javascript syntax for ? and :

Question

I have two questions:

1. I have a JavaScript function code:

var firstOrNull = function (elems){
    return (elems.length > 0 ) ? elems[0] : null;
} 

What does ? and : means in this code?

2. What is the meaning of this code:

var stopEvent = function(event){ event.stopPropagation() }

Answer 1

? and : pair indicate ternary operator in Javascript.

(elems.length > 0 ) ? elems[0] : null; line means if elems length is greater than zero then return elems[0] otherwise return null.

Answer 2

This is called the ternary operators

if(elements.length > 0){
   return elems[0];
} else {
return  null;
}

is equivalent to :

return (elems.length > 0 ) ? elems[0] : null;

ternary operators

Answer 3

1. This is Conditional Operator
2. stopPropagation method of javascript event. It uses to prevent further propagation of the current event.