JavaScript

JavaScript = ECMAScript + DOM (window.document) + BOM (window.document, window.navigator, window.location, window.history, window.screen, etc)

Good Tutorials

https://javascript.info/

Type

JavaScript Types = Primitive types + Reference Type

Primitive type

null, undefined, boolean, number, string, symbol (ES6)
immutable, fixed size memory

Reference Type

Object, array, function
dynamic size

typeof returns string.

Array

var a = Array(4); // array length of 4, but elements are all undefined
var a = Array('4'); // returns [4]
var a = Array()
var a = Array('a', 'b');
var a = ['a', 'b']
let arr = Array.from($('a')); // create from nodelist

can change length, like C# list: a.push('c')
can hold different data type
the length of the array is one more than the highest index. it's not always counting how many members in the array!
```
var arr = ['a', 'b', 'c'];
arr.length // 3
arr[100] = '100'
arr.length == 101 // !!!`
```
If you query a non-existent array index, you get undefined

Callback is usually: element, index, array while element is required

Value Most of the operations will mutate the array.

copy itself to itself, [start, end) not including the endIndex: copyWithin(targetIndex, startIndex, endIndex)
Reverse: Array.prototype.reverse()
Sort: arr.sort()
- if no function is provided, element is converted to string to sort, so [10, 5].sort() is still [10, 5]
- sort numbers: arr.sort((a, b) => a - b);
head: shift() + unshift()
tail: pop() + push()
Fill with value, [start, end) arr.fill
*Remove and insert in the middle (when deleteCount is 0): arr.splice(start, deleteCount[, insert args]), it will return the removed
clear content arr.length = 0; or arr.splice(0, arr.length);

immutable operation:

Concat
- return new array: arr.concat(arr2)
  - has flatten effect: [1].concat(2, [3,4]) -> [1,2,3,4]
- We could make it mutating too: in place (append b to a): Array.prototype.push.apply(a,b)
Slice, return shallow copy [start, end): arr.slice

Iterate

for: for(let i = 0, l = list.length; i < l; i++) {console.log(list[i]); }. Use break to break out.
for in values(): for (let elem in arr.values())
arr.forEach: cannot break unless throw an exception

Transform

filter: array.filter or _.filter or _.select
join to string: arr.join
reduce and reduceRight(from end): arr.reduce(callback, initial) # call back is (accumulator, currentValue, index, array)=> {})
- if no initial value given, first call, previousValue is arr[0] and currentValue is arr[1]
  [1,2,3].reduce((acc, cur)=> acc+cur); // 6
- the callback function returns value that'll pass to next accumulator. There is no need to assign acc = acc + cur in the callback function.

Query/Test

Check if it's array: Array.isArray()
- polyfill: return Object.prototype.toString.call(arg) === '[object Array]';
Test all: arr.every((element) => true) or _.all or _.every
Test some: arr.some or _.some or _.any
Find first, else return undefined: array.find or array.findIndex
Includes:
- (ES7): arr.includes or _.contains
- indexOf or lastIndexOf

Tips:

create array:
- ES6: Array(length).fill().map((_,i)=>i+1)
- ES6: Array.from({length: 10}, (v, i) => i);
- Lodash: _.fill(Array(10), 1);

Number

Special Numbers:
- Infinity and -Infinity
- NaN
  - any operation with NaN will rerturn Nan
  - NaN doesn't equal to anything, including itself
Hex and Octal:
- If number starts with 0 and is a valid octal number, it'll be a octal number: var octalNum = 070; // 56
- hex number: var hexNum = 0xA2 or 0xf1
Convert anything to number:
- Number()
  - Number(null) === 0
  - Number(undefined) === NaN
  - Number('') === 0
  - Number('a') === NaN
  - Number('0x1') === 1
- parseInt
  - parseInt parses until found invalid character, + simply convert the string to int/float and it returns NaN if there is ANY invalid character.
    parseInt(' 1') === 1
    parseInt('') === NaN returns NaN if cannot convert
    parseInt(' 123abc4') === 123
  - always use radix: parseInt('09', 10) Otherwise '09' will be treated as hex and result as 0
- paserFloat
  - It always convert to oct. If starts with 0x, it returns 0.
Get digits: String(321)split('')
Check if it's integer: Number.isInteger polyfill on MDN

Tips: Random number

Math.random() -> [0, 1)

function getRandomArbitrary(min, max) {
  return Math.random() * (max - min) + min;
}

function getRandomInt(min, max) {
  min = Math.ceil(min);
  max = Math.floor(max);
  return Math.floor(Math.random() * (max - min)) + min; //The maximum is exclusive and the minimum is inclusive
}

Tips: format number

link: https://codesandbox.io/s/js-tips-format-number-frd6v

function formatNumber(num, decimals) {
  if (decimals < 0) {
    throw new Error('Decimals cannot be smaller than 0');
  }

  const factor = Math.pow(10, decimals);
  return Math.round(num * factor) / factor;
}

String

string is value tpye.

var a = 'a';
  a.x = 'x';
  a.x // will be undefined
  var b = new String('b');
  b.x = 'x';
  b.x // will be 'x'

It's unicode of 16bit
u1235 is unicode nnnn
charAt(index), indexOf
match(regex) returns matches
- NOTE if regex has /g: 1. it'll return first match; 2. you can match multiple times. See http://www.2ality.com/2013/08/regexp-g.html
replace(regex/g, 'to replace') specials '$1', etc.
substr(index, length) vs substring(index, endIndex) not including endIndex`
- if negative number:
  - slice(index) index will become index+length: 'abc'.slice(-1) -> 'abc'.slice(2) -> 'c'
  - substr(index, length) index becomes index+length; length will become 0
  - substring will treat negative as 0
Can mix different type using '+' since JavaScript is weak typed. The non-string type is auto converted to string.
So, if you add a string to a number (or other value) everything is converted in to a string first
```
this year is " + 2013 => "this year is 2013"
  '3'+4+5 => '345'
  4+5+'3' => '93'
```
To convert to string, use either toString or String()
- String(null) === 'null'
- String(undefined) === 'undefined'
In place sort string with numbers: '8902'.split('').sort().join('')
Use String.prototype.localeCompare() for string comparison in sort. Return negative number if it's before. You can also use 'A' > 'a'

Special reference type: String, Boolean, Number

it's created on the fly then get destroyed.

var a = 'abc';
a.subString(0); // a is value type, why it has methods?
// It's doing something like this:
var tmp = new String('abc');
var s = tmp.subString(0);
tmp = null;

Date

Only using new will return the object; Others will return the number or string.

new Date()
new Date(2005, 0, 3) local time; year and month is required, month is 0 based
Date.UTC(2005, 0, 3) UTC time; Return number; year and month is required, month is 0 based

Use moment.js

const now = moment();
const future = moment('2030-03-30');
const diffInDays = now.diff(future, 'days');

Boolean

Sometimes you'll need to check boolean, it's better to force converting to boolean by:

// if you don't do !! you'll get undefined because you're accessing something doesn't exist
const hasMissingChannels = !!this.props.channels.missingChannels && this.props.channels.missingChannels.length > 0;

null undefined

Special objects: null and undefined

null: a pointer to nothing (that's why typeof null === 'object'. Most case it can be replaced by undefined;
undefined: it's like null in other program.
- If you access a.name and a is {} then it returns undefined
- If arr[out_of_bound_index], it returns undefined

boolean: any value can be converted to boolean

// as false
false
null
undefined
'' //the empty string, for example var thisIsFlase = Boolean('')
0 // the number
NaN // the number

// Anything except the above is true, for example:
'0' // the string
[] // the empty array
{} // the empty object

Check is null or empty:

function isNullOrEmpty(str) {
    return !str;
}
// if you can use lodash/underscore
function isNullOrEmpty(str) {
    return _.isEmpty(str); // it's actually checking str.length
}

// Be careful with _.isEmpty as it's checking length, so don't use it to check number
_.isEmpty(10) === true;


function isNullOrEmpty(arr) {
    return !arr || arr.length === 0;
}

_.isEmpty():

check the length of array or string
CANNOT check number: _.isEmpty(10) === true
check if object has enumerable own-properties

Check null:

use typeof instance.currentPosition !== 'undefined'
Why not using instance.currentPosition === undefined? it can throw error
CoffeeScript:
- coffeescript: ? or ?. (the latter can soak up so a.address?.zip returns undefined instead of typeerror)
- CoffeeScript's existential operator ? returns true unless a variable is null or undefined, which makes it analogous to Ruby's nil?

Comparison

Equality operator ==: doesn't compare type, i.e, it converts the type first (performs type coercion) then compare
"" == false // returns true
Strict Equality Operator ===: doesn't not perform type coercion recommended
'' == false // return false
when comparison includes reference type, the comparison (both == and ===) performs pointer comparison.
{} != {}

Global Objects / Methods

Set: unique values of any type, has
Map: dictionary, any type can be key, size, get/set

Methods:

isNaN, etc
let id = setTimeout(()=>console.log('future'), 1000), clearTimeout(id)
let id = setInterval(()=>console.log('again'), 1000), clearInterval(id) Use setTimout to emulate interval, as timeout is pushed to the queue, where interval might run in overlap.
window.print display print window
location: window.location is the same as document.location
encodeURI
- replaces all characters except: ; , / ? : @ & = + $ - _ . ! ~ * ' ( ) # a-z 0-9

encodeURIComponent

replaces all characters except- _ . ! ~ * ' ( ) a-z 0-9

Use it to encode the value part: because it'll encode = and & so it's not supposed to encode the whole param string encodeURIComponent("var1=value1&var2=value2").

var arr = [];
for(var i=0;i<256;i++) {
var char=String.fromCharCode(i);
if(encodeURI(char)!==encodeURIComponent(char)) {
  arr.push({
    character:char,
    encodeURI:encodeURI(char),
    encodeURIComponent:encodeURIComponent(char)
  });
}
}
console.table(arr);

Object

the property name will always be string, even you created like this {a:'b'}
- Use dot notation when accessing properties
- Use subscript notation [] when accessing properties with a variable.
  const isJedi = luke.jedi; // dynamic funciton getProp(obj, prop) { return obj[prop]; } const isJedi = getProp(luke, prop);
for (let prop in obj) will iterate all enumerable props (including the prototype ones) in arbitrar order
- All properties that we create by simply assigning to them are enumerable.
- The standard properties in Object.prototype are all nonenumerable. Ex, toString is in prototype but it's not enumerable 'toString' in {} // -> true
- use hasOwnProperty(key) to check if it's in instance
  for (let name in map) { if (map.hasOwnProperty(name)) { // ... this is an own property } }
for (let value of arr) (ES6). It's using iterator (next: ()=> {value: x, done: false})
The in operator check if the specified property is in the specified object.
- use hasOwnProperty(key) to check if it's in instance
```js
let obj = {
a: 'a'
};
console.log('a' in obj); // true
obj.a = undefined; console.log('a' in obj); // true
delete obj.a; console.log('a' in obj); // false
```
typeof, check if it's a basic type plus others, it only returns those string: 'undefined', 'null', 'boolean', 'string', 'number', 'object', 'symbol', 'function'
- so typeof [] === 'object
- BUT typeof null === 'object'; See MDN explanation
- usage: check method defined in prototype: if (typeof this.sayName !== 'function')
a instanceof Constructor, check if it's a reference type
- Use constructor, not string: so [] instanceof Object, not 'object'

(TODO) Check if an object is a type

Why not instanceof?

Why not typeof? Because typeof returns only those 5 types, and array is Object type. For example, typeof [] === 'object', you cannot tell if it's array. See here

function is(type, obj) {
  var clas = Object.prototype.toString.call(obj).slice(8, -1);
  // [object String]
  return obj !== undefined && obj !== null && clas === type;
}
is('String', 'test'); // true
is('String', new String('test')); // true

object detection, check if it has property or method

if (document.getElementsByName)

      // or
      if (typeof document.getElementsByName === 'function')
      if (typeof document.getElementsByName !== 'undefined')

JSON.stringify(obj, ['fliter', 'list]) can filter

Custom getter setter:

var pile = {
  elements: ["eggshell", "orange peel", "worm"],
  get height() {
    return this.elements.length;
  },
  set height(value) {
    console.log("Ignoring attempt to set height to", value);
  }
};

// or
Object.defineProperty(TextCell.prototype, "heightProp", {
  get: function() { return this.text.length; }
});

FP operations

Freeze: const frozen = Object.freeze(obj)

Create shallow copy:

var o = {
  x: 1,
  y: 2
};

// in ES2017+, using object spread:
var p = { ...o };
p.y = 3;

// in ES2015+:
var p = Object.assign( {}, o );
p.y = 3;

Prototype

Constructor

A constructor is just a function called with new.

function Paper(type) {
  this.type = type;
}

let note = new Paper('notebook');

What happens?

create new object
this bound to new object
call the constructor function
return the object

// let note = new Paper('notebook');

// ->
let note = {};
Paper.call(note, 'notebook');
return note;

Prototype Chain

every instance created with the same constructor will share the same prototype: object derived from Object.prototype.
How to create prototype-less object? (so that for .. in works without check)
```
let map = Object.create(null);
console.log("toString" in map); // → false
```

Example

function Rabiit() {
}
Rabbit.prototype.teeth = "small";
console.log(killerRabbit.teeth); // → small
killerRabbit.teeth = "long, sharp, and bloody";
console.log(killerRabbit.teeth); // → long, sharp, and bloody

Prototype chain:

// o ---> Object.prototype ---> null
// The newly created object o has Object.prototype as its [[Prototype]]
// o has no own property named 'hasOwnProperty'
// hasOwnProperty is an own property of Object.prototype. 
// So o inherits hasOwnProperty from Object.prototype
// Object.prototype has null as its prototype.
var o = {a: 1};

// b ---> Array.prototype ---> Object.prototype ---> null
// Arrays inherit from Array.prototype 
// (which has methods indexOf, forEach, etc.)
// The prototype chain looks like:
var b = ['yo', 'whadup', '?'];

// f ---> Function.prototype ---> Object.prototype ---> null
// Functions inherit from Function.prototype 
// (which has methods call, bind, etc.)
function f() {
  return 2;
}

var b = Object.create(o);
// b ---> o ---> Object.prototype ---> null
console.log(b.a); // 1 (inherited)

Inheritence

call parent constructor
set prototype to parent prototype

function RTextCell(text) {
  TextCell.call(this, text);
}
RTextCell.prototype = Object.create(TextCell.prototype);
RTextCell.prototype.constructor = RTextCell;

ES6, it's just syntax sugar, remains prototype based.

class Polygon {
  constructor(height, width) {
    this.height = height;
    this.width = width;
  }
}

class Square extends Polygon {
  constructor(sideLength) {
    super(sideLength, sideLength);
  }

Function

arguments

Always has arguments (note it's not this.arguments).
- arguments is a build-in object, it's array like.
- To convert it to array:
  - Array.from(arguments)
  - const a = [...arguments];
  - Array.prototype.slice.call(arguments)
Because of 'arguments', there is no function overloading.
function.length required params length
Function.prototype.bind(thisArgs, arg1, arg2...) creates a new function that binds this and currying parameters.
- so 'this' is always 'thisArgs'
- use when in setTimeout callback to refer this, that's also why => in ES6 don't need to bind anymore
Funnction.prototype.apply(scope, paramsArray) and Function.prototype.call(scope, param1, param2, etc)
- apply uses array
- call uses comma
- Math.max.apply(null, [1,10,-1])
Assignment makes a copy of the value only if it's a primitive type (like Number, Boolean, String, etc...). Otherwise, assignment just copies a reference to the same object (Object, Array, etc...). A new object is not created with assignment.
```
var a = {};
  var b = {name:'b'};
  var a = b;
  console.log(a.name == 'b')
```

Arguments are always copied by value. Even if the type is reference.

function setName(obj) {
      obj.name = 'Andrew';
      obj = new Object(); // local copy of obj now points to new object
      obj.name = 'Bella';
  }

  let obj = {};
  setName(obj);
  console.log(obj.name); // 'Andrew'

Return value

Function output: funciton always return a value. If no return statement or return;, it returns undefined.

Declaration vs Expression

Declaration (will be hoisted)

  alert(sum(10,10));
  function sum(num1, num2) {
      return num1 + num2;
  }

  // error: alert(sum(10,10));
  let sum = function(num1, num2) {
      return num1 + num2;
  };
  alert(sum(10,10));

Expression:

//anonymous function expression
var a = function() {
    return 3;
}

//named function expression
var a = function bar() {
    return 3;
}

//self invoking function expression
(function sayHello() {
    alert("hello!");
})();

IIFE: immediately-invoked function expression

Why? It's useful when you have some work to do, some initialization maybe. You need to do it only once and you don't want to leave any globals lying around after the work is finished.

How? It's used to avoid hoisting and creating scope. A function creates a scope.

  (function(){
  var a = 1;
  var b = 2;
  alert(a + b);
  })();

  // or use a named version
  (function foo() {
    alert('in foo');
  })();

Function Name

Why you can pass function name to map function?

let getJSON = (url)=>{}
[].map(getJSON); // see getJSON

Actually you’re NOT omitting function parenthesis, what you mean here is passing the function reference so that the map knows which function to call. See http://stackoverflow.com/questions/5520155/settimeout-callback-argument/5520190#5520190

Closure

Closure: when the inner function makes reference to a variable to the outer/surrounding function, this is called closure.

More formal definition: A closure = a function + lexical environment within which that function was declared.

Usage: promise chain, currying function, this-that pattern，'private' data in module or function (because function creates scope)

function runningCounter(start) {
  var val = start;

  return function current(increment = 1){
      val = val + increment;
      return val;
  };
}

var score = runningCounter( 0 );

score(); // 1
score(); // 2

// 'private' data
let application = (function() {
  let _components = [];

  return {
    getComponentCount: function() {
      return _components.length;
    }
  };
})();

Common error when creating closure in loop:

// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Closures
function showHelp(help) {
  document.getElementById('help').innerHTML = help;
}

function setupHelp() {
  var helpText = [
      {'id': 'email', 'help': 'Your e-mail address'},
      {'id': 'name', 'help': 'Your full name'},
      {'id': 'age', 'help': 'Your age (you must be over 16)'}
    ];

  for (var i = 0; i < helpText.length; i++) {
    var item = helpText[i];
    document.getElementById(item.id).onfocus = function() {
      showHelp(item.help);
    }
  }
}
setupHelp();

// fix: use let to create local scope

Scope

Who can create scope?

function scope: created by funciton.
- var is limited in the scope.
- the scope implicitly defines a reference to this
ES6: block scope
- let is local to the block scope, not the function scope.
- the scope does not implicitly defines a reference to this
ES6: lambda scope
- the scope does not implicitly defines a reference to this. So this refers to enclosing scope.

Declared variable is a property of the scope. Undeclared variable too. So, both var a = 10; b = 11; is accessible in global window.

var a = 10;
console.log('a' in window); // true
console.log(window.a); // 10

b = 11;
console.log('b' in window); // true
console.log(window.b); // 10

for loop:

var creates one binding as well as a variable (and the declaration part will be hoisted), so when i is accessed in a callback function, it always refered to the final i value.
let creates binding each iteration so it works normally

let arr = [];
for (var i=0; i < 3; i++) {
    arr.push(() => i);
}
console.log(arr.map(x => x())); // [3,3,3]
console.log(i); // 3

How does js lookup variable?

Similar to stack, look for local then global. Local variables 'shallow' the global ones.
If there is a local same name identifier, the search will stop.

Examples:

x = "global";

// function scope:
(function() {
    console.log(x); // undefined (x get hoisted, search will stop)

    var x = 'local';
}());

// block scope
{
    console.log(x); // ReferenceError: x is not defined (x get hoisted but it's not initialized)
    let x = 'local';
}

Hoisted

What will be hoisted?

var

only the declaration part
and it'll be initialized as undefined

console.log(a); // undefined
var a = 12;

// will become this:
var a;
console.log(a);
a = 12;

function declaration: function foo() {}

hoisted(); // Output: "This function has been hoisted."

function hoisted() {
  console.log('This function has been hoisted.');
};

// expression is NOT hoisted
expression(); //Output: "TypeError: expression is not a function

var expression = function() {
  console.log('Will this work?');
};

let const
- only the delcaration part
- it will be uninitialized, trying to use it will cause 'Reference error: y is not defined'.
- This is temporal dead zone (anything above the actual statement). See: https://stackoverflow.com/a/31222689

this

Rules:

Normally, this is bind to 'Call-site', the invoking object.
In arrow function, this is bind to the context 'Where is defined'. If no parent defines scope, then the context is window
You can use bind to change this manually.
in event handler, this is bind to invoking target. (NOTE if you define event handler using arrow function then it's bind to window)

Examples:

Normally, 'this' is the invoking object. If no invoking object, is the global object. (window or global in Node).

//
// Test
//
function speak(line) {
  console.log(this.type);
}
let a = {type: "white", speak: speak};
a.speak(); // white

//
// test
//
// this binds to window, because it's defined when executing
Vector.prototype.plus = (other)=> { 
  return new Vector(this.x + other.x, this.y + other.y);
}

//
// test
//
var global = function(){ console.log(this); }();
// 立即执行函数，由于没有给他object，于是this就是global (window)

//
// test
//
function Foo() {
}
Foo.method = function() {
  function point(x, y) {
    console.log(this);
    this.x = x;
    this.x = y;
  }
  point(3,5); 
}
// this is window.
// Why? because Foo is defined in window.
Foo.method();

When using bind, this is set to a fixed value when it's defined. Or use apply or call to dynamically change context.
Arrow function doesn't bind this, arguments.

Arrow function this is determined by where is defined. And it refers to the enclosing execution context. You can think it's using the this-that pattern. (Can use babel to verify) 箭头函数从封闭它的（函数或全局）作用域采用 this 绑定.

// test
// Why? Code executed by setTimeout() is called from an execution context separate from the function from which setTimeout was called.

function Person() {
  this.age = 0;

  setInterval(function growUp() {
    // In non-strict mode, the growUp() function defines `this` 
    // as the global object, which is different from the `this`
    // defined by the Person() constructor.
    console.log(this.age);
    this.age++;
  }, 1000);
}
var p = new Person();

// fix
function Person() {
  this.age = 0;

  setInterval(()=> {
    console.log(this.age);
    this.age++;
  }, 1000);
}
var p = new Person();

// ex
foo.prototype.say = () => {  
  console.log(this === window); // => true
  // ()=> is defiend in is in window.
};

// ex
var button = document.getElementById('myButton');  
button.addEventListener('click', () => {  
  console.log(this === window); // => true
});

Arrow function is not suitable to define methods:

So arrow function cannot be used as constructor, because there is no this in it.

var obj = {
  i: 10,
  b: () => console.log(this.i, this),
  c: function() {
    console.log(this.i, this);
  }
}
obj.b(); // prints undefined, Window {...} (or the global object)
// Why? it's defiend in obj =, the enclosing context is window.
obj.c(); // prints 10, Object {...}

// fix: ES6 shorthand method
var obj = {
  b() {
    console.log(this.i, this)
  }
};

Default binding. It means binding loss: it happens whenever you’re accessing a method through a reference instead of directly through its owner object.

Why? Because the invoking site is window.
So it also means, when you pass function as callback (which implicitly do the assignment in local scope, and when invoking, none is invoking it.), it's window.
How to fix? this-that pattern or arrow function.

// test
let john = {
  name: 'John',
  // test 1
  greet: function(person) {
    console.log("Hi " + person + ", my name is " + this.name);
  },
  items: [1, 2, 3],
  // test 2
  processItems: function() {
    this.items.forEach(function(item) {
      this.markItemAsProcessed(item);
    });
  },
  processed: [],
  markItemAsProcessed: function(item) {
    this.processed.push(item);
  }
};

// test 1
john.greet();
let fx = john.greet;
fx("Mark"); // this.name will be referencing global object because fx is property of global, global is the invoking object.

// test 2
john.processItems(); // this.markItemAsProcessed is not a function

// fix: that pattern (using closure)
  processItems: function() {
    let that = this;
    this.items.forEach(function(item) {
      that.markItemAsProcessed(item);
    });
  },
// fix: arrow function
  processItems: function() {
    this.items.forEach((item)=> {
      this.markItemAsProcessed(item);
    });
  },

// other examples

// ex
function foo() {
  setTimeout(() => {
    console.log('id:', this.id);
  }, 100);
}
var id = 21;
foo(); // id: undefined ()=> is defined in foo(), which is defined in window
foo.call({id: 42}); // id: 42

// ex
function Timer() {
  this.s1 = 0;
  this.s2 = 0;
  setInterval(() => this.s1++, 1000);
  setInterval(function () {
    this.s2++;
  }, 1000);
}
var timer = new Timer();
setTimeout(() => console.log('s1: ', timer.s1), 3100); // 3 (new Timer() defines this for arrow function, and it's the function scope)
setTimeout(() => console.log('s2: ', timer.s2), 3100); // 0 (this is the invoking scope)

Reference:

Error Exception

catch: there is no selective catch. catch(e)

You can create your own error and use instanceOf Constructor

catch (e) {
  if (e instanceof InputError)
    console.log("Not a valid direction. Try again.");
  else
    throw e;
}

Assertion

function AssertionFailed(message) {
  this.message = message;
}
AssertionFailed.prototype = Object.create(Error.prototype);

function assert(test, message) {
  if (!test)
    throw new AssertionFailed(message);
}

assert(1 === 2, 'msg');

i18n

navigator.languages returns the user's preferred languages //["en-US", "zh-CN", "ja-JP"]
navigator.language is the first element of the above returned array

How to test?

Use chrome://settings/languages#lang and (important) make sure that the language you selected is the top choice (the preferred language). The navigator value will change accordingly

Pattern

default value if it doesn't exist

Use ES6 default parameter instead. Previously: Use || NOTE this pattern doesn't work if the value is ''.

for timestamp, count of json["#{projectName}"]
      existing = total["#{timestamp}"] || 0
      total["#{timestamp}"] = existing + count

Console & Debug

debugger;

If the result is 0, the elements are not in the DOM: console.log($(".theElements").length);

_ Debug JQuery: http://fixingthesejquery.com/#slide23

How to open a new window without being blocked?

If the window is not a user click resullt, it'll be blocked.

var win = window.open('', 'Window Name'); // given a name so it'll always refresh this window instead of opening new every time.
win.document.body.innerHTML = '<p>loading...</p>';

// do async work
async.do().then(()=> {
  // Replace the content
  win.document.body.innerHTML = images;
  win.focus();
});

Reference:

How to read binary file from the browser?

https://github.com/jDataView/jDataView

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