Why do you need the + between variables in javascript?

Question

Why does this line work

$('#body-image').css("background-image", 'url('+ backgroundimage +')');

but not this one

$('#body-image').css("background-image", 'url('backgroundimage')');

or this one

$('#body-image').css("background-image", 'url(backgroundimage)');

Answer 1

backgroundimage is a JavaScript variable. The concatenation operator in JavaScript is +, so to put a string together with a variable, you do 'some string ' + someVariable. Without the +'s, JavaScript wouldn't know what to do with your variable (and in your third example, wouldn't even know that it was a variable).

Answer 2

You need to concat the string with the variable backgroundimage. So you use "+" for this.

That's why this doesn't work.

$('#body-image').css("background-image", 'url('backgroundimage')');

And the secont doesn't work because there is no image called 'backgroundimage'.

$('#body-image').css("background-image", 'url(backgroundimage)');