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In general it’s a good practice to check for the existence of something before blindly using it by faith and hoping it works. There are various times during the execution of scripts when a variable may not be defined, it may be null, or it might be an empty string. These are three things that are easily conflated. A good way to look at this is thinking of these as having increasing levels of existence (getting a bit philosophical here for a moment…):

foo0;             // existence level 0 (creates the error "not defined")
var foo1;         // existence level 1 ("undefined" - variable declared but not defined/initialized)
var foo2 = null;  // existence level 2 (variable initialized, but isn't an Object, Number, String, etc)
var foo3 = "";    // existence level 3 (variable initialized to an empty String)
var foo4 = "bar"; // existence level 4 (variable initialized to String "foo")

Generally it would be handy if we had some way to filter out everything but the very last line. We simply want to check for these cases without the script entirely blowing up, as it does with the first line:

foo;  // ReferenceError: foo is not defined

We’re not particularly doing anything useful with foo here, but notice that the script fails out anyway. At this point any code that follows will not be executed. Your first instinct might be “Oh! I know how to contain these errors! We’ll use a try-catch!”:

try {
  foo;
} catch(e) {
  e.message;  // "foo is not defined"
}

The script still fails, but not critically, so your script continues to execute. But this turns out to aversely affect performance. The basic lesson here is that try-catch can be useful in some situations, but shouldn’t be used where alternatives are available.

typeof foo

JavaScript has quite a useful remedy for this:

typeof foo; // "undefined"

Unlike everything else in JavaScript, typeof will deal with whatever you throw at it, including undefined variables. So you can use it as a simple check before using a variable that might not exist:

if(typeof foo !== "undefined") {
  // do something with foo
}

Note that this is easy to confuse with the undefined keyword, which in this case doesn’t help us one bit, as it gives us a fatal error:

if(foo !== undefined) {  // ReferenceError: foo is not defined

}

This filters out our first two cases, since they both evaluate to “undefined”:

typeof foo; // "undefined"

var bar;
typeof bar; // "undefined"

But this turns out not to work well for our other conditions, which evaluate differently:

typeof null;  // "object" (what?!)
typeof "";    // "string"

So we need to add other “if” conditions to check.. but there turns out to be a better way!

Exploiting loose typing

JavaScript is a loosely typed language, which means that it will “automagically” cast variables into other types when necessary (i.e. when adding a Number to a String), sometimes resulting in the unexpected. For instance, whenever we use the “if” statement, the expected input is a Boolean true/false value. If JavaScript gets anything other than a Boolean, such as a String or a Number, instead of blowing up completely (as in strictly typed languages such as C), it’ll cast the variable into a Boolean for you.

“How handy!” you might think. Except in the following unexpected cases:

if("0") {
  // this will run because "0" is true
}

if("false") {
  // this will run because "false" is true
}

To see what JavaScript will cast a value to without having to use an if statement, we could create a new Boolean value with the following:

Boolean(0);    // false
Boolean("0");  // true

Or we can use the less intuitive but quick way of using double exclamation marks (this will probably award you cleverness points in someone’s book.. hopefully those points actually matter):

!!0;    // false
!!"0";  // true

This works because !foo converts foo to a Boolean but negates its original value, turning it on its head. !!foo converts foo to a Boolean and flips it back to its expected value, which is the same value that’s evaluated by our if statement.

Using this ALMOST gives us the answer we’re looking for:

!!foo0; // ReferenceError: foo0 is not defined

var foo1;
!!foo1; // false

var foo2 = null;
!!foo2; // false (same as !!null)

var foo3 = "";
!!foo3; // false (same as !!"")

var foo4 = "bar";
!!foot4; // true (same as !!"bar")

Excluding the first example, now we can test for uninitialized variables (foo1), null variables (foo2), and empty strings (foo3) all with just an if statement:

if(foo1) {
  // do something with foo1
}

if(foo2) {
  // do something with foo2
}

if(foo3) {
  // do something with foo3
}

Dang… so close! But we can’t yet test the first case without an error:

if(foo0) {  // ReferenceError: foo0 is not defined

}

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