JS Puzzles · Const

this binding

JavaScript’s this for regular functions comes from the call form, not the definition. Use bind to create a callable function with a fixed this and optional initial arguments.

How it works

For regular functions, this is selected when the function is called. Calling through object.method() makes the object the receiver; extracting the method removes that receiver, while bind creates a callable function with a fixed this. Arrow functions instead use the surrounding lexical this.

Syntax

let bound = func.bind(context, [arg1], [arg2], ...);

Parameters

Name	Type	Description
`context`	`any`	The object or value to use as `this` when the bound function is called.
`args`	`any[]`	Optional leading arguments supplied to the original function before call-time arguments.

Returns

A bound variant of the original function with the selected this value and any preset leading arguments.

Key points

For ordinary functions, invocation style determines this; the function's definition location does not.
bind returns another callable function-like value rather than changing the original function.
Calling the bound result behaves like calling the original function with the selected context.
bind can preset both the receiver and leading arguments.
Use bind when an object method must be handed to a callback without losing its receiver.

Common mistakes

Passing a method as a callback can lose its object receiver, so this may no longer refer to the original object.
A bound function keeps the chosen context and initial arguments from the moment bind is called.

Example: Method calls and bind

const user = { name: "Ada", show() { return this.name; } };
const detached = user.show;
console.log(detached());
console.log(user.show());

const bound = user.show.bind(user);
console.log(bound());

The method call supplies the object as this; detaching the function loses that receiver, while bind preserves it.

When to use it

Preserve a method receiver

Passing an object method as a timer or callback.

Fix the calling context

Reusing one function with a chosen object context.

Partially apply arguments

Supplying arguments before a callback is invoked.

bind()
arrow functions
object methods
setTimeout

Source: Function Binding. Checked 2026-10-15.