**Xiang Wang @ 2019-01-21 17:11:00**

# Functions
[官网参考](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions)
[语法](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function)

## 待补充
* [ ] Arrow functions
* [ ] Method definitions
* [ ] Rest parameters
* [ ] The arguments object
* [ ] getter
* [ ] setter

## default functions
```
function multiply(a, b=1) {
}
```
### 注意事项
* 但是如果你传入了`b=undefined`, 那么b仍然等于1
* 每次调用函数，默认变量都会重新生成，而不会继续使用这个变量(和python不同)
```
    function append(value, array=[]) {
        array.push(value);
        return array
    }

    append(1); //[1]
    append(2); //[2], 如果是python, 就会返回 [1, 2]

    function callSomething(thing = something()) {
     return thing;
    }

    let numberOfTimesCalled = 0;  // 可以利用something来记录函数调用次数
    function something() {
      numberOfTimesCalled += 1;
      return numberOfTimesCalled;
    }

    callSomething(); // 1
    callSomething(); // 2
```

* 后面的默认值，可以调用前面的参数
```
    function greet(name, greeting, message = greeting + ' ' + name) {
        return [name, greeting, message];
    }

    greet('David', 'Hi');  // ["David", "Hi", "Hi David"]
    greet('David', 'Hi', 'Happy Birthday!');  // ["David", "Hi", "Happy Birthday!"]
```

* 参数只会按照顺序赋值给变量，而不根据是否有default来
```
    function f(x = 1, y) { 
      return [x, y]; 
    }

    f(); // [1, undefined]
    f(2); // [2, undefined]
```

* [ ] [destructuring assignment](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Default_parameters#Destructured_parameter_with_default_value_assignment)