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  3. JavaScript — Check if a Value is a Number

JavaScript — Check if a Value is a Number

In some situations, you must ensure that a given value is of a certain type. For example, you may have an options object supporting different properties. Some of them are expected to be numbers. You’re validating the input values and ensuring the given values are numbers and values in your expected range.

This tutorial shows you how to determine whether a given value is a number (or numeric) in JavaScript. This also works with TypeScript, Node.js, and all the other runtimes.

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Determine if a Given Value is a JavaScript Number

A naive approach to check whether a given value is a number is using JavaScript’s typeof operator. The typeof operator returns a variable’s type as a string. For this tutorial, you can compare the returned type value against the 'number' type.

Here’s a sample isNumber function checking the type of a given input:

/**
 * Determine whether the given `value` is a number.
 *
 * @param {*} value
 *
 * @returns {value is Number}
 */
function isNumber (value) {  
  return typeof value === 'number'
}

Now you can use the isNumber function like this:

isNumber(0) // true  
isNumber(1) // true  
isNumber(NaN) // true  
isNumber(1 / 0) // true  
isNumber(Number.NEGATIVE_INFINITY) // true  
isNumber(Number.POSITIVE_INFINITY) // true

isNumber('1') // false

Notice that the created isNumber function returns true for ±Infinity and NaN, too. Let’s refine that.

Check for a Finite Number

You may want to restrict the number check to finite numbers, excluding ±Infinity and NaN. You can do that using the built-in Number.isFinite function. This function determines whether a given input value is a finite number.

/**
 * Determine whether the given `value` is a finite number. A finite number   
 * is neither positive Infinity, nor negative Infinity, nor NaN.
 *
 * @param {*} value
 *
 * @returns {value is Number}
 */
function isFiniteNumber (value) {  
  return Number.isFinite(value)
}

Notice: we’re using Number.isFinite here because it’s more reliable than the global isFinite function. The difference between both: Number.isFinite won’t coerce a given input to a number and then compare it against ±Infinity and NaN.

Now you can use the isFiniteNumber function like this:

isFiniteNumber(0) // true  
isFiniteNumber(1) // true

isFiniteNumber(NaN) // false  
isFiniteNumber('1') // false  
isFiniteNumber(1 / 0) // false  
isFiniteNumber(Number.NEGATIVE_INFINITY) // false  
isFiniteNumber(Number.POSITIVE_INFINITY) // false

Determine if a Given Value is Numeric in JavaScript

You may want to detect numeric values, too. Our examples above used the string '1' as a test value. The functions behave as expected and mark the string value as „not a number“. But in some situations you may want to detect and cast a numeric value to a number.

Here’s a sample function detecting whether a given input is numeric, meaning it’s already a number or can be parsed to a number:

/**
 * Determine whether the given `value` is numeric.
 *
 * @param {*} value
 *
 * @returns {Boolean}
 */
function isNumeric (value) {  
  const num = parseFloat(value)

  return !isNaN(num) && Number.isFinite(num)
}

Now you can use the isNumeric function like this:

isNumeric(0) // true  
isNumeric(1) // true  
isNumeric('1') // true

isFiniteNumber(NaN) // false  
isFiniteNumber(1 / 0) // false  
isFiniteNumber(Number.NEGATIVE_INFINITY) // false  
isFiniteNumber(Number.POSITIVE_INFINITY) // false

Enjoy!

Marcus Pöhls

Marcus is a fullstack JS developer. He’s passionate about the hapi framework for Node.js and loves to build web apps and APIs. Creator of Futureflix and the “learn hapi” learning path.

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