变量提升与匿名函数

 a存在变量提升，所以a等于undefined

var a = 1;
let b = 4;
(function() {
    console.log(a);//undefined
    var a = 3;
    let b = 4;
    console.log(a, b);//3,4
})()

var a = 1;
let b = 4;

function try1() {
    var a = 3;
    console.log(a);//undefined
    let b = 4;
    console.log(a, b);//3,4
};
try1()

 作用域内未定义a

var a = 1;
let b = 4;
(function() {
    console.log(a); //1
    let b = 4;
    console.log(a, b); //3,4
})()

var a = 1;
let b = 4;

function try1() {
    console.log(a); //1
    let b = 4;
    console.log(a, b); //3,4
};
try1()

 b不存在变量提升,所以报错

var a = 1;
let b = 4;
(function() {
    console.log(a, b);
    var a = 3;
    let b = 4;
    console.log(a, b);
})()
//报错

var a = 1;
let b = 4;
(function() {
    var a = 3;
    console.log(a, b);
    let b = 4;
    console.log(a, b);
})()
//报错

特殊情况

var a;
let b = 4;
function try1() {
    console.log(a); //undefined
    let b = 4;
    console.log(a, b); //undefined,4
};
try1()
a = 1

匿名函数可以拆成一个函数定义和一个函数执行，函数定义会提升，函数执行保持原来位置

例如：

var name = '小吴同学';
(function() {
    if (typeof name === "undefined") {
        name = '小邵先生';
        console.log('第一次' + name);
    } else {
        console.log('第二次' + name);
    }
    console.log(name);
})()
// 第二次小吴同学

var name = '小吴同学';
(function() {
    if (typeof name === "undefined") {
        var name = '小邵先生';
        console.log('第一次' + name);
    } else {
        console.log('第二次' + name);
    }
    console.log(name);
})()
//第一次小邵先生