Understanding the 'this' keyword in JavaScript

'this' in JavaScript

In JavaScript it is more complicated because where this refers to depends not only on how the function is defined but also on the form of calling it.

Take a look how this works depending on invocation place and form.

Global context

Used in a global context bound to the global object, like window in a web browser.

this; // window

Inside object method

Used inside of an object method bound to the closest enclosing object. The object is the new context of the this keyword. Note that you should not change function () to ES6 syntax fun: () => this.context because it will change the context.

const obj = {
  context: "object",
  fun: function() {
    return this.context;
  }
};

obj.fun(); // object

In this example with a nested object, this still refers to its closest context.

const nestedObj = {
  context: "parent",
  child: {
    context: "child",
    fun: function() {
      return this.context;
    }
  }
};

nestedObj.child.fun(); // child

Context-less function

Used inside a function that has no context (has no object as parent) bound to the global context, even if the function is definded inside the object.

Note that we use var context instead of let/const context because let and const change the variable enclosed context. var is always closest to the global execution context. let and const declare variables only in a local block scope.

var context = "global";

const obj = {
  context: "object",
  funA: function() {
    function funB() {
      const context = "function";
      return this.context;
    }
    return funB(); // invoked without context
  }
};

obj.funA(); // global

Inside constructor function

Used inside a function that is the constructor of the new object bound to it.

var context = "global";

function Obj() {
  this.context = "Obj context";
}

const obj = new Obj();
obj.context; // Obj context

Inside function defined on prototype chain

Used inside a function defined on the prototype chain to creating an object bound to it.

const ProtoObj = {
  fun: function() {
    return this.name;
  }
};

const obj = Object.create(ProtoObj);
obj.name = "foo";
obj.fun(); // foo

Inside call() and apply()  functions

call() and apply() are JavaScript functions. With these, an object can use methods belonging to another object. call() takes arguments separately where apply() takes them as an array.

this is here bound to new context changed in call() and apply() methods.

const objA = {
  context: "objA",
  fun: function() {
    console.log(this.context, arguments);
  }
};

const objB = {
  context: "objB"
};

objA.fun(1, 2); // objA, \[1, 2\]
objA.fun.call(objB, 1, 2, 3); // objB, \[1, 2, 3\]
objA.fun.apply(objB, \[1, 2, 3, 4\]); // objB, \[1, 2, 3, 4\]

Inside bind() function

bind() is also a JavaScript method. It creates a new function that will have this set to the first parameter passed to bind()**.**

const objA = {
  context: "objA",
  fun: function() {
    console.log(this.context, arguments);
  }
};

const objB = {
  context: "objB"
};

const fun = objA.fun.bind(objB, 1, 2);
fun(3, 4); // objB, \[1, 2, 3, 4\]

Inside event handlers

Used in any event handler (for example, addeventListener, onclick, attachEvent), it is bound to the DOM element the event was attached to.

document.querySelector(".foo").addEventListener("click", function() {
  this; // refers to the \`foo\` div element
});

ES6 arrow function

Used inside the arrow function it is always bound to its lexical scope. In the arrow function, you can't re-assign the this in any way.

const globalArrowFunction = () => this;

globalArrowFunction(); // Window

const obj = {
  context: "object",
  funA: () => this,
  funB: function() {
    return () => {
      return this.context;
    };
  }
};

obj.funA(); // Window
obj.funB()(); // object

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