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[js] 第63天 举例子说明javascript的变量声明提升和函数声明提升 #281

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[js] 第63天 举例子说明javascript的变量声明提升和函数声明提升#281
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@haizhilin2013

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@haizhilin2013
haizhilin2013
opened on Jun 17, 2019
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第63天 举例子说明javascript的变量声明提升和函数声明提升

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xn213

xn213 commented on Jun 17, 2019

@xn213
xn213
on Jun 17, 2019 · edited by xn213
var getName = function(){
  console.log(4)
}

function getName() {
  console.log(5)
}

getName() // 4 函数声明优先级高于var声明,  故 4 覆盖了 5
AnsonZnl

AnsonZnl commented on Jun 18, 2019

@AnsonZnl
AnsonZnl
on Jun 18, 2019 · edited by AnsonZnl
Contributor

变量声明

console.log(a);// undefinde
var a= "hello world";
console.log(a);//  "hello world"

函数声明

f()
function f(){
    console.log('hello world');// "hello world"
}

变量声明只提升声明 不提升赋值操作，函数声明 函数体整体被提升。

getName();
var  getName = function(){
    console.log("小明");
}
function getName(){
    console.log("大明");
}
getName();

提升之后变成：

var getName;
function getName(){
    console.log("大明");
}
getName();//”大明“
getName= function(){
    console.log("小明");
}
getName();//"小明"
wenyejie

wenyejie commented on Jun 18, 2019

@wenyejie
wenyejie
on Jun 18, 2019
var getName = function(){
  console.log(4)
}

function getName() {
  console.log(5)
}

getName() // 4 函数声明优先级高于var声明,  故 4 覆盖了 5

不是4的优先级是高于5, 而是5的优先级高于4,
5先声明, 但是后来它被4覆盖而已

forever-z-133

forever-z-133 commented on Jun 18, 2019

@forever-z-133
forever-z-133
on Jun 18, 2019

先声明函数名，再声明 var 变量名，然后按顺序从上到下赋值。

var a = 1;
function a() { }
console.log(a);  // 1
yxkhaha

yxkhaha commented on Jun 18, 2019

@yxkhaha
yxkhaha
on Jun 18, 2019
console.log(a);//undefined 变量a提升
var a = 10;

fn();//666 函数能在声明之前调用说明函数声明提升了
function fn() {
   console.log('666')
}
haizhilin2013
added
jsJavaScript
on Jun 18, 2019
Vi-jay

Vi-jay commented on Jul 31, 2019

@Vi-jay
Vi-jay
on Jul 31, 2019
console.log(a);//a会被提升到作用域最前面声明 var a = undefined
var a = 1;
console.log(a);
var a = 1;

// 变量和函数都会被提升 函数优先
function a() {
  
}
smile-2008

smile-2008 commented on Dec 31, 2020

@smile-2008
smile-2008
on Dec 31, 2020
var getName = function(){
  console.log(4)
}

function getName() {
  console.log(5)
}

getName() // 4 函数声明优先级高于var声明,  故 4 覆盖了 5
xiaoqiangz

xiaoqiangz commented on Jun 16, 2022

@xiaoqiangz
xiaoqiangz
on Jun 16, 2022

// 函数表达式 -- 匿名函数
var sum = function() {
console.log('sum2')
}
// 函数声明 --函数名 变量提升
function sum() {
console.log('sum')
}
sum()

var name = 'xxx'
function variable() {
  if (name == undefined) {
    var name = 'qqq'
    console.log(`hello ${name}`)
  } else {
    console.log(`hello ${name}`)
  }
}
variable()
Frontendnightmare

Frontendnightmare commented on Jun 14, 2023

@Frontendnightmare
Frontendnightmare
on Jun 14, 2023

@AnsonZnl

变量声明

console.log(a);// undefinde
var a= "hello world";
console.log(a);//  "hello world"

函数声明

f()
function f(){
    console.log('hello world');// "hello world"
}

变量声明只提升声明 不提升赋值操作，函数声明 函数体整体被提升。

getName();
var  getName = function(){
    console.log("小明");
}
function getName(){
    console.log("大明");
}
getName();

提升之后变成：

var getName;
function getName(){
    console.log("大明");
}
getName();//”大明“
getName= function(){
    console.log("小明");
}
getName();//"小明"
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          [js] 第63天 举例子说明javascript的变量声明提升和函数声明提升 · Issue #281 · haizlin/fe-interview