JavaScript: Checking If a Variable Is Defined

There are quite a few times when writing JavaScript that we need to check if a variable has been set. Keep in mind that we want to check if a variable has been set, not necessarily what its value is.

We can do that by using this little code snippet:

1if (typeof myVariableName === 'undefined') {
2    // The myVariableName is undefined!
3}

Notice that undefined is in quotes. That is because the typeof operator will return a string. Just a little piece of information for the tool belt. The inverse of this is:

1if (typeof myVariableName !== 'undefined') {
2    // The myVariableName is defined. We can do whatever we want with it. Hooray!
3}

Do note that this method should be used to check if a variable has been defined. There are other ways to determine if a variable has been assigned to. The easiest way is just to assign the variable a default value when you initialize it, say null, then explicitly check for the default value to determine if the value has been changed.

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