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.

Answer 2

1) 1 += 2 // equals ?

That is syntactically invalid. The left side must be a variable. For example.

var mynum = 1; mynum += 2; // now mynum is 3. 

mynum += 2; is just a short form for mynum = mynum + 2;

2)

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

Sum is now 15. Unrolling the forEach we have:

var sum = 0; sum += 1; // sum is 1 sum += 2; // sum is 3 sum += 3; // sum is 6 sum += 4; // sum is 10 sum += 5; // sum is 15