Overview

That is a short function.

x += 1;

x = x + 1; //This is equivalent to the first variable declaration.

Equally this:

result *= base;

is the same as:

result = result * base;

There are several shortcut operators like "+", "-", "*", and the recently added "**". That last one is the exponentiator operator.

2 ** 3 === 2 * 2 * 2; // '===' means strict equivalence (value and type).
result **= base === result = result ** base;

In a loop you write:

for(let i = 0; i < 20; i++) {
  /*
  * Do something
  * That 'i++ is just a shortcut (syntactic sugar) of 'i = i + i'.
  * By the way, the 'let' variable means 'i' 
  * will only be available inside the loop. If you try to 
  * console.log(i) outside of it, the compiler will return 'undefined'.
  */
}

text += "The number is " + i;

is equivalent to

text = text + "The number is " + i;

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