# 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](https://developer.mozilla.org/zh-TW/docs/Web/JavaScript/Reference/Operators/Operator_Precedence)

### 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](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Equality_comparisons_and_sameness)

### Existence and booleans
[Truthy](https://developer.mozilla.org/en-US/docs/Glossary/Truthy)
[Falsy](https://developer.mozilla.org/en-US/docs/Glossary/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`