Javascript get object function

Object methods, «this»

Objects are usually created to represent entities of the real world, like users, orders and so on:

And, in the real world, a user can act: select something from the shopping cart, login, logout etc.

Actions are represented in JavaScript by functions in properties.

Method examples

For a start, let’s teach the user to say hello:

let user = < name: "John", age: 30 >; user.sayHi = function() < alert("Hello!"); >; user.sayHi(); // Hello!

Here we’ve just used a Function Expression to create a function and assign it to the property user.sayHi of the object.

Then we can call it as user.sayHi() . The user can now speak!

A function that is a property of an object is called its method.

So, here we’ve got a method sayHi of the object user .

Of course, we could use a pre-declared function as a method, like this:

let user = < // . >; // first, declare function sayHi() < alert("Hello!"); >// then add as a method user.sayHi = sayHi; user.sayHi(); // Hello!

When we write our code using objects to represent entities, that’s called object-oriented programming, in short: “OOP”.

OOP is a big thing, an interesting science of its own. How to choose the right entities? How to organize the interaction between them? That’s architecture, and there are great books on that topic, like “Design Patterns: Elements of Reusable Object-Oriented Software” by E. Gamma, R. Helm, R. Johnson, J. Vissides or “Object-Oriented Analysis and Design with Applications” by G. Booch, and more.

Method shorthand

There exists a shorter syntax for methods in an object literal:

// these objects do the same user = < sayHi: function() < alert("Hello"); >>; // method shorthand looks better, right? user = < sayHi() < // same as "sayHi: function()" alert("Hello"); > >;

As demonstrated, we can omit «function» and just write sayHi() .

To tell the truth, the notations are not fully identical. There are subtle differences related to object inheritance (to be covered later), but for now they do not matter. In almost all cases, the shorter syntax is preferred.

“this” in methods

It’s common that an object method needs to access the information stored in the object to do its job.

For instance, the code inside user.sayHi() may need the name of the user .

To access the object, a method can use the this keyword.

The value of this is the object “before dot”, the one used to call the method.

let user = < name: "John", age: 30, sayHi() < // "this" is the "current object" alert(this.name); >>; user.sayHi(); // John

Here during the execution of user.sayHi() , the value of this will be user .

Technically, it’s also possible to access the object without this , by referencing it via the outer variable:

…But such code is unreliable. If we decide to copy user to another variable, e.g. admin = user and overwrite user with something else, then it will access the wrong object.

let user = < name: "John", age: 30, sayHi() < alert( user.name ); // leads to an error >>; let admin = user; user = null; // overwrite to make things obvious admin.sayHi(); // TypeError: Cannot read property 'name' of null

If we used this.name instead of user.name inside the alert , then the code would work.

“this” is not bound

In JavaScript, keyword this behaves unlike most other programming languages. It can be used in any function, even if it’s not a method of an object.

There’s no syntax error in the following example:

The value of this is evaluated during the run-time, depending on the context.

For instance, here the same function is assigned to two different objects and has different “this” in the calls:

let user = < name: "John" >; let admin = < name: "Admin" >; function sayHi() < alert( this.name ); >// use the same function in two objects user.f = sayHi; admin.f = sayHi; // these calls have different this // "this" inside the function is the object "before the dot" user.f(); // John (this == user) admin.f(); // Admin (this == admin) admin['f'](); // Admin (dot or square brackets access the method – doesn't matter)

The rule is simple: if obj.f() is called, then this is obj during the call of f . So it’s either user or admin in the example above.

We can even call the function without an object at all:

function sayHi() < alert(this); >sayHi(); // undefined

In this case this is undefined in strict mode. If we try to access this.name , there will be an error.

In non-strict mode the value of this in such case will be the global object ( window in a browser, we’ll get to it later in the chapter Global object). This is a historical behavior that «use strict» fixes.

Usually such call is a programming error. If there’s this inside a function, it expects to be called in an object context.

If you come from another programming language, then you are probably used to the idea of a «bound this «, where methods defined in an object always have this referencing that object.

In JavaScript this is “free”, its value is evaluated at call-time and does not depend on where the method was declared, but rather on what object is “before the dot”.

The concept of run-time evaluated this has both pluses and minuses. On the one hand, a function can be reused for different objects. On the other hand, the greater flexibility creates more possibilities for mistakes.

Here our position is not to judge whether this language design decision is good or bad. We’ll understand how to work with it, how to get benefits and avoid problems.

Arrow functions have no “this”

Arrow functions are special: they don’t have their “own” this . If we reference this from such a function, it’s taken from the outer “normal” function.

For instance, here arrow() uses this from the outer user.sayHi() method:

let user = < firstName: "Ilya", sayHi() < let arrow = () =>alert(this.firstName); arrow(); > >; user.sayHi(); // Ilya

That’s a special feature of arrow functions, it’s useful when we actually do not want to have a separate this , but rather to take it from the outer context. Later in the chapter Arrow functions revisited we’ll go more deeply into arrow functions.

Summary

• Functions that are stored in object properties are called “methods”.
• Methods allow objects to “act” like object.doSomething() .
• Methods can reference the object as this .

The value of this is defined at run-time.

• When a function is declared, it may use this , but that this has no value until the function is called.
• A function can be copied between objects.
• When a function is called in the “method” syntax: object.method() , the value of this during the call is object .

Please note that arrow functions are special: they have no this . When this is accessed inside an arrow function, it is taken from outside.

Tasks

Using «this» in object literal

Here the function makeUser returns an object.

What is the result of accessing its ref ? Why?

function makeUser() < return < name: "John", ref: this >; > let user = makeUser(); alert( user.ref.name ); // What's the result?

Answer: an error.

function makeUser() < return < name: "John", ref: this >; > let user = makeUser(); alert( user.ref.name ); // Error: Cannot read property 'name' of undefined

That’s because rules that set this do not look at object definition. Only the moment of call matters.

Here the value of this inside makeUser() is undefined , because it is called as a function, not as a method with “dot” syntax.

The value of this is one for the whole function, code blocks and object literals do not affect it.

So ref: this actually takes current this of the function.

We can rewrite the function and return the same this with undefined value:

function makeUser() < return this; // this time there's no object literal >alert( makeUser().name ); // Error: Cannot read property 'name' of undefined

As you can see the result of alert( makeUser().name ) is the same as the result of alert( user.ref.name ) from the previous example.

function makeUser() < return < name: "John", ref() < return this; >>; > let user = makeUser(); alert( user.ref().name ); // John

Now it works, because user.ref() is a method. And the value of this is set to the object before dot . .

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Return an Object From a Function in JavaScript

1. Return an Object From a Regular Function in JavaScript
2. Return an Object From an Anonymous Function in JavaScript
3. Return an Object From an Arrow Function in JavaScript

Whenever we say we want to return an object from a function, we return one object from another object (function). There are various ways to return an object using a function.

Let us now see how we can return an object from a function in JavaScript.

Return an Object From a Regular Function in JavaScript

There are various types of functions in JavaScript. Each of these functions is defined differently.

We will see each of these function types, and we will also see if we have to return an object using each of these functions in JavaScript and to return anything from a function, we always use a keyword known as return .

The regular function is a traditional way of defining a function in JavaScript. This type of function is present in the JavaScript programming language from its initial versions.

This function has three things, a function keyword, function name, and function body.

We have created a function called returnObj to return an object. This function aims to return an object.

We have created an obj object with two fields: the name and the company inside this function.

function returnObj()   var obj =   "name": "Adam",  "company": "Google",  >  return obj; >  var myObj = returnObj(); console.log(myObj); 

We must use a return keyword to return this object and return the obj created.

We must create a variable myObj and then call the returnObj function. After we call this function, whatever the function will return (in this case, an object) will be stored inside the myObj variable.

Finally, if you print the variable myObj , you will get the entire object as the output.

Return an Object From an Anonymous Function in JavaScript

An anonymous function is a function that does not have a name associated with it. This type of function only has a function keyword and a function body.

It was introduced in the ES6 version of JavaScript. Since this function does not have a name, we can’t call this function.

So, to call this function, we store this entire function into a variable first, and then we use this variable name as a function name to call this function, i.e., we add round brackets at the end.

Here, we have created a function that does not have a name. And we have created the same object obj which we have created earlier.

const myObj = function ()  var obj =   "name": "Adam",  "company": "Google",  >  return obj; >  console.log(myObj()); 

Finally, we will return this obj , and then after we call this function, the object we are returning from the function will be stored inside that variable.

Return an Object From an Arrow Function in JavaScript

The arrow function is also known as an anonymous function. The only difference between an arrow function and an anonymous function is that the arrow function does not use the function keyword while declaring a function.

Instead, it uses arrows (combination of equals and greater than sign) => to declare a function. This type of function was also introduced in the ES6 version of JavaScript.

Here, we have created an empty object, obj . We will create an arrow function that takes an object as a parameter ( entireObj ).

Inside the arrow function’s body, we will set the name and company properties of the entireObj object with some values. Then we will return the entireObj using the return keyword.

var obj = <>; const myObj = (entireObj)=>  entireObj.name = "Adam";  entireObj.company = "Google";  return entireObj; >  console.log(myObj(obj)); 

In the end, we will call the function myObj . Don’t forget to pass the empty object we created at the start as the parameter while calling the myObj function.

The result of this operation, i.e., the object, will be stored inside the myObj variable and then printed on the console.

Sahil is a full-stack developer who loves to build software. He likes to share his knowledge by writing technical articles and helping clients by working with them as freelance software engineer and technical writer on Upwork.

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