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, which is guaranteed to return a string:
if (typeof myVar !== 'undefined')
Direct comparisons against undefined are troublesome as undefined can be overwritten.
window.undefined = "foo";
"foo" == 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?
}
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, which is guaranteed to return a string:
if (typeof myVar !== 'undefined')
Direct comparisons against undefined are troublesome as undefined can be overwritten.
window.undefined = "foo";
"foo" == 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?
}
I personally use
myVar === undefined
Warning: Please note that === is used over == and that myVar has been previously declared (not defined).
I do not like typeof myVar === "undefined". I think it is long winded and unnecessary. (I can get the same done in less code.)
Now some people will keel over in pain when they read this, screaming: "Wait! WAAITTT!!! undefined can be redefined!"
Cool. I know this. Then again, most variables in Javascript can be redefined. Should you never use any built-in identifier that can be redefined?
If you follow this rule, good for you: you aren't a hypocrite.
The thing is, in order to do lots of real work in JS, developers need to rely on redefinable identifiers to be what they are. I don't hear people telling me that I shouldn't use setTimeout because someone can
window.setTimeout = function () {
alert("Got you now!");
};
Bottom line, the "it can be redefined" argument to not use a raw === undefined is bogus.
(If you are still scared of undefined being redefined, why are you blindly integrating untested library code into your code base? Or even simpler: a linting tool.)
Also, like the typeof approach, this technique can "detect" undeclared variables:
if (window.someVar === undefined) {
doSomething();
}
But both these techniques leak in their abstraction. I urge you not to use this or even
if (typeof myVar !== "undefined") {
doSomething();
}
Consider:
var iAmUndefined;
To catch whether or not that variable is declared or not, you may need to resort to the in operator. (In many cases, you can simply read the code O_o).
if ("myVar" in window) {
doSomething();
}
But wait! There's more! What if some prototype chain magic is happening…? Now even the superior in operator does not suffice. (Okay, I'm done here about this part except to say that for 99% of the time, === undefined (and ****cough**** typeof) works just fine. If you really care, you can read about this subject on its own.)
Videos
So I'm usually more of a server side developer, but lately I've been working with more of the client code at work. I understand what undefined and null are in JavaScript, but I find myself always checking for both of them. In fact, when checking if a String property exists, I end up writing this:
if(value !== undefined && value !== null && value !== '')
I figure there is a better way than this, and it's probably because I'm not 100% clear of when to check for what. So if someone could help fill me in here on the rules of when to check for undefined vs null, that would be great.
TL;DR: Use value != null. It checks for both null and undefined in one step.
In my mind, there are different levels of checking whether something exists:
0) 'property' in object - Returns true if the property exists at all, even if it's undefined or null.
-
object.property !== undefined- Returns true if the property exists and is not undefined. Null values still pass. -
object.property != null- Return true if the property exists and is not undefined or null. Empty strings and 0's still pass. -
!!object.property- Returns true if the property exists and is "truthy", so even 0 and empty strings are considered false.
From my experience, level 2 is usually the sweet spot. Oftentimes, things like empty strings or 0 will be valid values, so level 3 is too strict. On the other hand, levels 0 and 1 are usually too loose (you don't want nulls or undefineds in your program). Notice that level 1 uses strict equality (!==), while level 2 uses loose equality (!=).
I would just say
if (value) {
// do stuff
}because
'' || false
// false
null || false
// false
undefined || false
//falseEdit:
Based on this statement
I end up writing this: if(value !== undefined && value !== null && value !== '')
I initially assumed that what OP was really looking for was a better way to ask "is there a value?", but...
if someone could help fill me in here on the rules of when to check for undefined vs null, that would be great.
If you're looking to see if something is "truthy":
if (foo.bar) {
alert(foo.bar)
}This won't alert if value is '', 0, false, null, or undefined
If you want to make sure something is a String so you can use string methods:
if (typeof foo.bar === 'string') {
alert(foo.bar.charAt(0))
}This won't alert unless value is of type 'string'.
So.. "when to check for undefined vs null"? I would just say, whenever you know that you specifically need to check for them. If you know that you want to do something different when a value is null vs when a value is undefined, then you can check for the difference. But if you're just looking for "truthy" then you don't need to.