# Prerequisites

These articles are meant to help you learn how to use three.js. They assume you know how to program in JavaScript. They assume you know what the DOM is, how to write HTML as well as create DOM elements in JavaScript. They assume you know how to use es6 modules via import and via `<script type="module">` tags. They assume you know how to use import maps. They assume you know some CSS and that you know what CSS selectors are. They also assume you know ES5, ES6 and maybe some ES7. They assume you know that the browser runs JavaScript only via events and callbacks. They assume you know what a closure is.

Here's some brief refreshers and notes

## es6 modules

es6 modules can be loaded via the `import` keyword in a script or inline via a `<script type="module">` tag. Here's an example

    <script type="importmap">
    {
      "imports": {
        "three": "./path/to/three.module.js",
        "three/addons/": "./different/path/to/examples/jsm/"
      }
    }
    </script>

    <script type="module">
    import * as THREE from 'three';
    import {OrbitControls} from 'three/addons/controls/OrbitControls.js';

    ...

    </script>

See more details at the bottom of this article.

## `document.querySelector` and `document.querySelectorAll`

You can use `document.querySelector` to select the first element that matches a CSS selector. `document.querySelectorAll` returns all elements that match a CSS selector.

## You don't need `onload`

Lots of 20yr old pages use HTML like

    <body onload="somefunction()">

That style is deprecated. Put your scripts at the bottom of the page.

    <html>
      <head>
        ...
      </head>
      <body>
         ...
      </body>
      <script>
        // inline javascript
      </script>
    </html>

or use the `defer` property.

## Know how closures work

    function a(v) {
      const foo = v;
      return function() {
         return foo;
      };
    }

    const f = a(123);
    const g = a(456);
    console.log(f());  // prints 123
    console.log(g());  // prints 456

In the code above the function `a` creates a new function every time it's called. That function closes over the variable `foo`. Here's more info.

## Understand how `this` works

`this` is not magic. It's effectively a variable that is automatically passed to functions just like an argument is passed to function. The simple explanation is when you call a function directly like

    somefunction(a, b, c);

`this` will be `null` (when in strict mode or in a module) where as when you call a function via the dot operator `.` like this

    someobject.somefunction(a, b, c);

`this` will be set to `someobject`.

The parts where people get confused is with callbacks.

     const callback = someobject.somefunction;
     loader.load(callback);

doesn't work as someone inexperienced might expect because when `loader.load` calls the callback it's not calling it with the dot `.` operator so by default `this` will be null (unless the loader explicitly sets it to something). If you want `this` to be `someobject` when the callback happens you need to tell JavaScript that by binding it to the function.

     const callback = someobject.somefunction.bind(someobject);
     loader.load(callback);

this article might help explain `this`.

## ES5/ES6/ES7 stuff

### `var` is deprecated. Use `const` and/or `let`

There is no reason to use `var` EVER and at this point it's considered bad practice to use it at all. Use `const` if the variable will never be reassigned which is most of the time. Use `let` in those cases where the value changes. This will help avoid tons of bugs.

### Use `for(elem of collection)` never `for(elem in collection)`

`for of` is new, `for in` is old. `for in` had issues that are solved by `for of`

As one example you can iterate over all the key/value pairs of an object with

    for (const [key, value] of Object.entries(someObject)) {
      console.log(key, value);
    }

### Use `forEach`, `map`, and `filter` where useful

Arrays added the functions `forEach`, `map`, and `filter` and are used fairly extensively in modern JavaScript.

### Use destructuring

Assume an object `const dims = {width: 300, height: 150}`

old code

    const width = dims.width;
    const height = dims.height;

new code

    const {width, height} = dims;

Destructuring works with arrays too. Assume an array `const position = [5, 6, 7, 1]`;

old code

    const y = position[1];
    const z = position[2];

new code

    const [, y, z] = position;

Destructuring also works in function arguments

    const dims = {width: 300, height: 150};
    const vector = [3, 4];

    function lengthOfVector([x, y]) {
      return Math.sqrt(x * x + y * y);
    }

    const dist = lengthOfVector(vector);  // dist = 5

    function area({width, height}) {
      return width * height;
    }
    const a = area(dims);  // a = 45000

### Use object declaration short cuts

old code

     const width = 300;
     const height = 150;
     const obj = {
       width: width,
       height: height,
       area: function() {
         return this.width * this.height
       },
     };

new code

     const width = 300;
     const height = 150;
     const obj = {
       width,
       height,
       area() {
         return this.width * this.height;
       },
     };

### Use the rest parameter and the spread operator `...`

The rest parameter can be used to consume any number of parameters. Example

     function log(className, ...args) {
       const elem = document.createElement('div');
       elem.className = className;
       elem.textContent = args.join(' ');
       document.body.appendChild(elem);
     }

The spread operator can be used to expand an iterable into arguments

    const position = [1, 2, 3];
    someMesh.position.set(...position);

or copy an array

    const copiedPositionArray = [...position];
    copiedPositionArray.push(4); // [1,2,3,4]
    console.log(position); // [1,2,3] position is unaffected

or to merge objects

    const a = {abc: 123};
    const b = {def: 456};
    const c = {...a, ...b};  // c is now {abc: 123, def: 456}

### Use `class`

The syntax for making class like objects pre ES5 was unfamiliar to most programmers. As of ES5 you can now use the `class` keyword which is closer to the style of C++/C#/Java.

### Understand getters and setters

Getters and setters are common in most modern languages. The `class` syntax of ES5 makes them much easier than pre ES5.

### Use arrow functions where appropriate

This is especially useful with callbacks and promises.

    loader.load((texture) => {
      // use texture
    });

Arrow functions bind `this` to the context in which you create the arrow function.

    const foo = (args) => {/* code */};

is a shortcut for

    const foo = (function(args) {/* code */}).bind(this));

See link above for more info on `this`.

### Promises as well as async/await

Promises help with asynchronous code. Async/await help use promises.

It's too big a topic to go into here but you can read up on promises here and async/await here.

### Use Template Literals

Template literals are strings using backticks instead of quotes.

    const foo = `this is a template literal`;

Template literals have basically 2 features. One is they can be multi-line

    const foo = `this
    is
    a
    template
    literal`;
    const bar = "this\nis\na\ntemplate\nliteral";

`foo` and `bar` above are the same.

The other is that you can pop out of string mode and insert snippets of JavaScript using `${javascript-expression}`. This is the template part. Example:

    const r = 192;
    const g = 255;
    const b = 64;
    const rgbCSSColor = `rgb(${r},${g},${b})`;

or

    const color = [192, 255, 64];
    const rgbCSSColor = `rgb(${color.join(',')})`;

or

    const aWidth = 10;
    const bWidth = 20;
    someElement.style.width = `${aWidth + bWidth}px`;

# Learn JavaScript coding conventions.

While you're welcome to format your code any way you chose there is at least one convention you should be aware of. Variables, function names, method names, in JavaScript are all lowerCasedCamelCase. Constructors, the names of classes are CapitalizedCamelCase. If you follow this rule your code will match most other JavaScript. Many linters, programs that check for obvious errors in your code, will point out errors if you use the wrong case since by following the convention above they can know when you're using something incorrectly.

    const v = new vector(); // clearly an error if all classes start with a capital letter
    const v = Vector();     // clearly an error if all functions start with a lowercase letter.

# Consider using Visual Studio Code

Of course use whatever editor you want but if you haven't tried it consider using Visual Studio Code for JavaScript and after installing it setup eslint. It might take a few minutes to setup but it will help you immensely with finding bugs in your JavaScript.

Some examples

If you enable the `no-undef` rule then VSCode via ESLint will warn you of many undefined variables.

Above you can see I mis-spelled `doTheThing` as `doThing`. There's a red squiggle under `doThing` and hovering over it it tells me it's undefined. One error avoided.

If you're using `<script>` tags to include three.js you'll get warnings using `THREE` so add `/* global THREE */` at the top of your JavaScript files to tell eslint that `THREE` exists. (or better, use `import` 😉)

Above you can see eslint knows the rule that `UpperCaseNames` are constructors and so you should be using `new`. Another error caught and avoided. This is the `new-cap` rule.

There are 100s of rules you can turn on or off or customize. For example above I mentioned you should use `const` and `let` over `var`.

Here I used `var` and it warned me I should use `let` or `const`

Here I used `let` but it saw I never change the value so it suggested I use `const`.

Of course if you'd prefer to keep using `var` you can just turn off that rule. As I said above though I prefer to use `const` and `let` over `var` as they just work better and prevent bugs.

For those cases where you really need to override a rule you can add comments to disable them for a single line or a section of code.

# If you really need to support legacy browsers use a transpiler

Most modern browsers are auto-updated so using all these features will help you be productive and avoid bugs. That said, if you're on a project that absolutely must support old browsers there are tools that will take your ES5/ES6/ES7 code and transpile the code back to pre ES5 Javascript.