js 预编译环节的变量提升
有些东西需要先告诉你怎么用,再去理解定义
关于变量的提升
function test () {
console.log(a)
var a = 100
console.log(a)
};
test () // undefined // 100
执行步骤
function test () {
var a = undefined
console.log(a)
a = 100
console.log(a)
}
关于函数的提升
function test () {
console.log(a)
function a () {}
console.log(a)
function b () {}
console.log(b)
};
test() // function a () {} // function a () {} // function b () {}
执行步骤
function test () {
var a = undefined
var b = undefined
a = function a () {}
b = function b () {}
console.log(a)
console.log(a)
console.log(b)
}
关于变量和函数的混合提升
function test () {
console.log(a)
var a = 100
console.log(a)
function a () {}
console.log(a)
function b () {}
console.log(b)
};
test() // function a () {} // 100 // 100 // function b () {}
执行步骤
function test () {
var a = undefined
var b = undefined
a = function a () {}
b = function b () {}
console.log(a)
a = 100
console.log(a)
console.log(a)
console.log(b)
}
关于形参和变量混合提升
function test (a) { // 定义函数中用来接收参数 这个就是形参
console.log(a)
var a = 100
console.log(a)
};
// 调用函数的时候传具体的参数 这个参数就是实参
test(1) // 1 // 100
执行步骤
function test (a) {
var a = undefined
a = a // 右边的a是形参
console.log(a)
a = 100
console.log(a)
}
关于形参和函数的混合提升
function test (a) {
console.log(a)
function b () {}
function a () {}
console.log(a)
console.log(b)
};
test(1) // functioin a () {} // function a () {} // function b () {}
执行步骤
function test (a) {
var a = undefined
var b = undefined
a = a // 右边的a是形参
b = function b () {}
a = function a () {}
console.log(a)
console.log(a)
console.log(b)
}
关于形参和变量和函数的混合提升
function test (a) {
console.log(a);
var a = 100;
console.log(a)
function a () {}
console.log(a)
function b () {}
console.log(b)
};
test(1) // function a () {} // 100 // 100 // function b () {}
执行步骤
function test (a) {
var a = undefined
var b = undefined
a = a // 右边的a是形参
a = function a () {}
b = function b () {}
console.log(a)
a = 100
console.log(a)
console.log(a)
console.log(b)
}
if 判断语句不影响变量提升 (if是执行语句,而变量提升属于预编译环节;js先编译后执行)
function test () {
console.log(a)
if (false) {
var a = 100
console.log(a)
}
};
test() // undefined
执行步骤
function test () {
var a = undefined
console.log(a)
if (false) {
a = 100
console.log(a)
}
}
总结:
函数内部js的预编译环节执行顺序如下
1. 将形参,变量提升到该函数的顶部(执行期上下文顶部)
2. 将形参,函数赋值到对应的变量 (先形参,后函数)