Slaying `this` in Javascript

Jim Cummins

Twitter/Github: @jimthedev

Slides: http://tiny.cc/slaying-this

Developer

Commitizen author

Developers

`this`

Stand up!

Air squats

function myFunction() {
  console.log('Hello world');
}








myFunction();

Function declaration

Instructions to be executed when the function is called.

Functions in Javascript

Call site

Tells Javascript to actually call the function.

Call stack

(empty)

> myFunction

Ignore

Ignore

Ignore

Execute!

Return

Execute

Execute

Execute

(global)

Let's see a quick example of the keyword `this` in Javascript.

For many developers new to Javascript...

Why does `this`
seem difficult?  

Slightly different semantics in other programming languages.

"Today I woke up feeling well rested. This is going to be a great day!"

But...Even context is a hard word to define.

Why does context
seem difficult?  

In the above sentence, the context is implicit.

Mark went to the store today. He bought 5 apples.

Determining context in plain English

Mark went to the store today and saw Juan. He bought 5 apples.

Mark went to the store today and he bought 5 apples. While there Mark saw Juan.

In this context, he refers to Mark.

In this context, he refers to ???

In this context, he refers to Mark.

Implicit &

Unambiguous

Explicit &

Unambiguous

Implicit &

Ambiguous

Why does context matter in Javascript?

Context matters so that we can determine what the `this` keyword actually refers to.

In many languages, context is only provided to`this` implicitly.

In Javascript we have the power to set context explicitly using apply(), call(), or bind().

Types of Contexts

Implicit

Explicit

New

Default

Implicit Binding

When `this` gets set because a function is a member of an object.

Explicit Binding

When `this` gets set as a result of using .call(), .apply(), or .bind()

New Binding

When `this` gets set as a result of calling new Foo();

Default Binding

The result of no other binding being set.

Explicit: .apply(), .call()

Explicitly set the context of a given function call. 

Overrides any implicit binding that might have been set.

Explicit: .bind()

Bind gives us the benefits of apply() or call() but instead of immediately invoking the function it creates a new function.

Thank you!

Questions? Comments? Concerns?

http://javascriptissexy.com/understand-javascripts-this-with-clarity-and-master-it/

http://ryanmorr.com/understanding-scope-and-context-in-javascript/

https://github.com/getify/You-Dont-Know-JS/blob/master/this%20%26%20object%20prototypes/ch1.md

Sources

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By jimthedev

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