为什么JavaScript在第二种情况下不查找全局范围

Question

foo will lookup b value with 2 as expected when executing in console

function foo() {
    console.log(b)
}
var b = 2
foo() // 2 for console.log

but when I do this

function foo() {
    console.log(b)
    var b = 2
    console.log(b)
}
var b = 2
foo() // undefined for first console.log

it won't lookup b in global scope anymore why ?

Answer 1

It is because of what is called Javascript Hoisting, this is how javascript sees your code:

function foo() {
  var b; // javascript hoisted this variable on top
  console.log(b) // so here you see underfined
  b = 2;
  console.log(b);
}

Now because javascript hoised variable b on top of function, your global variable b was never used and hence statement immendiately after hoisted variable showed undefined . So if you remove new declaration ( var keyword) from your function, you should still have access to global b variable.

BTW, it is good practice to declare all your variables on top of function like:

function foo() {
  var b = 2;
  console.log(b);
}

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To know more on the topic, see this:

• http://www.adequatelygood.com/JavaScript-Scoping-and-Hoisting.html