Check If a String is in Uppercase in JavaScript or Node.js

JavaScript is missing a method to detect whether a given input is a string value in uppercase format. What’s existing are methods allowing us to create our own isUpperCase utility method.

This tutorial shows you how to create a method detecting whether an input is in uppercase.

Node.js Series Overview

Determine Whether a JavaScript String is in UPPERCASE

Detecting whether a given input is an uppercase string requires you to create the uppercased version first. Then, you’re comparing the input value against the uppercase version. This check returns true if the values are equal, other false.

JavaScript provides a toUpperCase method on string values. That’s what we’ll use for our isUpperCase method.

You can ensure a string value by wrapping a given input using the globally available String(input) function. This is useful because we can call toUpperCase only on string inputs.

Here’s a sample utility function determining whether a given input is an uppercased string:

/**
 * Determine whether the given `input` is a string in uppercase.
 *
 * @param {*} input
 *
 * @returns {Boolean}
 */
function isUpperCase (input) {  
  return input === String(input).toUpperCase()
}

You may then use the isUpperCase utility function like this:

isUpperCase('FUTURE STUDIO')  
// true

isUpperCase('Future Studio')  
// false

isUpperCase(null)  
// false

isUpperCase()  
// false

Please notice, JavaScript doesn’t treat null or undefined as “nullish” values when using them to create a string. The String() function wraps the given value into a string, like 'null' and 'undefined' (as strings):

String()  
// ''

String(null)  
// 'null'

String(undefined)  
// 'undefined'

This can lead to unexpected behavior in your application when working with nullish values. Depending on your expected behavior you may need to add checks or fallbacks for undefined or null input values to the isUpperCase utility from above.

Use the @supercharge/strings Package

I’m the maintainer of the @supercharge/strings package providing convenient string utilities. It provides a useful Str#isUpper method determining whether a given string is in uppercase:

const Str = require('@supercharge/strings')

Str('FUTURE STUDIO').isUpper()  
// true

Str('Future Studio').isUpper()  
// false

Str(null).isUpper()  
// true

Str().isUpper()  
// true

The @supercharge/strings package treats undefined and null values as empty strings. And empty strings are uppercase by default. That’s the consistent behavior you want in your app 🤝


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