What is this in javascript: "var var1 = var1 || []"

Question

I just want to increase my core javascript knowledge.

Sometimes I see this statement but I don't know what it does:

var var1 = var1 || [];

What does it means and/or what's it for, and how do you use it?

Thank you.

Answer 1

Basically, it looks to see if a variable var1 already exists and is "truthy". If it is, it assigns the local var1 variable its value; if not, it gets assigned an empty array.

This works because the JavaScript || operator returns the value of the first truthy operand, or the last one, if none are truthy. var1 || var2 returns var1 if it's truthy, or var2 otherwise.

Here are some examples:

var somevar;
somevar = 5 || 2; // 5
somevar = 0 || 2; // 2
somevar = 0 || null; // null

Values that aren't "truthy": false, 0, undefined, null, "" (empty string), and NaN. Empty arrays and objects are considered truthy in JavaScript, unlike in some other languages.

Answer 2

It assigns an empty array to var1, if the boolean representation of it is false (for example it hasn't been initialized).