Check if number is odd or even in JavaScript

function isOdd(number) {
  return Math.abs(number % 2) === 1;
}

function isEven(number) {
  return number % 2 === 0;
}

In JavaScript, you can easily check whether an integer is even or odd by using the remainder / modulo % operator. 
If a number divided by 2 gives a remainder of 1 , it is odd, if it gives a remainder of 0 , it is even. You can implement this simply as follows:

function isOdd(number) {
  return number % 2 === 1;
}

function isEven(number) {
  return number % 2 === 0;
}

This is quite simple, but unfortunately not always appropriate. For example, it does not work correctly for negative numbers:

console.log(isOdd(-5)); // false but it should be true

To handle negative numbers, you just need to modify the isOdd function to take the absolute value of the result.

function isOdd(number) {
  return Math.abs(number % 2) === 1;
}

Another problem that may arise with JavaScript is that our input parameter is not a number. Therefore, you may want to first check if the input value is a number and if not then return undefined instead of true or false :

function isOdd(number) {
    if (Number.isFinite(number) === false) {
        return undefined;
    }
    return Math.abs(number % 2) === 1;
}

However, this is still not perfect. In JavaScript, the value of a form input field is always a string , even if its type is set to number . Let's look at the following example:

<input type="text" name="name" id="input-field" />

And the JavaScript code to handle the input:

document.addEventListener("DOMContentLoaded", () => {
    const input = document.querySelector("#input-field");

    input.addEventListener("change", (event) => {
        const userInput = event.target.value;
        console.log(userInput + ' is odd? ' + isOdd(userInput));
    });
});

In this case, even if the user enters a valid number, the Number.isFinite check will result that the return value of our function will always be undefined.

For this reason, you also need to include a conversion to a number in the code:

function isOdd(number) {
    if (Number.isFinite(parseInt(number)) === false) {
        return undefined;
    }
    return Math.abs(number % 2) === 1;
}

Finally, this is what the two functions look like in TypeScript format:

function isOdd(number:number):boolean|undefined {
    if (Number.isFinite(parseInt(number)) === false) {
        return undefined;
    }
    return Math.abs(number % 2) === 1;
}

function isEven(number:number):boolean|undefined {
  return number % 2 === 0;
}