How do I loop through or enumerate a JavaScript ob

Question

I have a JavaScript object like the following:

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

Now I want to loop through all p elements (p1, p2, p3...) And get their keys and values. How can I do that?

I can modify the JavaScript object if necessary. My ultimate goal is to loop through some key value pairs and if possible I want to avoid using eval.

Answer 1

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]))

Answer 2

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);
});

Answer 3

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

Object.entries(p);

Answer 4

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

Answer 5

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.

Answer 6

Object.entries() function:

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

for (var i in Object.entries(p)){
	var key = Object.entries(p)[i][0];
	var value = Object.entries(p)[i][1];
	console.log('key['+i+']='+key+' '+'value['+i+']='+value);
}