A brief discussion on the improvement of variable and function declarations in JavaScript

Phenomena:

1. In JavaScript, the declaration of variables and functions will be promoted to the top for execution.

2. The promotion of functions is higher than the promotion of variables.

3. If an external variable with the same name is declared using var inside the function, the function will no longer search upwards.

4. Anonymous functions will not be promoted.

5. Functions in different <script> blocks do not affect each other. </script>

Example:

Function declaration is promoted higher than variable declaration

//同时声明变量a和函数a
var a;
function a() {} 
alert(typeof a); //显示的是"function"，初步证明function的优先级高于var。

//先声明函数后声明变量，证明上边的例子不是function覆盖了变量
function a() {}
var a; 
alert(typeof a); //显示的仍是"function",而不是"undefined"，即function的优先级高于var。

//声明了变量的同时赋值
function a() {}
var a = 1;
alert(typeof a); //number,此时不是function了。
//说明："var a=1"相当于"var a;a=1",即先声明，后赋值，"a=1"相当于把a重新赋值了，自然就是number!

The function uses var to define the same value as the external one Variables, functions will no longer look up external variables

var value = 'hello';
function show() {
  alert(value);
  if (!value) {
    var value = 'function';
  }
  alert(value);
}
show() //此处调用函数依次弹出 "undefined", "function"

//上例相当于var value = 'hello';
function show() {
　　var value; //注意这行
  alert(value);
  if (!value) {
    value = 'function'; //这行去掉var
  }
  alert(value);
}
show()//1.如果上列中show内部定义value未用var,则会用到外部的变量，弹出"hello", "hello"。 
//2.如果函数内部未定义value,也能获取到外部的value值。

Anonymous functions will not look up

getName()
var getName = function () {
  alert('closule')
}
function getName() {
  alert('function')
}
getName()

//上边的代码相当于
function getName() { //函数向上提升
  alert('function')
}
getName()
var getName = function () {
  alert('closule')
}
getName()

//代码执行分别弹出 "function", "closule"

Functions in different <script> blocks do not affect each other</script>

<script>
  getName()
  var getName = function () {
    alert(&#39;closule&#39;)
  }
<script>
<script>
  function getName() {
    alert(&#39;function&#39;)
  }
<script>

//代码执行报错：TypeError: getName is not a function
//因为第一个<script>块中getName()函数未定义,匿名函数又不会向上提升</script>

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