What does a + prefix do in this context?

Question

Here is a line of code from underscore. What is that plus prefix for in this line?

if (obj.length === +obj.length) { // plus prefix?

Answer 1

The plus prefix converts the variable into a number. Basically, the obj.length === +obj.length is a sanity check that obj.length really is a number. If the obj.length was not a number, and for example a string "foo", then "foo" === +"foo" would equate to false since +"foo" comes out as NaN.