Check If a String Is in Lowercase in JavaScript or Node.js

The JavaScript stdlib is missing a method to detect whether a given input is a string value in lowercase format. Yet, we can build our own isLowerCase method using the existing methods.

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

Node.js Series Overview

  1. String Replace All Appearances
  2. Remove All Whitespace From a String in JavaScript
  3. Generate a Random ID or String in Node.js or JavaScript
  4. Remove Extra Spaces From a String in JavaScript or Node.js
  5. Remove Numbers From a String in JavaScript or Node.js
  6. Get the Part Before a Character in a String in JavaScript or Node.js
  7. Get the Part After a Character in a String in JavaScript or Node.js
  8. How to Check if a Value is a String in JavaScript or Node.js
  9. Check If a String Includes All Strings in JavaScript/Node.js/TypeScript
  10. Check if a Value is a String in JavaScript and Node.js
  11. Limit and Truncate a String to a Given Length in JavaScript and Node.js
  12. Split a String into a List of Characters in JavaScript and Node.js
  13. How to Generage a UUID in Node.js
  14. Reverse a String in JavaScript or Node.js
  15. Split a String into a List of Lines in JavaScript or Node.js
  16. Split a String into a List of Words in JavaScript or Node.js
  17. Detect if a String is in camelCase Format in Javascript or Node.js
  18. Check If a String Is in Lowercase in JavaScript or Node.js
  19. Check If a String is in Uppercase in JavaScript or Node.js
  20. Get the Part After First Occurrence in a String in JavaScript or Node.js
  21. Get the Part Before First Occurrence in a String in JavaScript or Node.js
  22. Get the Part Before Last Occurrence in a String in JavaScript or Node.js
  23. Get the Part After Last Occurrence in a String in JavaScript or Node.js
  24. How to Count Words in a File
  25. How to Shuffle the Characters of a String in JavaScript or Node.js
  26. Append Characters or Words to a String in JavaScript or Node.js (Coming soon)
  27. Check if a String is Empty in JavaScript or Node.js (Coming soon)
  28. Ensure a String Ends with a Given Character in JavaScript or Node.js (Coming soon)
  29. Left-Trim Characters Off a String in JavaScript or Node.js (Coming soon)
  30. Right-Trim Characters Off a String in JavaScript or Node.js (Coming soon)
  31. Lowercase the First Character of a String in JavaScript or Node.js (Coming soon)
  32. Uppercase the First Character of a String in JavaScript or Node.js (Coming soon)
  33. Prepend Characters or Words to a String in JavaScript or Node.js (Coming soon)

Determine Whether a JavaScript String is in Lowercase

You can detect whether a given input is a lowercase string in JavaScript using a comparison of the given value against its lowercased pendant. JavaScript provides a toLowerCase method on string values. That’s what we’ll use for our isLowerCase 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 toLowerCase only on string inputs.

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

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

You may then use the isLowerCase utility function like this:

isLowerCase('future studio')  
// true

isLowerCase('Future Studio')  
// false

isLowerCase(null)  
// false

isLowerCase()  
// false

Please notice, JavaScript doesn’t treat null or undefined as “nullish” values when creating a string of them using the String() function. It creates a string representation of the given value:

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 for undefined and null values to the isLowerCase 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#isLower method determining whether a given string is in lowercase:

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

Str('future studio').isLower()  
// true

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

Str(null).isLower()  
// true

Str().isLower()  
// true

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


Mentioned Resources

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