Set Default Variable Value in JavaScript

Did you know that you can set a default variable value in JavaScript?

Here is an example declaration statement, where foo is set to bar if bar is defined, but otherwise set to Default Value:

var bar;

const foo = bar || `Default Value`;

Because bar is undefined in the above example, foo is set to Default Value.

In the above example, a string is used for the default value, but any data type can be used to the right of the || operator.

Here is another way of visualizing how this works:

const foo = undefined || `Default Value`;

If the value to the left of the || operator evaluates to undefined, then the variable is set to the value to the right of the || operator.

But if the value on the left does not evaluate to undefined, then the variable will be set to the value on the left.

Conclusion

There are technically a few other ways to go about doing this, but I’ve found this method to be very versatile.

This method also works if the value on the left side of the || operator evaluates to false instead of undefined.