How does global variables in JavaScript work? [duplicate]

问题

This question already has answers here:

Surprised that global variable has undefined value in JavaScript (6 answers)

Closed 5 years ago.

I m a newbie to javascript. I usually program in Java. I am confused by this following code snippet.

<script>
 x = "foo";

function bar(p){
  if (p){
    document.writeln("x = " + x);
  } else {
    var x = "baz";
  }
}

bar("baz");

</script>

When I run the above code snipped its printing

 x = undefined

Why does it print undefined, since x is a global variable it should print foo right ? Can anyone explain ?

回答1:

since x is a global variable it should print foo right

It would if it wasn't shadowed by the var x = "baz"; declaration further up in your function; due to hoisting it will execute the function as if you wrote

function bar(p){
  var x; // = undefined

  if (p){
    document.writeln("x = " + x);
  } else {
    x = "baz";
  }
}

To make the code do what you want, you could simply write x = "baz"; instead of var x = "baz";.

回答2:

try this output is x = foo

var x="foo";
function bar(p){

  if (p){
    document.writeln("x = " + x);
  } else {
    x = "baz";
  }
}

bar("baz");

来源：https://stackoverflow.com/questions/22705181/how-does-global-variables-in-javascript-work