JavaScript

Built-in types / objects

Primitive built-in types

The values of the primitive built-in types is data that is not stored with or in an objects.

ECMAScript defines the following primitive built-in types:

Type	Range of values
Undefined	one value: `undefined`
Null	one value: `null`
Boolean	`true` and `false`
Number	IEEE-754-2008 64-bit double-precision numbers
BigInt	(?, not defined by ECMAScript ?)
String	A sequence of zero through 2⁵⁶-1 16-bit unsigned integer values («elements»)
Symbol	The set of strings that may be used as the key of an object property

All primitive built-in types except null and undefined have object wrappers so that their values can be stored in or with objects.

The respective wrapper objects' valueOf() method returns the primitive value again.

TODO: compare with built-in objects.

Object

Then there is the non-primitive built-in type Object.

TODO

call vs apply

Destructing assignments

function x() { return {num: 42, txt: "hello world"}; }

const {num, txt, def='unassigned'} = x();
console.log(num);
console.log(txt);
console.log(def);

const {num: ft, txt: hw} = x()
console.log(ft);
console.log(hw);

function y({a, b}) { console.log(a * b);
y( {a: 7, b: 6} );

Modules

A module is a JavaScript unit that exports objects (such as functions or variables) (export keyword).

See also <script type="module">…</script>.

Modules are fetched using CORS.

ES Modules vs CommonJS

The ECMS standard (ES) is the official standard to package JavaScript for reuse. The corresponding keywords are export and import.

CommonJS is the original way to package JavsScript for Node.js. The corresponding keywords are module.exports and require().

Node version 13.2.0 has ES modules (as well?).

In order for Node.js to treat a module as ES module, there are three possibilities:

use .mjs file suffix
use the type field with a value of module in package.json
use the --input-type=module command line option

The suffix .cjs is used to mark a file as a CommonJS module.

ES Module:

import pkg from 'moduleName'

CommonJS:

module.exports.sym = …;

const pkg = require('moduleName')