e. over arrays, objects, and array-like objects. These methods are convenient to use and are entirely compatible with any browser.

If you work with jQuery, you may use jQuery.each() method. It can be used to seamlessly iterate over both objects and arrays:
```
$.each(obj, function(key, value) {
 console.log(key, value);
});
```
In Underscore.js you can find method _.each(), which iterates over a list of elements, yielding each in turn to a supplied function (pay attention to the order of arguments in iteratee function!):
```
_.each(obj, function(value, key) {
 console.log(key, value);
});
```
Lo-Dash provides several methods for iterating over object properties. Basic _.forEach() (or it's alias _.each()) is useful for looping through both objects and arrays, however (!) objects with length property are treated like arrays, and to avoid this behavior it is suggested to use _.forIn() and _.forOwn() methods (these also have value argument coming first):
```
_.forIn(obj, function(value, key) {
 console.log(key, value);
});
```
_.forIn() iterates over own and inherited enumerable properties of an object, while _.forOwn() iterates only over own properties of an object (basically checking against hasOwnProperty function). For simple objects and object literals any of these methods will work fine.

Generally all described methods have the same behaviour with any supplied objects. Besides using native for..in loop will usually be faster than any abstraction, such as jQuery.each(), these methods are considerably easier to use, require less coding and provide better error handling.

Question 18

To get to the value: $.each(obj, function (key, value) { console.log(value.title); });

Question 19

Preface:

Object properties can be own (the property is on the object itself) or inherited (not on the object itself, on one of its prototypes).
Object properties can be enumerable or non-enumerable. Non-enumerable properties are left out of lots of property enumerations/arrays.
Property names can be strings or Symbols. Properties whose names are Symbols are left out of lots of property enumerations/arrays.

Here in 2018, your options for looping through an object's properties are (some examples follow the list):

for-in [MDN, spec] — A loop structure that loops through the names of an object's enumerable properties, including inherited ones, whose names are strings
Object.keys [MDN, spec] — A function providing an array of the names of an object's own, enumerable properties whose names are strings.
Object.values [MDN, spec] — A function providing an array of the values of an object's own, enumerable properties.
Object.entries [MDN, spec] — A function providing an array of the names and values of an object's own, enumerable properties (each entry in the array is a [name, value] array).
Object.getOwnPropertyNames [MDN, spec] — A function providing an array of the names of an object's own properties (even non-enumerable ones) whose names are strings.
Object.getOwnPropertySymbols [MDN, spec] — A function providing an array of the names of an object's own properties (even non-enumerable ones) whose names are Symbols.
Reflect.ownKeys [MDN, spec] — A function providing an array of the names of an object's own properties (even non-enumerable ones), whether those names are strings or Symbols.
If you want all of an object's properties, including non-enumerable inherited ones, you need to use a loop and Object.getPrototypeOf [MDN, spec] and use Object.getOwnPropertyNames, Object.getOwnPropertySymbols, or Reflect.ownKeys on each object in the prototype chain (example at the bottom of this answer).

With all of them except for-in, you'd use some kind of looping construct on the array (for, for-of, forEach, etc.).

Examples:

for-in:

// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const name in o) {
 const value = o[name];
 console.log(`${name} = ${value}`);
}

Object.keys (with a for-of loop, but you can use any looping construct):

// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const name of Object.keys(o)) {
 const value = o[name];
 console.log(`${name} = ${value}`);
}

Object.values:

// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const value of Object.values(o)) {
 console.log(`${value}`);
}

Object.entries:

// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const [name, value] of Object.entries(o)) {
 console.log(`${name} = ${value}`);
}

Object.getOwnPropertyNames:

// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const name of Object.getOwnPropertyNames(o)) {
 const value = o[name];
 console.log(`${name} = ${value}`);
}

Object.getOwnPropertySymbols:

// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const name of Object.getOwnPropertySymbols(o)) {
 const value = o[name];
 console.log(`${String(name)} = ${value}`);
}

Reflect.ownKeys:

// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const name of Reflect.ownKeys(o)) {
 const value = o[name];
 console.log(`${String(name)} = ${value}`);
}

All properties, including inherited non-enumerable ones:

// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (let depth = 0, current = o; current; ++depth, current = Object.getPrototypeOf(current)) {
 for (const name of Reflect.ownKeys(current)) {
 const value = o[name];
 console.log(`[${depth}] ${String(name)} = ${String(value)}`);
 }
}

.as-console-wrapper {
 max-height: 100% !important;
}

Question 20

Nice addition of enumerable/non-enumberable object properties.

Question 21

You can just iterate over it like:

for (var key in p) {
 alert(p[key]);
}

Note that key will not take on the value of the property, it's just an index value.

Question 22

This is repeated and not even entirely correct. You need to have a check of hasOwnProperty to make this work properly

Question 23

I initially downvoted this based on the above comment until i realized that this answer came first, therefore is not "repeated". It is possibly incomplete but works just fine for many cases.

Question 24

In ECMAScript 5 you have new approach in iteration fields of literal - Object.keys

More information you can see on MDN

My choice is below as a faster solution in current versions of browsers (Chrome30, IE10, FF25)

var keys = Object.keys(p),
 len = keys.length,
 i = 0,
 prop,
 value;
while (i < len) {
 prop = keys[i];
 value = p[prop];
 i += 1;
}

You can compare performance of this approach with different implementations on jsperf.com:

Browser support you can see on Kangax's compat table

For old browser you have simple and full polyfill

UPD:

performance comparison for all most popular cases in this question on perfjs.info:

object literal iteration

Question 25

Performance

Today 2020年03月06日 I perform tests of chosen solutions on Chrome v80.0, Safari v13.0.5 and Firefox 73.0.1 on MacOs High Sierra v10.13.6

Conclusions

solutions based on for-in (A,B) are fast (or fastest) for all browsers for big and small objects
surprisingly for-of (H) solution is fast on chrome for small and big objects
solutions based on explicit index i (J,K) are quite fast on all browsers for small objects (for firefox also fast for big ojbects but medium fast on other browsers)
solutions based on iterators (D,E) are slowest and not recommended
solution C is slow for big objects and medium-slow for small objects

enter image description here

Details

Performance tests was performed for

small object - with 3 fields - you can perform test on your machine HERE
'big' object - with 1000 fields - you can perform test on your machine HERE

Below snippets presents used solutions

function A(obj,s='') {
	for (let key in obj) if (obj.hasOwnProperty(key)) s+=key+'->'+obj[key] + ' ';
 return s;
}
function B(obj,s='') {
	for (let key in obj) s+=key+'->'+obj[key] + ' ';
 return s;
}
function C(obj,s='') {
 const map = new Map(Object.entries(obj));
	for (let [key,value] of map) s+=key+'->'+value + ' ';
 return s;
}
function D(obj,s='') {
 let o = { 
 ...obj,
 *[Symbol.iterator]() {
 for (const i of Object.keys(this)) yield [i, this[i]]; 
 }
 }
 for (let [key,value] of o) s+=key+'->'+value + ' ';
 return s;
}
function E(obj,s='') {
 let o = { 
 ...obj,
 *[Symbol.iterator]() {yield *Object.keys(this)}
 }
 for (let key of o) s+=key+'->'+o[key] + ' ';
 return s;
}
function F(obj,s='') {
	for (let key of Object.keys(obj)) s+=key+'->'+obj[key]+' ';
 return s;
}
function G(obj,s='') {
	for (let [key, value] of Object.entries(obj)) s+=key+'->'+value+' ';
 return s;
}
function H(obj,s='') {
	for (let key of Object.getOwnPropertyNames(obj)) s+=key+'->'+obj[key]+' ';
 return s;
}
function I(obj,s='') {
	for (const key of Reflect.ownKeys(obj)) s+=key+'->'+obj[key]+' ';
 return s;
}
function J(obj,s='') {
 let keys = Object.keys(obj);
	for(let i = 0; i < keys.length; i++){
 let key = keys[i];
 s+=key+'->'+obj[key]+' ';
 }
 return s;
}
function K(obj,s='') {
 var keys = Object.keys(obj), len = keys.length, i = 0;
 while (i < len) {
 let key = keys[i];
 s+=key+'->'+obj[key]+' ';
 i += 1;
 }
 return s;
}
function L(obj,s='') {
 Object.keys(obj).forEach(key=> s+=key+'->'+obj[key]+' ' );
 return s;
}
function M(obj,s='') {
 Object.entries(obj).forEach(([key, value]) => s+=key+'->'+value+' ');
 return s;
}
function N(obj,s='') {
 Object.getOwnPropertyNames(obj).forEach(key => s+=key+'->'+obj[key]+' ');
 return s;
}
function O(obj,s='') {
 Reflect.ownKeys(obj).forEach(key=> s+=key+'->'+obj[key]+' ' );
 return s;
}
// TEST
var p = {
 "p1": "value1",
 "p2": "value2",
 "p3": "value3"
};
let log = (name,f) => console.log(`${name} ${f(p)}`)
log('A',A);
log('B',B);
log('C',C);
log('D',D);
log('E',E);
log('F',F);
log('G',G);
log('H',H);
log('I',I);
log('J',J);
log('K',K);
log('L',L);
log('M',M);
log('N',N);
log('O',O);

This snippet only presents choosen solutions

And here are result for small objects on chrome

enter image description here

Question 26

I guess it depends on context, but that's nerdy stuff. No disrespect or denying the effort put into this answer, but for general consumer apps I always find those pedant performance tests out of real life context. Ok, one can save like 5ms here and there. Not a single user will notice this. This could probably make a difference back in 80s-90s when CPU cycle was expensive, but today this is practically irrelevant in 99.9% of the time. Developer's time is much more expensive than the outcome of those micro optimisations especially when UX improvements are negligible or non-existent.

Question 27

for(key in p) {
 alert( p[key] );
}

Note: you can do this over arrays, but you'll iterate over the length and other properties, too.

Question 28

When using a for loop like that, key will just take on an index value, so that will just alert 0, 1, 2, etc... You need to access p[key].

Question 29

It is the slowest method of array iteration in JavaScript. You can check this on your computer - Best way to iterate over Arrays in JavaScript

Question 30

@Pencroff: the problem is that the question is not about looping through arrays... ;)

Question 31

This is something I don't understand on stackoverflow. Richard gave the correct answer, and he was the first one giving that answer, but he did not get any +1? @Bryan var p = {"p1":"q","p2":"w"}; for(key in p) { alert( key ); } is popping "p1" and "p2" in alerts, so whats wrong about that???

Question 32

I think the main difference is the quality: the other answers not only tell how, but also tell the caveats (e.g., the prototype) and how to deal with those caveats. IMHO, those other answers are better than mine :).

Question 33

Since es2015 is getting more and more popular I am posting this answer which include usage of generator and iterator to smoothly iterate through [key, value] pairs. As it is possible in other languages for instance Ruby.

Ok here is a code:

const MyObject = {
 'a': 'Hello',
 'b': 'it\'s',
 'c': 'me',
 'd': 'you',
 'e': 'looking',
 'f': 'for',
 [Symbol.iterator]: function*() {
 for (const i of Object.keys(this)) {
 yield [i, this[i]];
 }
 }
};
for (const [k, v] of MyObject) {
 console.log(`Here is key ${k} and here is value ${v}`);
}

All information about how can you do an iterator and generator you can find at developer Mozilla page.

Hope It helped someone.

EDIT:

ES2017 will include Object.entries which will make iterating over [key, value] pairs in objects even more easier. It is now known that it will be a part of a standard according to the ts39 stage information.

I think it is time to update my answer to let it became even more fresher than it's now.

const MyObject = {
 'a': 'Hello',
 'b': 'it\'s',
 'c': 'me',
 'd': 'you',
 'e': 'looking',
 'f': 'for',
};
for (const [k, v] of Object.entries(MyObject)) {
 console.log(`Here is key ${k} and here is value ${v}`);
}

You can find more about usage on MDN page

Question 34

This looks totally superfluous/unneeded to me. Would you add it to every object in your system? I thought the point of providing an iterator was so that you could do `for( const [k, v] of myObject )'. It just looks like extra code providing little additional value.

Question 35

Single line and more readable code can be..

Object.entries(myObject).map(([key, value]) => console.log(key, value))

Question 36

good answer and it's more readable than the above solutions but can you explain what happens on the .map(([key, value]) in your answer?

Question 37

@Nivethan the output of Object.entries will be an Array of Arrays. i.e [ ['key1', 'value'], ['key2', 'value'] ] So, map will loop over outer array with each array elements passing into its callback function one by one. So, here I have used Array de-structuring syntax ([key, value]) => {} instead of (element) => {}, where element is an array.

Question 38

After looking through all the answers in here, hasOwnProperty isn't required for my own usage because my json object is clean; there's really no sense in adding any additional javascript processing. This is all I'm using:

for (var key in p) {
 console.log(key + ' => ' + p[key]);
 // key is key
 // value is p[key]
}

Question 39

Whether the JSON object is clean or not is irrelevant. If at any other time some code sets a property on Object.prototype, then it will be enumerated by for..in. If you are sure you are not using any libraries that do that, then you don't need to call hasOwnProperty.

Question 40

It can be completely clean if created with Object.create(null)

Question 41

via prototype with forEach() which should skip the prototype chain properties:

Object.prototype.each = function(f) {
 var obj = this
 Object.keys(obj).forEach( function(key) { 
 f( key , obj[key] ) 
 });
}
//print all keys and values
var obj = {a:1,b:2,c:3}
obj.each(function(key,value) { console.log(key + " " + value) });
// a 1
// b 2
// c 3

Question 42

Be careful with the prototype: obj = { print: 1, each: 2, word: 3 } produces TypeError: number is not a function. Using forEach to match the similar Array function may reduce the risk somewhat.

Question 43

It's interesting people in these answers have touched on both Object.keys() and for...of but never combined them:

var map = {well:'hello', there:'!'};
for (let key of Object.keys(map))
 console.log(key + ':' + map[key]);

_{You can't just for...of an Object because it's not an iterator, and for...index or .forEach()ing the Object.keys() is ugly/inefficient.

I'm glad most people are refraining from for...in (with or without checking .hasOwnProperty()) as that's also a bit messy, so other than my answer above, I'm here to say...}

You can make ordinary object associations iterate! Behaving just like Maps with direct use of the fancy for...of
DEMO working in Chrome and FF (I assume ES6 only)

var ordinaryObject = {well:'hello', there:'!'};
for (let pair of ordinaryObject)
 //key:value
 console.log(pair[0] + ':' + pair[1]);
//or
for (let [key, value] of ordinaryObject)
 console.log(key + ':' + value);

So long as you include my shim below:

//makes all objects iterable just like Maps!!! YAY
//iterates over Object.keys() (which already ignores prototype chain for us)
Object.prototype[Symbol.iterator] = function() {
 var keys = Object.keys(this)[Symbol.iterator]();
 var obj = this;
 var output;
 return {next:function() {
 if (!(output = keys.next()).done)
 output.value = [output.value, obj[output.value]];
 return output;
 }};
};

Without having to create a real Map object that doesn't have the nice syntactic sugar.

var trueMap = new Map([['well', 'hello'], ['there', '!']]);
for (let pair of trueMap)
 console.log(pair[0] + ':' + pair[1]);

In fact, with this shim, if you still wanted to take advantage of Map's other functionality (without shimming them all in) but still wanted to use the neat object notation, since objects are now iterable you can now just make a Map from it!

//shown in demo
var realMap = new Map({well:'hello', there:'!'});

For those who don't like to shim, or mess with prototype in general, feel free to make the function on window instead, calling it something like getObjIterator() then;

//no prototype manipulation
function getObjIterator(obj) {
 //create a dummy object instead of adding functionality to all objects
 var iterator = new Object();
 //give it what the shim does but as its own local property
 iterator[Symbol.iterator] = function() {
 var keys = Object.keys(obj)[Symbol.iterator]();
 var output;
 return {next:function() {
 if (!(output = keys.next()).done)
 output.value = [output.value, obj[output.value]];
 return output;
 }};
 };
 return iterator;
}

Now you can just call it as an ordinary function, nothing else is affected

var realMap = new Map(getObjIterator({well:'hello', there:'!'}))

or

for (let pair of getObjIterator(ordinaryObject))

There's no reason why that wouldn't work.

Welcome to the future.

Question 44

Case in point. So long as people scroll down and find it helpful, that's all that matters. Usually it's me trying to do something, not liking the stuff I see online, end up figuring it out, then I come back to share. It's good doco, I've actually come across my own answers before googling things I completely forgot about!

Question 45

@HelpMeStackOverflowMyOnlyHope Personally I do not like modifying the prototypes of objects I did not define myself.

Question 46

@JanusTroelsen did you even read the whole answer?

For those who don't like to shim, or mess with prototype in general, feel free to make the function on window instead, calling it something like getObjIterator() then;

Question 47

Note that this technique doesn't work on plain objects, but useful nonetheless.

Question 48

it does work for plain objects, that's literally the whole point (as well as the variable names like ordinaryObject for emphasis that the magic still works for those types). Did you check the demos; what isn't working for you, @noɥʇʎԀʎzɐɹƆ? (P.S. your SE profile image is boss)

Question 49

Using a for-of on Object.keys()

Like:

let object = {
 "key1": "value1",
 "key2": "value2",
 "key3": "value3"
};
for (let key of Object.keys(object)) {
 console.log(key + " : " + object[key])
}

Question 50

You can also use Object.keys() and iterate over the object keys like below to get the value:

var p = {
 "p1": "value1",
 "p2": "value2",
 "p3": "value3"
};
Object.keys(p).forEach((key)=> {
 console.log(key +' -> '+ p[key]);
});

Question 51

You saved my time, Thank you

Question 52

Happy to know:)

Question 53

var p = {
 "p1": "value1",
 "p2": "value2",
 "p3": "value3"
};
for (var key in p) {
 if (p.hasOwnProperty(key)) {
 console.log(key + " = " + p[key]);
 }
}

<p>
 Output:<br>
 p1 = values1<br>
 p2 = values2<br>
 p3 = values3
</p>

Question 54

Object.keys(obj) : Array

retrieves all string-valued keys of all enumerable own (non-inherited) properties.

So it gives the same list of keys as you intend by testing each object key with hasOwnProperty. You don't need that extra test operation than and Object.keys( obj ).forEach(function( key ){}) is supposed to be faster. Let's prove it:

var uniqid = function(){
			var text = "",
					i = 0,
					possible = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
			for( ; i < 32; i++ ) {
					text += possible.charAt( Math.floor( Math.random() * possible.length ) );
			}
			return text;
		}, 
		CYCLES = 100000,
		obj = {}, 
		p1,
		p2,
		p3,
		key;
// Populate object with random properties
Array.apply( null, Array( CYCLES ) ).forEach(function(){
	obj[ uniqid() ] = new Date()
});
// Approach #1
p1 = performance.now();
Object.keys( obj ).forEach(function( key ){
	var waste = obj[ key ];
});
p2 = performance.now();
console.log( "Object.keys approach took " + (p2 - p1) + " milliseconds.");
// Approach #2
for( key in obj ) {
	if ( obj.hasOwnProperty( key ) ) {
		var waste = obj[ key ];
	}
}
p3 = performance.now();
console.log( "for...in/hasOwnProperty approach took " + (p3 - p2) + " milliseconds.");

In my Firefox I have following results

Object.keys approach took 40.21101451665163 milliseconds.
for...in/hasOwnProperty approach took 98.26163508463651 milliseconds.

PS. on Chrome the difference even bigger http://codepen.io/dsheiko/pen/JdrqXa

PS2: In ES6 (EcmaScript 2015) you can iterate iterable object nicer:

let map = new Map().set('a', 1).set('b', 2);
for (let pair of map) {
 console.log(pair);
}
// OR 
let map = new Map([
 [false, 'no'],
 [true, 'yes'],
]);
map.forEach((value, key) => {
 console.log(key, value);
});

Question 55

if you don't feel like letting go of the {} notation, you can still use of without creating Maps

Question 56

In latest ES script, you can do something like this:

let p = {foo: "bar"};
for (let [key, value] of Object.entries(p)) {
 console.log(key, value);
}

Question 57

Works as a stand-alone, but does not work if this function returns a value for each for condition

Question 58

Pass your object to Object.keys(). This will return an array containing all the keys in the object. You can then loop through the array using map. Using obj[key] where obj is your object and key is the current value in the map iteration, you can get the value for that key/property.

const obj = { name: "Jane", age: 50 };
Object.keys(obj).map( key => {
 console.log(key, obj[key]);
});

Question 59

Only JavaScript code without dependencies:

var p = {"p1": "value1", "p2": "value2", "p3": "value3"};
keys = Object.keys(p); // ["p1", "p2", "p3"]
for(i = 0; i < keys.length; i++){
 console.log(keys[i] + "=" + p[keys[i]]); // p1=value1, p2=value2, p3=value3
}

Question 60

The Object.keys() method returns an array of a given object's own enumerable properties. Read more about it here

var p = {
 "p1": "value1",
 "p2": "value2",
 "p3": "value3"
};
Object.keys(p).map((key)=> console.log(key + "->" + p[key]))

Question 61

Here is another method to iterate through an object.

 var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
Object.keys(p).forEach(key => { console.log(key, p[key]) })

Question 62

This is pretty cool, however for large objects, the for method might be more performant.

Question 63

Multiple way to iterate object in javascript

Using for...in loop

 var p = {
 "p1": "value1",
 "p2": "value2",
 "p3": "value3"
};
for (let key in p){
 if(p.hasOwnProperty(key)){
 console.log(`${key} : ${p[key]}`)
 }
}

Using for...of loop

 var p = {
 "p1": "value1",
 "p2": "value2",
 "p3": "value3"
};
for (let key of Object.keys(p)){
 console.log(`key: ${key} & value: ${p[key]}`)
}

Using forEach() with Object.keys, Object.values, Object.entries

var p = {
 "p1": "value1",
 "p2": "value2",
 "p3": "value3"
};
Object.keys(p).forEach(key=>{
 console.log(`${key} : ${p[key]}`);
});
Object.values(p).forEach(value=>{
 console.log(value);
});
Object.entries(p).forEach(([key,value])=>{
 console.log(`${key}:${value}`)
})

Question 64

Loops can be pretty interesting when using pure JavaScript. It seems that only ECMA6 (New 2015 JavaScript specification) got the loops under control. Unfortunately as I'm writing this, both Browsers and popular Integrated development environment (IDE) are still struggling to support completely the new bells and whistles.

At a glance here is what a JavaScript object loop look like before ECMA6:

for (var key in object) {
 if (p.hasOwnProperty(key)) {
 var value = object[key];
 console.log(key); // This is the key;
 console.log(value); // This is the value;
 }
}

Also, I know this is out of scope with this question but in 2011, ECMAScript 5.1 added the forEach method for Arrays only which basically created a new improved way to loop through arrays while still leaving non iterable objects with the old verbose and confusing for loop. But the odd part is that this new forEach method does not support break which led to all sorts of other problems.

Basically in 2011, there is not a real solid way to loop in JavaScript other than what many popular libraries (jQuery, Underscore, etc.) decided to re-implement.

As of 2015, we now have a better out of the box way to loop (and break) any object type (including Arrays and Strings). Here is what a loop in JavaScript will eventually look like when the recommendation becomes mainstream:

for (let [key, value] of Object.entries(object)) {
 console.log(key); // This is the key;
 console.log(value); // This is the value;
}

Note that most browsers won't support the code above as of June 18th 2016. Even in Chrome you need to enable this special flag for it to work: chrome://flags/#enable-javascript-harmony

Until this becomes the new standard, the old method can still be used but there are also alternatives in popular libraries or even lightweight alternatives for those who aren't using any of these libraries.

levik levik 118k28 gold badges77 silver badges92 bronze badges · Accepted Answer · 2009-03-26 06:12:13Z

You can use the for-in loop as shown by others. However, you also have to make sure that the key you get is an actual property of an object, and doesn't come from the prototype.

Here is the snippet:

var p = {
 "p1": "value1",
 "p2": "value2",
 "p3": "value3"
};
for (var key in p) {
 if (p.hasOwnProperty(key)) {
 console.log(key + " -> " + p[key]);
 }
}

For-of with Object.keys() alternative:

var p = {
 0: "value1",
 "b": "value2",
 key: "value3"
};
for (var key of Object.keys(p)) {
 console.log(key + " -> " + p[key])
}

Notice the use of for-of instead of for-in, if not used it will return undefined on named properties, and Object.keys() ensures the use of only the object's own properties without the whole prototype-chain properties

Using the new Object.entries() method:

Note: This method is not supported natively by Internet Explorer. You may consider using a Polyfill for older browsers.

const p = {
 "p1": "value1",
 "p2": "value2",
 "p3": "value3"
};
for (const [key, value] of Object.entries(p)) {
 console.log(`${key}: ${value}`);
}

In javascript, every object has a bunch of built-in key-value pairs that have meta-information. When you loop through all the key-value pairs for an object you're looping through them too. hasOwnPropery() filters these out.
Actually, For...in is not deprecated. For each...in is. But I really like the term archaeologists...I'm going to have to start using that.
for anyone concerned about performance, for..in + hasOwnProperty is marginally faster in normal use (jsben.ch/pCDAk), with Object.keys being significantly faster beyond a few thousand properties (jsben.ch/ptzsL).

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