Get relative file path in JavaScript

relativePath.js

/**
 * Given a source directory and a target filename, return the relative
 * file path from source to target.
 * @param source {String} directory path to start from for traversal
 * @param target {String} directory path and filename to seek from source
 * @return Relative path (e.g. "../../style.css") as {String}
 */
function getRelativePath(source, target) {
	var sep = (source.indexOf("/") !== -1) ? "/" : "\\",
		targetArr = target.split(sep),
		sourceArr = source.split(sep),
		filename = targetArr.pop(),
		targetPath = targetArr.join(sep),
		relativePath = "";
	
	while (targetPath.indexOf(sourceArr.join(sep)) === -1) {
		sourceArr.pop();
		relativePath += ".." + sep;
	}
	
	var relPathArr = targetArr.slice(sourceArr.length);
	relPathArr.length && (relativePath += relPathArr.join(sep) + sep);
	
	return relativePath + filename;
}

console.log(getRelativePath("/users/eric/src/csslint", "/users/eric/style.css"));

In NodeJS you could use path.relative(from, to)

Hi. First off, thanks for a so minimalist function! But any clue why if I feed it e.g. like this...

var dirPath = 'file:///D:/Rai/Projects/LS/ls_script/ScriptResources/ls/docs/'
var pagePath = 'file:///D:/Rai/Projects/LS/ls_script/ScriptResources/ls_script/docs/index.html'
const relPath = getRelativePath(dirPath, pagePath);

...it gives me: ../../docs/ when clearly: ../../ls_script/docs/ should be the expected in this case? Sadly I've not enough JS knowledge to can realize where the issue might be and let alone fix it! But hopefully someone has, so there it is and, if so, thanks in advance!