js 变量及函数声明提升

a();  // a is not a function, a 应该是 undefined， a 的声明被提升

b(); // Uncaught ReferenceError: b is not defined

var a = function b(){console.log(1)}

a() // 1, a 是 [Function: b]

b() // Uncaught ReferenceError: b is not defined

同理

const C = class CC{}

new C(); // CC {}

new CC(); // Uncaught ReferenceError: CC is not defined