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14 lines
1.3 KiB
Markdown
14 lines
1.3 KiB
Markdown
## Bound (Fat Arrow) Functions
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In JavaScript, the `this` keyword is dynamically scoped to mean the object that the current function is attached to. If you pass a function as a callback or attach it to a different object, the original value of `this` will be lost. If you’re not familiar with this behavior, [this Digital Web article](https://web.archive.org/web/20150316122013/http://www.digital-web.com/articles/scope_in_javascript) gives a good overview of the quirks.
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The fat arrow `=>` can be used to both define a function, and to bind it to the current value of `this`, right on the spot. This is helpful when using callback-based libraries like Prototype or jQuery, for creating iterator functions to pass to `each`, or event-handler functions to use with `on`. Functions created with the fat arrow are able to access properties of the `this` where they’re defined.
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```
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codeFor('fat_arrow')
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```
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If we had used `->` in the callback above, `@customer` would have referred to the undefined “customer” property of the DOM element, and trying to call `purchase()` on it would have raised an exception.
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The fat arrow was one of the most popular features of CoffeeScript, and ES2015 [adopted it](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions); so CoffeeScript 2 compiles `=>` to ES `=>`.
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