JavaScript utility to split a string into array that includes the delimiter and can ignore case

splitWithDelimiter.js

/*
* Example splitWithDelimiter({query: 'the', suggestion: 'The quick brown fox jumps over the lazy dog.', shouldIgnoreCase: true})
* Evaluates to ['The', ' quick brown fox jumps over ', 'the', ' lazy dog.'];
*/
function splitWithDelimiter ({delimiter, target, shouldIgnoreCase}) {
    const result = [];
    const formattedTarget = shouldIgnoreCase ? target.toLowerCase() : target;
    const formattedDelimiter = shouldIgnoreCase ? delimiter.toLowerCase() : delimiter;
    const doesStartWithDelimiter = formattedTarget.startsWith(formattedDelimiter);
    const doesEndWithDelimiter = formattedTarget.endsWith(formattedDelimiter);
    const mismatchedFragments = formattedTarget.split(formattedDelimiter);
    let resultLength = 0;

    if (doesStartWithDelimiter) {
        result.push(target.substring(0, delimiter.length));
        resultLength = delimiter.length;
    }

    mismatchedFragments.forEach((fragment, index) => {
        if (!fragment) return;

        result.push(target.substring(resultLength, resultLength + fragment.length));
        resultLength += fragment.length;

        if (index < mismatchedFragments.length && resultLength < target.length) {
            const characters = target.substring(resultLength, resultLength + delimiter.length);

            result.push(characters);
            resultLength += delimiter.length;
        }
    })

    if (doesEndWithDelimiter) {
        result.push(target.substring(target.length - delimiter.length, target.length));
    }
  
  return result;
}