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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;
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
+= in JavaScript (as well as in many other languages) adds the right hand side to the variable on the left hand side, storing the result in that variable.  Your example of 1 +=2 therefore does not make sense.  Here is an example:
var x = 5;
x += 4; // x now equals 9, same as writing x = x + 4;
x -= 3; // x now equals 6, same as writing x = x - 3;
x *= 2; // x now equals 12, same as writing x = x * 2;
x /= 3; // x now equals 4, same as writing x = x / 3;
In your specific example the loop is summing the numbers in the array data.
+= operator is used to concatenate strings or add numbers.
It will increment your sum variable with the amount next to it.
var sum = 0;
var valueAdded = 5; 
sum += valueAdded;
  
sum = 5
Assignment operators syntax is: variable = expression;
For this reason 1 += 2 -> 1 = 1 + 2 is not a valid syntax as the left operand isn't a variable. The error in this case is ReferenceError: invalid assignment left-hand side.
x += y is the short form for x = x + y, where x is the variable and x + y the expression. 
The result of the sum is 15.
      sum = 0;
      sum = sum + 1; // 1
      sum = sum + 2; // 3
      sum = sum + 3; // 6
      sum = sum + 4; // 10
      sum = sum + 5; // 15
Other assignment operator shortcuts works the same way (relatively to the standard operations they refer to).
                Fixed with a small edit.  It had previously said "no" instead of "now", which is what Mark was replying to.
– Keith Irwin
                Jul 5, 2017 at 8:48
you are going to add 1+2.
But this will give you a syntax error.
Assume if a variable is int type int a=1;
a+=2; means a=1+2; and increase the value of a from 1 to 3.
1 += 2 won't throw an error but you still shouldn't do it. In this statement you are basically saying "set 1 equal to 1 + 2" but 1 is a constant number and not a variable of type :number or :string so it probably wouldn't do anything.  Saying
var myVariable = 1
myVariable += 2
console.log(myVariable)
would log 3 to the console, as x += y is just short for x = x + y

var data = [1,2,3,4,5]
var sum
data.forEach(function(value){
  sum += value
would make sum = 15 because:
sum += 1 //sum = 1
sum += 2 //sum = 3
sum += 3 //sum = 6
sum += 4 //sum = 10
sum += 5 //sum = 15