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best way to get the key of a key/value javascript object

Posted by: admin November 29, 2017 Leave a comment

Questions:

If I have a JS object like:

var foo = { 'bar' : 'baz' }

If I know that foo has that basic key/value structure, but don’t know the name of the key, what’s the easiest way to get it? for ... in? $.each()? I hope there’s something better….

Answers:

If you want to get all keys, ECMAScript 5 introduced Object.keys. This is only supported by newer browsers but the MDC documentation provides an alternative implementation (which also uses for...in btw):

if(!Object.keys) Object.keys = function(o){
     if (o !== Object(o))
          throw new TypeError('Object.keys called on non-object');
     var ret=[],p;
     for(p in o) if(Object.prototype.hasOwnProperty.call(o,p)) ret.push(p);
     return ret;
}

Of course if you want both, key and value, then for...in is the only reasonable solution.

Questions:
Answers:

You would iterate inside the object with a for loop:

for(var i in foo){
  alert(i); // alerts key
  alert(foo[i]); //alerts key's value
}

Or

Object.keys(foo)
  .forEach(function eachKey(key) { 
    alert(key); // alerts key 
    alert(foo[key]); // alerts value
  });

Questions:
Answers:

You can access each key individually without iterating as in:

var obj = { first: 'someVal', second: 'otherVal' };
alert(Object.keys(obj)[0]); // returns first
alert(Object.keys(obj)[1]); // returns second

Questions:
Answers:

Given your Object:

var foo = { 'bar' : 'baz' }

To get bar, use:

Object.keys(foo)[0]

To get baz, use:

foo[Object.keys(foo)[0]]

Assuming a single object

Questions:
Answers:

I was having the same problem and this is what worked

//example of an Object
var person = {
    firstName:"John",
    lastName:"Doe",
    age:50,
    eyeColor:"blue"
};

//How to access a single key or value
var key = Object.keys(person)[0];
var value = person.firstName;

Questions:
Answers:

I don’t see anything else than for (var key in foo).

Questions:
Answers:

There is no way other than for ... in. If you don’t want to use that (parhaps because it’s marginally inefficient to have to test hasOwnProperty on each iteration?) then use a different construct, e.g. an array of kvp’s:

[{ key: 'key', value: 'value'}, ...]

Questions:
Answers:

The easiest way is to just use Underscore.js:

keys

_.keys(object)
Retrieve all the names of the object’s properties.

_.keys({one : 1, two : 2, three : 3});
=> [“one”, “two”, “three”]

Yes, you need an extra library, but it’s so easy!

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Answers:

Since you mentioned $.each(), here’s a handy approach that would work in jQuery 1.6+:

var foo = { key1: 'bar', key2: 'baz' };

// keys will be: ['key1', 'key2']
var keys = $.map(foo, function(item, key) {
  return key;
});

Questions:
Answers:

use for each loop for accessing keys in Object or Maps in javascript

for(key in foo){
   console.log(key);//for key name in your case it will be bar
   console.log(foo[key]);// for key value in your case it will be baz
}

Note: you can also use

Object.keys(foo);

it will give you like this
output:

[bar];

Questions:
Answers:

Well $.each is a library construct, whereas for ... in is native js, which should be better

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Object.keys()
The Object.keys() method returns an array of a given object’s own enumerable properties, in the same order as that provided by a for…in loop (the difference being that a for-in loop enumerates properties in the prototype chain as well).

var arr1 = Object.keys(obj);

Object.values()
The Object.values() method returns an array of a given object’s own enumerable property values, in the same order as that provided by a for…in loop (the difference being that a for-in loop enumerates properties in the prototype chain as well).

var arr2 = Object.values(obj);

For more please go here

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for showing as a string, simply use:

console.log("they are: " + JSON.stringify(foo));