# JavaScript Arithmetic Operators

JavaScript’s **arithmetic operators** perform addition (+), subtraction (-), multiplication (*), and division (/). Mozilla has a great overview of them, but it fails to mention any of the edge cases that frequently trip up developers new to JavaScript.

## Addition (+)

The addition operator `+`

performs addition **or** concatenates strings. When you’re dealing with just numbers or just strings, the results are intuitive.

```
// Number + Number -> addition
1 + 2 // 3
5 + 50 // 55
// String + String -> concatenation
'hello' + ' there' // 'hello there'
'Java' + 'Script' // 'JavaScript'
```

But when you combine numbers and strings, the numbers will be turned into strings and concatenated. This is a typical gotcha when learning JavaScript.

```
// Number + String -> concatenation
1 + 'foo' = '1foo'
'5' + 5 = '55'
```

Another gotcha is that operators are evaluated from left to right. Consider the example below with its output.

```
1 + 2 + '3' + 4 + 5 // '3345'
```

Moving from left to right, we add `1 + 2`

which is `3`

. Then we see the string `'3'`

, so we have string concatenation, resulting in `'33'`

. Now that we have a string, we concatenate `4`

and `5`

as if they, too, were strings, resulting in `3345`

.

## Subtraction (-)

The subtraction operator subtracts two operands. If they are numbers, the behavior is expected.

```
10 - 5 // 5
5 - 10 // -5
```

If you perform subtraction with a string, the result will *usually* be `NaN`

(Not a Number).

```
5 - 'foo' // NaN
'bar' - 10 // NaN
```

**But** due to JavaScript’s type coercion, the interpreter will try to convert a string into a number. Which leads to unexpected results:

```
10 - '5' // 5
'5' - '10' // -5
'10' - '.' - '5' // NaN because '.' can't be converted into a number
```

**Want to improve your JavaScript? I have a list of recommended JavaScript books.**
<!– ## Division (/)