# [Javascript] bind()

### What does `bind()` do?

1. It binds a function to an object.
2. It allows you to reference the object using the keyword `this`.

### Without `bind()`

Here is an Object literal `obj`, which holds two properties, and a function `printProperties`, which prints the values of the keys `a` and `b`.

```
const obj = {
  a: 1,
  b: 2
};

function printProperties () {
   console.log(`${this.a} , ${this.b}`);
}
```

However, since the `printProperties` is not bound to any specific object yet, it is bound to the `Window` object. Attempting to call `printProperties` will result in the console logging `undefined , undefined`.

```
printProperties(); // undefined , undefined
```

The fact that `printProperties` is bound to the `Window` object can be demonstrated by redefining `printProperties` so that it logs `this`:

```
function printProperties () {
  console.log(this);
}
printProperties();
```

Here is a screenshot of the result:

![Screen Shot 2022-09-28 at 1.05.54 PM.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1664388360194/3RUfWv1Xz.png align="left")

### How to use `bind()`

Since `bind()` returns a "copy of the given function with the specified this value", it must be stored in a variable in order to be invoked properly.`obj` can be bound to `printProperties` as such:

```
// General Form: function_name.bind(object_name)
const boundPrintProperties = printProperties.bind(obj);
```

### With `bind()`

Now that the function `printProperties` is bound to `obj`, calling it will correctly print the intended values:

```
boundPrintProperties(); // 1 , 2
```

We can even bind `printProperties` to other objects.

Suppose we have another object named `newObj`. We can bind `newObj` with `printProperties` by using the `bind` method, then call it.

```
const newObj = {
  a: 3,
  b: 4
};

const newBoundPrintProperties = printProperties.bind(newObj);

newBoundPrintProperties(); // 3 , 4
```

`newBoundPrintProperties()` correctly prints `3 , 4`.

We can understand `bind()` as a method that "binds" a function to a specific object so that it can access the chosen object's methods and properties using the `this` keyword.