What's the significant use of unary plus and minus operators?

Solution 1:

The Unary + operator converts its operand to Number type. The Unary - operator converts its operand to Number type, and then negates it. (per the ECMAScript spec)

In practice, Unary - is used for simply putting negative numbers in normal expressions, e.g.:

var x = y * -2.0;

That's the unary minus operator at work. The Unary + is equivalent to the Number() constructor called as a function, as implied by the spec.

I can only speculate on the history, but the unary +/- operators behave similarly in many C-derived languages. I suspect the Number() behavior is the addition to the language here.