# @babel/plugin-transform-function-bind

> Compile the new function bind operator `::` to ES5.

## Detail

```js
obj::func
// is equivalent to:
func.bind(obj)

::obj.func
// is equivalent to:
obj.func.bind(obj)

obj::func(val)
// is equivalent to:
func.call(obj, val)

::obj.func(val)
// is equivalent to:
obj.func.call(obj, val)
```


## Example

### Basic

```js
const box = {
  weight: 2,
  getWeight() { return this.weight; },
};

const { getWeight } = box;

console.log(box.getWeight()); // prints '2'

const bigBox = { weight: 10 };
console.log(bigBox::getWeight()); // prints '10'

// Can be chained:
function add(val) { return this + val; }

console.log(bigBox::getWeight()::add(5)); // prints '15'
```


### Using with `document.querySelectorAll`

It can be very handy when used with `document.querySelectorAll`:

```js
const { map, filter } = Array.prototype;

let sslUrls = document.querySelectorAll('a')
                ::map(node => node.href)
                ::filter(href => href.substring(0, 5) === 'https');

console.log(sslUrls);
```


`document.querySelectorAll` returns a `NodeList` element which is not a plain array, so you normally can't use the `map` function on it, and have to use it this way: `Array.prototype.map.call(document.querySelectorAll(...), node => { ... })`. The above code using the `::` will work because it is equivalent to:

```js
const { map, filter } = Array.prototype;

let sslUrls = document.querySelectorAll('a');
sslUrls = map.call(sslUrls, node => node.href);
sslUrls = filter.call(sslUrls, href => href.substring(0, 5) === 'https');

console.log(sslUrls);
```

### Auto self binding
When nothing is specified before the `::` operator, the function is bound to its object:

```js
$('.some-link').on('click', ::view.reset);
// is equivalent to:
$('.some-link').on('click', view.reset.bind(view));
```

## Installation

```sh
npm install --save-dev @babel/plugin-transform-function-bind
```

## Usage

### Via `.babelrc` (Recommended)

**.babelrc**

```json
{
  "plugins": ["transform-function-bind"]
}
```

### Via CLI

```sh
babel --plugins transform-function-bind script.js
```

### Via Node API

```javascript
require("@babel/core").transform("code", {
  plugins: ["transform-function-bind"]
});
```

## References

* [Proposal](https://github.com/zenparsing/es-function-bind)
* [Babel Blog: Function Bind Syntax](/blog/2015/05/14/function-bind)