Scope of variables (Hoisting) in Javascript

问题

One of my friends was taking an online quiz and he asked me this question which I could not answer.

var global = false;

function test() {
  global = true;
  return false;
  function global() {}
}

console.log(global); // says false (As expected)
test();
console.log(global); // says false (Unexpected: should be true)

If we assume that functions are hoisted at the top along with var variables, let's try this one.

var foo = 1;
function bar() {
    return foo;
    foo = 10;
    function foo() {}
    var foo = 11;
}

bar();
console.log(foo); //says 1 (But should be 11) Why 1 this time ??

Here is a JSBin Demo and JSBIN Demo2 to play with.

PS: If we remove function global() {} from test(), then it runs fine. Can somebody help me understand why is this happening ?

回答1:

var statements and function declaration statements are "hoisted" to the top of their enclosing scope.
Therefore, the function global(){} in your function creates a local global name.

Assigning to global inside your functions binds to this local name. Here's how you can "rewrite" it using hoisting to understand how the compiler sees it:

function test() {
    var global = function() {};   // hoisted; 'global' now local
    global = true;
    return false;
}

回答2:

I'll answer the second part of your question,

If we assume that functions are hoisted at the top along with var variables
bar();
console.log(foo); //says 1 (But should be 11) Why 1 this time ??

You should try console.log(bar()); console.log(foo); instead. However, what hoisting does to your function is this:

function bar() {
    var foo;
    function foo() {}
    return foo;
    foo = 10;
    foo = 11;
}

So you should expect to get the function returned, since your variable assignments are after the return statement. And both the var and the function declaration make foo a local variable, so the global foo = 1 is never changed.

来源：https://stackoverflow.com/questions/21609493/scope-of-variables-hoisting-in-javascript

标签

javascript

global-variables

scope