Uncamelising a string in JavaScript

We are required to write a JavaScript function that takes in a string as the first argument and a separator character as the second argument.

The first string is guaranteed to be a camelCased string. The function should convert the case of the string by separating the words by the separator provided as the second argument.

For example −

If the input string is −

const str = 'thisIsAString';
const separator = '_';

Then the output string should be −

const output = 'this_is_a_string';

Using Custom Logic

This approach iterates through the string and identifies word boundaries by detecting uppercase letters:

const str = 'thisIsAString';
const separator = '_';

const separateCase = (str = '', separator = ' ') => {
    const arr = [];
    let left = 0, right = 0;
    
    for(let i = 0; i 

this_is_a_string


Using Regular Expression (Simpler Approach)

A more concise solution using regex to detect uppercase letters and replace them:

const uncamelize = (str, separator = '_') => {
    return str
        .replace(/([A-Z])/g, separator + '$1')
        .toLowerCase()
        .replace(new RegExp('^' + separator), ''); // Remove leading separator
};

console.log(uncamelize('thisIsAString', '_'));
console.log(uncamelize('camelCaseExample', '-'));
console.log(uncamelize('oneWord', '_'));


this_is_a_string
camel-case-example
one_word


Comparison

Common Use Cases

Uncamelizing strings is commonly used when:

• Converting JavaScript property names to database column names
• Generating CSS class names from camelCase variables
• Creating file names or URL slugs from camelCase strings

Conclusion

The regex approach is more concise and readable for uncamelizing strings. Use the custom logic approach when you need more control over the word detection process.