型別與運算子

Dynamic Type (弱型別)

You don't tell the engine what type of data a variable holds, it figures it out while your code is running
- Variables can hold different types of values because it's all figured out during execution

//Static Typing
bool isNew = 'hello'; // an error

//Dynamic Typing
Var isNew = true; // no errors
isNew = 'yup';
isNew = 1

Primitive Types (純值、基本型別)

A type of data that represents a single value. That is, not a object
JavaScript 有六種純值(Primitive Type)的型別。

• Undefined

  • Undefined represents lack of existence (you shouldn't set a variable to this)，也是 Javascript 給所有變數的初始值。

• Null

  • Null represents lack of existence (you can set a variable to this)

• Boolean

  • true or false

• Number

  • Floating point number (there's always some decimals). Unlike other programming languags, there's only one 'number' type… and it can make math weird.

• String

  • A sequence of characters (both '' and "" can be used)

• Symbol

  • Used in ES6 (the nex version of Javascript) We won't talk about this here…

Operator (運算子)

A special function that is syntactically (written) differently
Generally, operators take two parameters and return one result.
運算子都是函數

Operator precedence and associativity (運算子的優先性與相依性)

Operator precedence

Which operator function gets called first
Functions are called in order of precedence (HIGHER precedence wins)

Associativity

What order operator functions get called in: left-to-right or right-to-left
When functions have the same precedence

MDN - 運算子優先序

Coercion (強制型轉)

Converting a value from one type to another
This happens quite often in Javascript because it's dynamically typed.

var a = 1 + '2'
console.log(a) // 12 (String)

Comparision operators

應該永遠使用 === 來比較是否相同，除非特殊情況

MDN - 相等性比較

Existence and booleans

Truthy
Falsy

var a;

// do something..

if(a){
    console.log('a is truthy')
}else{
    console.log('a is falsy')
}

Default values

function greet(name){
    name = name || '<Your name here>'
    console.log('Hello' + name)
}
greet() // Hello <Your name here>

tags: WierdJavascript