In a = +a || 0, what does the '+' after '=' do?

Question

for example in the indexOf polyfill in MDN

fromIndex = +fromIndex || 0;

What does the + in front of fromIndex do?

Answer 1

Converts it into a number. It's the opposite of -fromIndex, so it has to convert into a number.

See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Arithmetic_Operators#-_.28Unary_Plus.29

+"0" 
>> 0
+true 
>> 1