在JavaScript中有時候我們會到看undefined,究竟什麼是undefined是什麼呢
其實undefined也是一個值.
If you are interested in finding out whether a variable has been declared regardless of its value, then using the in
operator is the safest way to go. Consider this example.
// global scope
var theFu; // theFu has been declared, but its value is undefined
typeof theFu; // "undefined"
But this may not be the intended result for some cases, since the variable or property was declared but just not initialized. Use the in
operator for a more robust check.
"theFu" in window; // true
"theFoo" in window; // false
If you are interested in knowing whether the variable hasn’t been declared or has the value undefined
, then use the typeof
operator.
if (typeof myVar != 'undefined')
The typeof
operator is guaranteed to return a string. Direct comparisons against undefined
are troublesome as undefined
can be overwritten.
window.undefined = "omg";
"omg" == undefined // true
As @CMS pointed out, this has been patched in ECMAScript 5th ed., and undefined
is non-writable.
if (window.myVar)
will also include these falsy values, so it’s not very robust:
false 0 "" NaN null undefined
Thanks to @CMS for pointing out that your third case – if (myVariable)
can also throw an error in two cases. The first is when the variable hasn’t been defined which throws a ReferenceError
.
// abc was never declared.
if (abc) {
// ReferenceError: abc is not defined
}
The other case is when the variable has been defined, but has a getter function which throws an error when invoked. For example,
// or it's a property that can throw an error
Object.defineProperty(window, "myVariable", {
get: function() { throw new Error("W00t?"); },
set: undefined
});
if (myVariable) {
// Error: W00t?
}