function count(x, y) {
?? ?return x + y;
}
count(3);//因為只傳遞了參數(shù)x,y的默認(rèn)值為undefined
//undefined + 3返回NaN

function count(x, y) {
?? ?x = x || 0;
?? ?y = y || 0;
?? ?return x + y;
}
count(3);//3

function count(x, y) {
?? ?x = x??0;
?? ?y = y??0;
?? ?return x + y;
}
count(3);//3

function count(x = 0, y = 0) {
?? ?return x + y;
}
count(3);

function count(x = 0, y = 0) {
?? ?let x;//報錯
?? ?const y;//報錯
}

let num = 1;
function count(x = num + 1, y = 0) {
?? ?return x;
}
count();//2

num = 99;
count();//100

function fn(x = 10, y = x) {
?? ?console.log(x, y);
}
fn(20);//20 20
fn();//10 10

function fn(x = y, y = 10) {
?? ?console.log(x, y);
}
fn();//報錯

let w = 10;
function fn(x = w) {
?? ?let w = 20;
?? ?return x;
}
fn();//10

//在()階段,x已經(jīng)賦值后,再改變w的值,也無法改變x的值
let w = 10;
function fn(x = 2) {
?? ?w = 20;
?? ?return x;
}
fn();//10

function count(...values) {
?? ?console.log(values);//[2, 5, 3]
?? ?return values.reduce((acc,curr) => acc + curr);
}
add(2, 5, 3);//10

function count(...value, a){}//報錯

function count(){}
console.log(count.name); //"count"

(new Function).name // "anonymous"

function foo() {};
foo.bind({}).name // "bound foo"

function foo() {};
foo.bind({}).name // "bound foo"

(function(){}).bind({}).name // "bound " // "bound "

(function(){}).name // ""

------------正常函數(shù)-------------
function count(x, y) {
?? ?return x + y;
}
------------箭頭函數(shù)-------------
let count =(x, y) => x + y;

let count = (x, y) => {
?? ?y = 100;
?? ?x = x * y;
?? ?return x + y;
}
count(3, 4);//400

let count = (x, y) => (x = 100, y = 10, x + y);
count(3, 4);//110

//報錯 會將{}識別為函數(shù)體
let count = id => {id: id, name: "yunjin"};

//不會報錯
let count = id => ({id: id, name: "yunjin"});