[2014-05-26] dev, javascript, jslang

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Integers lead an odd life in JavaScript. In the ECMAScript specification, they only exist conceptually: All numbers are always floating point and integers are ranges of numbers without decimal fractions (for details, consult “Integers in JavaScript” in “Speaking JavaScript”). In this blog post, I explain how to check whether a value is an integer.

There are many ways in which you could implement this check. At this moment, you may want to take a break and try to write your own solution: a function `isInteger(x)`

that returns `true`

if `x`

is an integer and `false`

, otherwise.

Let’s look at a few examples.

One can use the remainder operator (`%`

) to express the fact that a number is an integer if the remainder of dividing it by 1 is 0.

```
function isInteger(x) {
return x % 1 === 0;
}
```

I like this solution, because it is quite self-descriptive. It usually works as expected:

```
> isInteger(17)
true
> isInteger(17.13)
false
```

You have to be careful with the remainder operator, because the first operand determines the sign of the result: if it is positive, the result is positive, if it is negative, the result is negative.

```
> 3.5 % 1
0.5
> -3.5 % 1
-0.5
```

However, we are checking for zero, so that’s not an issue here. One problem remains: this function can return `true`

for non-numbers, because `%`

coerces its operands to numbers:

```
> isInteger('')
true
> isInteger('33')
true
> isInteger(false)
true
> isInteger(true)
true
```

That can be easily fixed by adding a type check:

```
function isInteger(x) {
return (typeof x === 'number') && (x % 1 === 0);
}
```

`Math.round()`

A number is an integer if it remains the same after being rounded to the “closest” integer. Implemented as a check in JavaScript, via `Math.round()`

:

```
function isInteger(x) {
return Math.round(x) === x;
}
```

This function works as it should:

```
> isInteger(17)
true
> isInteger(17.13)
false
```

It also handles non-numbers correctly, because `Math.round()`

always returns numbers and `===`

only returns `true`

if both operands have the same type.

```
> isInteger('')
false
```

If you wanted to make the code more explicit, you could add a type check (like we did in the previous solution). Furthermore, `Math.floor()`

and `Math.ceil()`

work just as well as `Math.round()`

.

Bitwise operators provide another way of converting a number to a “close” integer:

```
function isInteger(x) {
return (x | 0) === x;
}
```

This solution (along with other solutions based on bitwise operators) has one disadvantage: it can’t handle numbers beyond 32 bits.

```
> isInteger(Math.pow(2, 32))
false
```

`parseInt()`

`parseInt()`

also converts numbers to integers and can be used similarly to `Math.round()`

. Let’s find out whether that is a good idea.

```
function isInteger(x) {
return parseInt(x, 10) === x;
}
```

Like the `Math.round()`

solution, this implementation handles non-numbers well, but it does not correctly identify all numbers as integers:

```
> isInteger(1000000000000000000000)
false
```

Why? `parseInt()`

coerces its first parameter to string before parsing digits. It is not a good choice for converting numbers to integers.

```
> parseInt(1000000000000000000000, 10)
1
> String(1000000000000000000000)
'1e+21'
```

Above, `parseInt()`

stops parsing `'1e+21'`

before the first non-digit, `e`

, which is why it returns `1`

.

I received a few more interesting solutions via Twitter, check them out.

Complementing `Math.round()`

et al., ECMAScript 6 provides an additional way of converting numbers to integers: `Math.trunc()`

. That function removes a number’s decimal fraction:

```
> Math.trunc(4.1)
4
> Math.trunc(4.9)
4
> Math.trunc(-4.1)
-4
> Math.trunc(-4.9)
-4
```

Furthermore, ECMAScript 6 makes the task of checking for integers trivial, because it comes with a built-in function `Number.isInteger()`

.

- “Converting to Integer” (in “Speaking JavaScript”) covers the most common ways of converting numbers to integers.
- “Safe Integers” (in “Speaking JavaScript”) explains what range of integers can be safely used in JavaScript and what “safely used” means.