# How does += (plus equal) work?

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

`x += y` is shorthand for `x = x + y`. 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
`````` That is just a short form for:

``````sum = sum + value;
`````` `+=` 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;

``````

sum = 5 You have to know that:

• 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). . ...and don't forget what happens when you mix types:

``````x = 127;
x += " hours "
// x is now a string: "127 hours "
x += 1 === 0;
// x is still a string: "127 hours false"
`````` `a += b` is shorthand for `a = a +b` which means:

1) `1 += 2` // won't compile

2) 15 As everyone said above

``````var str = "foo"
str += " bar"
console.log(str) //will now give you "foo bar"
``````

Check this out as well https://www.sitepoint.com/shorthand-javascript-techniques/ that's just a shorthand notation in most languages.which means that

x=x+1;

we can do the same operation for x-=1,x*=1,x/=1; which means

## > x=x-1; x=x*1; x=x/1; `x+=y` is shorthand in many languages for `set x to x + y`. The sum will be, as hinted by its name, the sum of the numbers in `data`. NO `1+=2!=2` it means 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`; then `a+=2`; means `a=1+2`; and increase the value of a from 1 to 3.