奇怪的javascript范围问题

问题描述

var c = 1;

function myFunction(){
    c = 2;
    var c = 4;
    console.log(c);
}

console.log(c);
myFunction();
console.log(c);

为什么最后一个console.log吐出1？以下是我的大脑应该如何工作：

Why is the last console.log spitting out 1? Here is how it is supposed to work in my brain:

var c = 1; // create global variable called 'c'

function myFunction(){
    c = 2; // assign value of global 'c' to 2
    var c = 4; // create a new variable called 'c' which has a new address in memory (right?)  with a value of 4
    console.log(c); // log the newly created 'c' variable (which has the value of 4)
}

console.log(c); //log the global c
myFunction(); //log the internal c
console.log(c); //log the updated global c

推荐答案

在范围内您的console.log运行时，只存在全局变量。函数永远不会触及全局变量。

In the scope where your console.log runs, only the global variable exists. And the function never touches the global variable.

详细信息：

var c = 1; // create global variable called 'c'

function myFunction(){
    // the global is unavailable here, as you declared a local c.
    // Its declaration is hoisted to the top of the function, so
    // c exists here, and its value is undefined.
    c = 2; // assign value of local 'c' to 2
    var c = 4; // change local c to 4
    console.log(c); // log local c (4)
}

console.log(c); //log the global c
myFunction(); //log the internal c
console.log(c); //log the global c (which never changed)

由于上面提到的提升，你的功能表现如下好像代码是：

Due to the hoisting mentioned above, your function behaves as if the code was:

function myFunction(){
    var c; // declares a local c that shadows the global one
    c = 2;
    c = 4;
    console.log(c);
}

关于提升的一些参考

• 的例子中







Some references about hoisting

• MDN page for the var statement
• MDN scope cheatsheet
• How are javascript variables "hoisted" in these examples from MDN
• Javascript hoisting explained
• JavaScript Scoping and Hoisting

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