Object keys in javaScript and getting arrays of key names

In javaScript Object keys in javaScript are the property names of an object that correspond with a value that can be a primitive, or another nested object of one kind or another such as a function or Date object. There are a few things to know about object keys in javaScript, such as how to get an array of public key names from a given object, how to create and get hidden key names, and also the work with inherited keys also when it comes to the nature of the prototype property of objects.
In this post I will be writing about the basics of javaScript object keys, and also maybe some of the not so basic things to know about with object keys in general when working on a project. This includes the Object.keys method as well as other object prototype methods like Object.getOwnPropertyNames that I will be getting to when it comes to getting both public and hidden key names of an object.

1 - What to know about Object keys in javaScript

If you are new to javaScript you might want to read over this section. You might still want read over this anyway if you still think you might not know everything there is to know that is going on with objects in javaScript. However if you are a more advanced jaavScript user you might want to skip over this section.

In javaScript there are plain old Objects that can be created with the Object literal syntax, and then there are many other Objects that inherit from the plain old Object prototype such as Arrays, Functions, Dates, and so forth. In other words there is what is often referred to as an own property of an object, that is keys that are unique to a single given object, and then there are inherited key names that are not part of the single object but its prototype.

These Objects contain property names, or keys, that corresponds with values that can be Numbers, Strings, other primitives, or additional nested Objects of one type or another. So you have key names, and then you have values that correspond to these keys that can be both public, as well as privet or hidden. There are keys and values that are unique to a single object, and then there are keys and values that are not.

Often there might be a need to get an array of key names, or key values of an object. When doing so often it is desired to only get an array of keys and or values that are the own properties of an object. In addition in most situations I would only need to get public own key names or values. However in some situations I might want to also have everything that there is going on with the object.

1.1 - Enumerable, and non-Enumerable Object key names

Object keys can be enumerable meaning that it is possible for the key name to be easily acquired into an array of key names. However it is also possible for them to be non-enumerable as well, when this is the case it is still possible to to include them in an array of key names it just means that a method like Object.getOwnPropertyNames needs to be used to do so. It is also possible to set this value for an object key as well, more on that later.

1.2 - inherited Object keys, and Object Own keys

javaScript supports prototype based inheritance, a subject I will not be getting into detail with here. However for the sake of the content of this post I will quickly mention that some keys of any given Object in javaScript are inherited from a prototype object chain and thus are not part of what is often referred to as an Objects own properties. When an Array of key names is produced in javaScript typically these keys are not needed or desired in such an array, so it is a non issue, but it is still something to be aware of when it comes to Object keys in javaScript.

2 - Object.keys method

The Object.keys method can be used to get the objects enumerable own key names. In other words key names that will show up in a loop like that of a for in loop, and have not been created in a way that allows for them to be hidden from such a loop. The Object.keys method is a Static method of the Main javaScript Object Class Object rather than a prototype method. So in other words to use the Object keys method I just call the Object.keys static method and pass the object that I want an array of key names for. After calling the method the returned result will end up being an array of public keys for the given object.

2.1 - A Basic Object keys method example

So say I have an object that contains some public keys and corresponding values such as a x and y key along with number values for example. If just simply want an array of these key names i can pass the Object to the Object keys method as the first and only argument, and then assign the array that is returned to a variable. The variable will then contain my array of key names for this object of mine.

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var obj = {
x: 0,
y: 35,
attack: 12,
hp : {
current: 47,
max: 50
}
};
var keys = Object.keys(obj);
console.log(keys); // ["x", "y", "attack", "hp"]

So you get the basic idea, but lets look at a few more use cases examples just to help to get a better understanding of how useful this method is in all kinds of situations that will creep up now and then.

2.2 - Using array prototype methods with objects that have named keys

So then because the Object keys methods returns an array, then any array prototype method such as array forEach can be used with any plain old javaScript in general. So for example say I have an object with named keys, I can pass that object to the object keys method and then call the array foreach method off of the resulting array that is returned. Inside the body of the function that I pass to the foreach method as an argument I can access the key names for each public key, as well as the index value for each key name in the array. When it comes to getting the value for each key in the object I can use the key name with the bracket syntax with the original object.

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let obj = {
a: 1,
b: 2,
c: 3
};
Object.keys(obj).forEach((key, i, arr) => {
console.log(key, i, obj[key], arr[i]);
});
/*
a 0 1 a
b 1 2 b
c 2 3 c
*/

The array foreach method might not be the best example of this, however you get the general idea. Other array prototype methods can of course also be used with the resulting array such as array map, and array filter just to name a few. So lets look at some more advanced use case examples of the Object keys method in action to get a better feel of this method.

2.3 - Creating an array of arrays from and array of objects

Say you have an array of weird objects where values that you want are encoded into the key names. What you want to do is create an array of arrays where each nested array contains a value that is encoded in the key names of the objects that are in the array of objects. One way to go about doing that would be to use the Object keys method in conjunction with array map, and the string split method maybe.

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let points = [{x47: 0, y32: 0 }, {x13: 0, y7: 0 }, {x0: 0, y50: 0 } ];
let arr = points.map((obj) => {
return Object.keys(obj).map((key) => {
return key.split(/x|y/)[1];
})
});
console.log(arr);
// [ [ '47', '32' ], [ '13', '7' ], [ '0', '50' ] ]

So it goes without saying that is is useful to be aware of the Object keys method as well as methods like the array map method and the string split method when it comes to working out solutions for weird things like this that might come up now and then when working with javaScript code. I will nit be getting into array map, string split, and regular expressions that are ysed in this example of the Object keys method here. However those topics sure are worth reading up on if you have not done so before hand.

2.3 - using a polyfill for object keys

The object keys static method has decent browser support these days. However still in some situations the method might need to be pollyfilled, as a way to make sure that the method is there to work with in the event that a user visits a site with a browser that is out of date will not run into an issue with code braking as a result of the object keys method not being there.

Here is an Object keys polyfill that i just copied and pasted from the Mozilla page for object keys just so save you the trip there.

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// From https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/keys
if (!Object.keys) {
Object.keys = (function() {
'use strict';
var hasOwnProperty = Object.prototype.hasOwnProperty,
hasDontEnumBug = !({ toString: null }).propertyIsEnumerable('toString'),
dontEnums = [
'toString',
'toLocaleString',
'valueOf',
'hasOwnProperty',
'isPrototypeOf',
'propertyIsEnumerable',
'constructor'
],
dontEnumsLength = dontEnums.length;
return function(obj) {
if (typeof obj !== 'function' && (typeof obj !== 'object' || obj === null)) {
throw new TypeError('Object.keys called on non-object');
}
var result = [], prop, i;
for (prop in obj) {
if (hasOwnProperty.call(obj, prop)) {
result.push(prop);
}
}
if (hasDontEnumBug) {
for (i = 0; i < dontEnumsLength; i++) {
if (hasOwnProperty.call(obj, dontEnums[i])) {
result.push(dontEnums[i]);
}
}
}
return result;
};
}());
}

I have come to find that I prefer to use a user space option over polyfilling. One such example of this might be the lodash keys method. However when using the lodash keys method it is not just a question of using lodash and being done with it. Late versions of lodash might just reference the native object keys method actually, so it is still a good idea to keep an eye and how far back browser support goes with the version of lodash that you are suing if you choose to go that way with it.

3 - Using a for in loop

A for in loop can also be used to get the objects own key names, and for a time was the only way to do so when it comes to older javaScript specs. So with that said a for in loop is a more tired yet true way of getting the key names of an object in javaScript. The Object keys method was introduced in ecma 5 sec javaScript, so an alternative method such as this might only need to be used if for some reason you need to support a real old platform.

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var keys = [],
k;
for(k in obj){
keys.push(k);
}

For the most part the Object keys method is well supported these days. In addition there are of course ways of polyfilling the method, or using some kind of user space module to have a stand alone method for object keys that will work on older platforms. When I look at by browser stats for my website here i can nit say there is much need to worry about it, but the use of for in loops is one way to go about dealing with this concern if it does become a problem.

4 - Using Object.getOwnPropertyNames to get non enumerable own property object keys as well

In some situations I might have some properties in an Object own properties that are not enumerable. In a way these kinds of properties are still enumerable, it just means that it can not be done with a for in loop or Object.keys. The Object.getOwnProperyNames method can still be used to include these kinds of object keys in an array.

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var point = {
x: 15,
y: 25
};
Object.defineProperty(point, 'color', {
value: '#ff0000',
writable: false,
enumerable: false
});
console.log(Object.keys(point)); // ["x","y"]
console.log(Object.getOwnPropertyNames(point)); // ["x", "y", "color"]

5 - Conclusion

So that is it for my post on Object keys in javaScript. There are keys that are enumerable so a list of key names can be easily obtained by using Object.keys or a for loop, and then Object.getOwnPropertyNames to get all Object own property names. getting the public and or private key names of an object is just often one part of a certain something that needs to happen though. So being aware of other object static methods, and prototype methods helps when it comes to working out all kinds of solutions for various coding problems on an as needed basis.