function t()
{
var x = 1;
if(true)
{
var x = 2;
alert(x);
}
alert(x);
}
t();
Anyone knows the reason?
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4welcome to javascript :)Jacksonkr– Jacksonkr2011-08-17 01:47:48 +00:00Commented Aug 17, 2011 at 1:47
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3 Answers
Because JavaScript (well ECMAScript) does not have block scope (yet). Just function scope.
There's really just one variable declaration that is hoisted to the top of the function, so x=2 is overwriting the initial value of 1.
function t()
{
var x = 1;
// v---------immediately invoked function to create a new scope
(function() {
// new variable scope
if(true)
{
var x = 2;
alert(x); // 2
}
})();
alert(x); // 1
}
t();
2 Comments
anthony sottile
Not a very optimistic "yet". I doubt this will ever change. Imagine the nightmare of breaking decades(?) of code. EDIT: I stand corrected, your edited link provides some additional insight I did not consider.
user113716
@Anthony: If your code is running in the browser, then very true. Out of the browser in free-standing JS engines, there's reason to be optimistic (or even to enjoy it today). :o)
The 'var' keyword applies within a whole function, so the code you have will behave identically to this:
function t() {
var x = 1;
if (true) {
x = 2;
alert(x);
}
alert(x);
}
t();
Comments
Variables in Javascript are scoped to the function, not to blocks. You've got two vars, but there's really only one x.
Comments
