A super condensed JavaScript reference for Watch and Code students.
- Most of your time will be spent reading, not writing.
- Simulates working at a company or open source project.
- It's the fastest way to learn and improve.
- Learn how to ignore large parts of a codebase and get a piece-by-piece understanding.
- Read the docs (if they exist).
- Run the code.
- Play with the code to see how it behaves.
- Think about how the code might be implemented.
- Get the code into an editor so that you can modify it.
- Look at the file structure.
- Get a sense for the vocabulary.
- Make note of any unfamiliar concepts that you might need to research later.
- Do a quick read-through without diving into concepts from #2.
- Test one feature with the debugger.
- Document and add comments to confusing areas.
- Research items in #2 only if required.
- Repeat steps 4-6 if you want to understand more.
- Replicate parts of the app by hand (in the console).
- Make small changes and see what happens.
- Add a new feature.
There isn't a single word that describes this well, so I just think of it as a special variable that changes depending on the situation. Those different situations are captured below.
Case 1: In a regular function (or if you're not in a function at all), this points to window. This is the default case.
function logThis() {
console.log(this);
}
logThis(); // window
// In strict mode, `this` will be `undefined` instead of `window`.
// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_modeCase 2: When a function is called as a method, this points to the object that's on the left side of the dot.
/*
* You can also think of this as the "left of the dot" rule.
* For example, in myObject.myMethod(), `this` will be myObject
* because myObject is to the left of the dot.
*
* Of course, if you're using this syntax myObject['myMethod'](),
* technically it would be the "left of the dot or bracket" rule,
* but that sounds clumsy and generally terrible.
*
* If you have multiple dots, the relevant dot is the one closest
* to the method call. For example, if you have one.two.hi();
* `this` inside of hi will be two.
*/
var myObject = {
myMethod: function() {
console.log(this);
}
};
myObject.myMethod(); // myObjectCase 3: In a function that's being called as a constructor, this points to the object that the constructor is creating.
function Person(name) {
this.name = name;
}
var gordon = new Person('gordon');
console.log(gordon); // {name: 'gordon'}Case 4: When you explicitly set the value of this manually using bind, apply, or call, it's all up to you.
function logThis() {
console.log(this);
}
var explicitlySetLogThis = logThis.bind({name: 'Gordon'});
explicitlySetLogThis(); // {name: 'Gordon'}
// Note that a function returned from .bind (like `boundOnce` below),
// cannot be bound to a different `this` value ever again.
// In other words, functions can only be bound once.
var boundOnce = logThis.bind({name: 'The first time is forever'});
// These attempts to change `this` are futile.
boundOnce.bind({name: 'why even try?'})();
boundOnce.apply({name: 'why even try?'});
boundOnce.call({name: 'why even try?'});function outerFunction(callback) {
callback();
}
function logThis() {
console.log(this);
}
/*
* Case 1: The regular old default case.
*/
outerFunction(logThis); // window
/*
* Case 2: Call the callback as a method
* (You'll probably NEVER see this, but I guess it's possible.)
*/
function callAsMethod(callback) {
var weirdObject = {
name: "Don't do this in real life"
};
weirdObject.callback = callback;
weirdObject.callback();
}
callAsMethod(logThis); // `weirdObject` will get logged to the console
/*
* Case 3: Calling the callback as a constructor.
* (You'll also probably never see this. But in case you do...)
*/
function callAsConstructor(callback) {
new callback();
}
callAsConstructor(logThis); // the new object created by logThis will be logged to the console
/*
* Case 4: Explicitly setting `this`.
*/
function callAndBindToGordon(callback) {
var boundCallback = callback.bind({name: 'Gordon'});
boundCallback();
}
callAndBindToGordon(logThis); // {name: 'Gordon'}
// In a twist, we give `callAndBindToGordon` a function that's already been bound.
var boundOnce = logThis.bind({name: 'The first time is forever'});
callAndBindToGordon(boundOnce); // {name: 'The first time is forever'}
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