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Call, Apply And Bind In JavaScript

May 02, 2020

Table Of Contents

    In this post we are going to discuss some famous JavaScript methods: call, apply and bind, see how they work, what are they used for, and how to use them properly in order to get things working.

    Call, Apply And Bind

    They are all methods within the function prototype, basically doing the same thing: allow us to call a function with given this context and arguments, but in different ways.

    You can refer to this article in order to learn more about this object in JavaScript.

    Call Or Function.prototype.call

    call is used to immediately execute a function with given this context and arguments:

    const user = {
      name: "John",
      surname: "Doe",
    };
    
    function greet(welcomeText, tableNumber) {
      return welcomeText + ", " + this.name + " " + this.surname + ", your table number is: " + tableNumber;
    };
    
    greet.call(user, "Welcome", 10); // Prints "Welcome, John Doe, your table number is: 10"

    As you can see, call function receives user as the first parameter, therefore it is the context. Second and third parameters are passed directly to the greet function.

    Apply Or Function.prototype.apply

    apply behaves exactly the same way as call, with the only difference - it receives an array of the arguments as the second parameter:

    const user = {
      name: "John",
      surname: "Doe",
    };
    
    function getFullName(welcomeText, tableNumber) {
      return welcomeText + ", " + this.name + " " + this.surname + ", your table number is: " + tableNumber;
    };
    
    getFullName.apply(user, ["Welcome", 10]); // Prints "Welcome, John Doe, your table number is: 10"

    Were you able to spot the difference?

    Bind Or Function.prototype.bind

    The behavior of bind method is exactly the same as of apply with just only one difference. But it's really huge.

    It is not executed immediately, but returns a bound function:

    const user = {
      name: "John",
      surname: "Doe",
    };
    
    function getFullName(welcomeText, tableNumber) {
      return welcomeText + ", " + this.name + " " + this.surname + ", your table number is: " + tableNumber;
    };
    
    const boundFunction = getFullName.bind(user, "Welcome", 10); // "bind" returns bound function
    
    boundFunction(); // Prints "Welcome, John Doe, your table number is: 10"

    We can pass arguments at the same time we execute bind method, as we did above or at the moment when we invoke the bound function:

    const user = {
      name: "John",
      surname: "Doe",
    };
    
    function getFullName(welcomeText, tableNumber) {
      return welcomeText + ", " + this.name + " " + this.surname + ", your table number is: " + tableNumber;
    };
    
    const boundFunction = getFullName.bind(user); // "bind" returns bound function
    
    boundFunction("Welcome", 10); // Prints "Welcome, John Doe, your table number is: 10"

    Borrowing Methods

    Sometimes it’s desirable to reuse a function or method on a different object other than the object or prototype it was defined.

    By using call, apply and bind, we can easily borrow methods from different objects without having to inherit from them.

    Borrowing Max From The Math Object

    Imagine the following situation: you have an array of numbers and need to find a maximum number. You know that there is Math.max function that does exactly what we need, so why don't we use it?

    The problem is that Math.max does not accept an array, but numbers:

    Math.max(10, 20, 30, 1, 100); // "100"
    
    const numbers = [10, 20, 30, 1, 100];
    
    Math.max(numbers); // "NaN"

    apply to rescue:

    const numbers = [10, 20, 30, 1, 100];
    
    Math.max.apply(null, numbers); // "100"

    We pass null as the first argument, because we don't really need to pass context.

    The second argument, our numbers array gets converted to arguments add will be accepted as a valid parameter for Math.max function.

    Borrowing Join And Filter From Array

    Array.prototype.join.call("abc", ","); // "a,b,c"
    
    Array.prototype.filter.call("string1", function(val) {
        return val !== "1";
    }).join(""); // "string"

    As you can see it's cool to benefit from borrowing prototype methods, but there's one thing we can improve.

    We don't have to write Array.prototype each time we want to borrow something. We can benefit from literals.

    Using Literals

    Literals are used to represent values in JavaScript. These are fixed values, not variables, that you literally provide in your script

    We can replace Array.prototype with []:

    [].join.call("abc", ","); // "a,b,c"
    
    [].filter.call("string1", function(val) {
        return val !== "1";
    }).join(""); // "string"

    The code above can be a little improved, so we wouldn't have to operate [] directly.

    We can store a reference to literal in the variable:

    const join = [].join;
    
    join.call("abc", ","); // "a,b,c"
    
    
    const filter = [].filter;
    
    filter.call("string1", function(val) {
        return val !== "1";
    }).join(""); // "string"

    Continuing in the spirit of reducing verboseness, let’s borrow a method without having to write call and apply each time we want to use it:

    const boundJoin = Function.prototype.call.bind(Array.prototype.join);
    
    boundJoin('abc', ","); // "a,b,c"
    
    
    const boundFilter = Function.prototype.call.bind(Array.prototype.filter);
    
    boundFilter("string1", function(val) {
        return val !== "1";
    }).join(""); // "string"

    Important note: it's totally fine to change method names from boundJoin and boundFilter to join and filter. Those example names are used just to remind that bind isn't executed immediately, but returns a new function.

    Summary

    You should have a solid understanding of how call, apply and bind work by now.

    Those functions are often forgotten by developers so try to always keep them in mind and use if needed.

    • call is used to immediately execute a function with given this context and arguments
    • apply behaves exactly the same way as call, with the only difference - it receives an array of the arguments as the second parameter
    • bind behaves exactly the same way as apply, but not gets executed immediately. It returns a bound function
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