TIL

Lets learn something everyday.

Map

 var m = {};
 var a = {id: 1},
     b = {id: 2};
 m[x] = 'foo';
 m[x] = 'bar';
 
 m[x]?
 m[y]?

WeakMap

1. WeakMap take only object as keys.
2. They don't have size or clear method.
3. They don't have iterartor over keys, values or entries.

Array

Initialize an array of given length with default values:

Array(3).fill(null); // [null, null, null]

Destructuring

// Fail-soft destructuring with defaults
var [a = 1] = [];
a === 1;

Classes

Unlike function declarations, class declarations are not hoisted.

// A class only exists after execution reached its definition and it was evaluated.
// Accessing it beforehand leads to a ReferenceError

new Foo();

class Foo {}

Null

var a = null;

(!a && typeof a === "object"); // true

null is the only primitive value that is "falsy" but that also returns "object" from the typeof check.

Reference error

Unlike referencing undeclared variables, there is no ReferenceError thrown if you try to access an object property (even on the global window object) that doesn't exist.

A class body can only contain methods, but not data properties.

Prototypes having data properties is generally considered an anti-pattern, so this just enforces a best practice.

constructor, static methods, prototype methods

Rest vs Spread

spread elements 'expands' an array into its elements, and rest elements collects multiple elements and 'condenses' into a single element.

source (mdn)

Instance methods and properties

 class Bork {
    //Property initializer syntax
    instanceProperty = "bork";
    boundFunction = () => {
      return this.instanceProperty;
    }

    //Static class properties
    static staticProperty = "babelIsCool";
    static staticFunction = function() {
      return Bork.staticProperty;
    }
  }

Babel plugin

Function default values

function foo(opts) {
  opts = Object.assign({
    pow: ''
  }, opts);
}

Web performance

Faster Font Loading with Font Events

Reducers

Using combineReducers does "call all reducers", or at least all of the slice reducers it is wrapping.

Link

VIM

Convert tabs to spaces in a file

Select visually the area to apply the changes then,

:retab

Service workers

A service worker is run in a worker context: it therefore has no DOM access, and runs on a different thread to the main JavaScript that powers your app, so it is not blocking. It is designed to be fully async; as a consequence, APIs such as synchronous XHR and localStorage can't be used inside a service worker.

Javascript

Statements:

1. Declaration Statement var a = 5
2. Assignment Statement b = a
3. Expression statement a

All statements have completion values event if its undefined.