javascript - How does += (plus equal) work?

Question

I'm a bit confused with the += sign. How does it work?

1. 1 += 2 // equals ?

2. and this

   var data = [1,2,3,4,5];
   var sum = 0;
   data.forEach(function(value) {
       sum += value; 
   });
   sum = ?
   

Answer 1

1 += 2 is a syntax error (left-side must be a variable).

x += y is shorthand for x = x + y.