window.length

Most of the time, when dealing with vanilla JavaScript code, I try things out directly in the browser. I just open the console within my favorite developer tools and start poking around until I gain whatever I searched for.

Today I made a small mistake and accidentally called something similar to this:

myVariable - length

And as I hit return on my keyboard, I already saw my foolish mistake. The property access I did was not valid but a calculation with a not defined variable called length. Then I saw the output:

myVariable - length
-3

What? How did that happen? Did I declare length by mistake as a variable? Why does it not throw an error? Or - wait for a second - is this available as a property on the window object?

Yep, window.length exists. And as I apparently did not know anything about its existence, I asked the best online resource about it - the Mozilla Developer Network web docs.

window.length

Returns the number of frames (either <frame> or <iframe> elements) in the window.

window.length on MDN

Yes, it exists. Now you know too.