Strange Behavior on Global and Local Variable in JavaScript

问题

I tried following code:

var a = 5;

function x() {
  console.log(a);
}

x();

It runs as expected and prints 5.

But i changed the code so the global variable a will be overwrite as follows:

var a = 5;

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

x();

It prints undefined. It doesn't make sense for me since the overwrite should be happened right after console.log(a). So what is the problem?

回答1:

This is happening because your second a variable is being 'hoisted' to the top of the function and it hides the first a. What is actually happening is this:

var a = 5;

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

x();

Here is an article on hoisting from adequately good for further reading on the subject.

回答2:

var a = 5;

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

x();

you need to initalize variable a before console.log();

来源：https://stackoverflow.com/questions/15714752/strange-behavior-on-global-and-local-variable-in-javascript