April 3, 2022

How to Check if String is Positive Number in Javascript

By Krishnaa JavaScript Questions 0 Comments

In this tutorial, you will learn how to check if string is positive number in javascript. A number is considered to be a positive number if the value is greater than 0.

There are numerous ways to check if a string is a positive number. We are going to use one of the easiest solutions which involve the usage of the Math.sign() method and ternary (?) operator. The ternary operator is also known as the conditional operator which acts similar to the if-else statement.

Depending upon the kind of number passed to the Math.sign() method, it returns +/- 1. In the case of 0, it returns +/- 0 depending upon whether the 0 is negative or positive. If the number is positive, the + sign is explicitly omitted from returned values 1 and 0.

In the following example, we have one global variable and upon click of a button, we will check if the string is a positive number and display the result on the screen. Please have a look over the code example and the steps given below.

HTML & CSS

We have 3 elements in the HTML file (div, 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>
        <button>Check</button>
        <h1>Result</h1>
    </div>

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

body {
    text-align: center;
}

div {
    display: inline-block;
}

button {
    display: inline-block;
    padding: 10px 20px;
}

Javascript

We have selected the button element and h1 element using the document.querySelector() method and stored them in btnCheck and output variables respectively.
We have a global variable myVar which holds a string as its value.
We have attached a click event listener to the button element.
In the event handler function, we are calling isPositive() method and passing myVar as a parameter. This method will verify if the string is a positive number and will return either true or false.
In the isPositive() method, we are converting myVar to number type using Number() constructor. Also, we are using Math.sign() to check if the string is a positive number.
Depending upon the returned value, we will assign “Yes” or “No” to the result variable.
We are displaying the result in the h1 element using the innerText property.

let btnCheck = document.querySelector("button");
let output = document.querySelector("h1");

let myVar = "6";

btnCheck.addEventListener("click", () => {
  let result = isPositive(myVar) ? "Yes" : "No";
  output.innerText = result;
});

function isPositive(str) {
  const num = Number(str);
  return Math.sign(num) == 1;
}

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