How to Check if a Number is Positive or Negative in Javascript

In this tutorial, you will learn how to check if a number is positive or negative in javascript. The numbers below zero are negative numbers and similarly, the numbers above zero are positive numbers. For example, 1 is a positive number and -1 is a negative number.

A negative number is always prefixed with a minus (-) sign and a positive number is optionally prefixed with a plus (+) sign. Detecting whether a number is positive or negative is very easy, but it can be tricky if the number is coming from an input field because the value present in the input field is of string type. So first, we have to convert it into a number type using the Number() method and then perform the check.

In the following example, we have an input field in which we will enter a random number. Upon click of a button, we will check whether it’s negative or positive and display the result on the screen.  Please have a look over the code example and the steps given below.

HTML & CSS

  • We have 4 elements in the HTML file (div, input, button, and h1). The div element is just a wrapper for the rest of the elements.
  • The innerText for the button element is “Check” and for the h1 element, it is “Result”.
  • We have done some basic styling using CSS and added the link to our style.css stylesheet inside the head element.
  • We have also included our javascript file script.js with a script tag at the bottom.
<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <link rel="stylesheet" href="style.css">
  <title>Document</title>
</head>

<body>

  <div class="container">
    <input type="number" placeholder="Enter Number">
    <button>Check</button>
    <h1>Result</h1>
  </div>

  <script src="script.js"></script>
</body>

</html>
.container {        
    text-align: center;
}

button {
  margin-top: 10px;
  padding: 10px 20px;
}

input {
  display: block;  
  margin: 10px auto;
  padding: 10px 20px;
}

input[type=number]::-webkit-inner-spin-button, 
input[type=number]::-webkit-outer-spin-button { 
  -webkit-appearance: none; 
  margin: 0; 
}

Javascript

  • We have selected the input element, button element, and h1 element using the document.querySelector() method and stored them in input, btnCheck, and output variables respectively.
  • We have attached a click event listener to the button element.
  • In the event handler function, we are passing the value of the input element to the Number() method and storing the number in the num variable.
  • The result variable is undefined initially.
  • We are using the if statement to verify whether the number is negative or positive. Depending upon the result of the check, we are setting the result variable equal to “Positive”, ”Negative”, or “Zero”.
  • We are displaying the result in the h1 element using the innerText property.
let input = document.querySelector("input");
let btnCheck = document.querySelector("button");
let output = document.querySelector("h1");

btnCheck.addEventListener("click", () => {
  let num = Number(input.value);
  let result;

  if (num == 0) result = "Zero";
  else if (num > 0) result = "Positive";
  else if (num < 0) result = "Negative";

  output.innerText = result;
});
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