Javascript

Reference

On this wiki

• Node.js
• Npm
• Typescript

Language

• JavaScript and HTML DOM reference on w3schools.com
• Web API on mozilla.org
• Mozilla tutorials
• Here are examples of everything new in ECMAScript 2016, 2017, and 2018
• Sparse and Dense arrays

Test

• Jest — Delightful JavaScript Testing

Typescript

• Typescript is a typed-superset of Javascript.

Syntax

JS type

// Strings
var carname = "Volvo XC60";

// Numbers
var x = 3.14;
var y = 34;

JS operators

// Arithmetic operators
x = y + 2;
x = y - 2;
x = y * 2;
x = y / 2;
x = y % 2;       // Modulus (division remainder)
x = ++y;         // Pre-increment
x = y++;         // Post-increment
x = --y;         // Pre-decrement
x = y--;         // Post-decrement

// Assignment operators
x = y;
x += y;
x -= y;
x *= y;
x /= y;
x %= y;

// String operators
text3 = text1 + text2;     // Concatenation
text1 += text2;

// Comparison operators
x == 8;    // equal to
x === 5;   // equal value and equal type
x != 8     // not equal
x !== 5    // not equal value or not equal type
x > 5;
x < 5;
x >= 5;
x <= 5;

// Ternary operator
voteable = (age < 18) ? "Too young" : "Old enough";

// Logical operators
x && y;     // logical and
x || y;     // logical or
!x;         // not

// Bitwise operators
x = 5 & 1;  // bitwise and
x = 5 | 1;  // bitwise or
x = ~5;     // bitwise not
x = 5 ^ 1;  // bitwise xor
x = 5 << 1; // bitwise left shift
x = 5 >> 1; // bitwise right shift

// typeof
typeof "John";            // returns string

// delete
var person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};
delete person.age;        // or delete person["age"] -- delete property from an object

// in
var cars = ["Saab", "Volvo", "BMW"];
"length" in cars          // true if property is in the specified object
"firstname" in person

// instanceof
cars instanceof Array;    // true if object is instance of specified class

// void
void(0);                  // evaluate expressions and returns undefined

JS statements

// switch
switch(fruits) {
    case "Banana":
        text = "Banana is good!";
        break;
    default:
        text = "I have never heard of that fruit...";
}

// while
while (i < 5) {
    // ...
}

JS functions

var myfct = function(arg1, arg2) {
     return arg1 + ", " + arg2;
}
myfct("Hello", "World");

jQuery

jQuery is a fast, small, and feature-rich JavaScript library for HTML manipulation...

Dollar $ symbol

In Javascript, jQuery library is often exported as the dollar symbol, $!

Document ready

A page can't be manipulated safely until the document is "ready." [1] jQuery detects this state of readiness for you. Code included inside $( document ).ready() will only run once the page Document Object Model (DOM) is ready for JavaScript code to execute.

// A $( document ).ready() block.
$( document ).ready(function() {
    console.log( "ready!" );
});

Developer frequently use the shorthand version $( ... ), although it is more confusing: [2]

// Shorthand for $( document ).ready()
$(function() {
    console.log( "ready!" );
});

How-to

Wait until a function is loaded

We fire a setTimeout event until the function is available [3]:

$(function(){
    $("#loadingGIF").show();
    WaitForFunction();
});

function WaitForFunction()
{
    // Here we should use (typeof FUNCTION_TO_WAIT_ON_HERE === "function")
    // See https://stackoverflow.com/questions/12206083/whats-the-difference-between-typeof-variable-function-and-jquery-isfunc
    if (!$.isFunction(FUNCTION_TO_WAIT_ON_HERE)) {
       setTimeout( WaitForFunction, 100);
       return;
    }
    Function_Loaded();
}

function Function_Loaded(){
      $("#loadingGIF").hide();
}

Note that the code is not recursive because setTimeout does not call the given function but add it to a timer event queue.

Simulate click on a button

Use jQuery [4]:

$('#mybutton').trigger('click');
$('#mybutton').click();          // Simpler

View object attributes

for (var attr in obj) { console.log ("Attribute " + attr + " is: " + obj[attr]); }

Add property to an undefined object

If we want to add a property to an object that is not yet defined, just assign the value {} to it.

var myobj = {};
myobj.someProperty = "value";

// or...

window.myobj = {};
window.myobj.someProperty = "value";

How to sleep / wait for a delay

Just don't, because it would block the execution thread. Instead, fire a time event with setTimeout [5]:

setTimeout(function(){alert("hi")}, 1000);

Build

Grunt: the Javascript Task Runner

• https://gruntjs.com/getting-started

grunt          // Run default task (usually build), equiv. to "make"

Debug

Print message to console

Use console

console.error(message); //gives you the red errormessage
console.log(message); //gives the default message
console.warn(message); //gives the warn message with the exclamation mark in front of it
console.info(message); //gives an info message with an 'i' in front of the message

Firefox

• In Firefox, Ctrl-Shift-C to open up the console and debugger tools.\
• To browse an object, go to the console, and type the name of an object to open it up in the object explorer. For instance, to browse from the global context window:

>> windows

Chrome

• F12 to open up Developers tools (Ctrl-Shift-C also works but show another tab?).
• Keyboard: while in developer tools, Ctrl-P then > to have a drop down list and fuzzy search (eg. des to select Show Overrides).

Tips

To grok a mockingbird

Or how to make a function recursive, but keep functional composition [6] (also about memoization).