23 June, 2021

JavaScript Notes

I use node.js to run JavaScript code in the command line. Install the latest version to get new JavaScript features.

Create a new project with npm init -y

Use Module Import Syntax

To use the new module syntax, the package.json file must be updated:

"type": "module",

Create file index.js

import formatDate from './format-date.js' 

console.log(formatDate(new Date()))

Create the file format-date.js

function pad(text) {
  return `00${text}`.substring(`${text}`.length)
}

export default function formatDate(date) {
  return `${date.getFullYear()}-${pad(date.getMonth()+1)}-${pad(date.getDate())} ${pad(date.getHours())}-${pad(date.getSeconds())}`
}

Call node index to test-run the script.

Variable

Declare variables with var, let and const keywords.

Objects

let s = Symbol()
let a = {
  b: true,
  x: 1,
  y: 1.2,
  z: 'text',
  k: ['a', 'b'],
  [s]: 'hidden',
  f: function() {
       return `${this.b}, ${this.z} ...`
     }
}

for (let i in a) {
  console.log(i)
}

Object attribute defined by Symbol() is hidden from JSON.stringify and for ... in loops.

Strings

Variable declaration:

let x = 'text'
let y = `more ${x}`
parseInt, parseFloat

var new Date(1411229689063)

text.replaceAll('DEV_', 'd_') // not available before 15 in node
text.replace(/DEV_/g, 'd_')

Number('1234') - Convert string to number 1.234.toFixed(2) - Formats a number to decimal with to digits after the decimal pointer.

Arrays

Array.isArray(a1) - Returns boolean, true when object a1 is an array

includes(e1) - Returns boolean

push(e1, e2, e3) - Adds one or more elements to the end

unshift(e4) - Adds one element to the front

some(e => e.weight == 4) - Returns boolean when any element has weight 4

indexOf, filter, slice, splice

Functions

Functions must have a name when not immediately called. Functions cannot be overloaded with different parameter count.:

function f() {}

Immediately called functions can be without a name:

(function () {})()

Using void operator to immediately call a function:

void function () {}()

Declare a variable by function expression. That function returns a function and call that:

let f = function () {
  return function () {}
}

f()()

Lambdas

Declare a lambdad variable and call it:

let lambda = _ => console.log('lambda')
lambda()

Declare a function that returns a lambda and call that:

let g = function () {
  return _ => void 0
}

g()()
let h = _ => _ => void 0

h()()

Classes

class A {
  x = 1
  y = 'a'
  z = null

  constructor() {
    z = 'constructor'
  }
}

Bind this

In a class with a callback funtion which uses member variables we have to use bind. When bind is not done, the additionalFilter uses the list instance for this:

class SomeComponent {
  parameter = 'something'

  constructor(list) {
    list.additionalFilter = this.additionalFilter.bind(this)
  }

  /* called by list */
  additionalFilter(entries) {
    // use this.parameter
  }
}

Async

Functions:

function delayResult(x) {
  return new Promise(r => {
    setTimeout(() => {
      r(x)
    }, 1000)
  })
}

async function f() {
  let a = await delayResult(x)
  ...
}

Lambda:

const f = async () => {
  ...
}

Keywords

typeof,

Build Environment

.editorconfig

root = true

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