Javascript变量范围未定义

Question

I want to define a variable to default value if not already defined but found a strange issue.

var x = x || {} var x = x || {} works whereas x = x || {} x = x || {} gives an error.

Output from firebug.

>>> a = a || {};
ReferenceError: a is not defined
[Break On This Error]   
a = a || {};
with(_... {}; }; (line 2)
>>> var b = b || {};
undefined

>>> b;
Object {}
>>> a;
ReferenceError: a is not defined
[Break On This Error]   

why does the first one give error while the second seems to go through.

Answer 1

Just use var . Except for a bug on old IE versions, adding the var saves you an error message and makes it clear that you're not assuming it is declared elsewhere.

On those old versions of IE,

x = (typeof x !== 'undefined' && x) || {}

a typeof check prevents the "undeclared variable" error.