index.js

var loaderUtils = require("loader-utils");
var path = require('path');
var jsesc = require('jsesc');

module.exports = function (content) {
    this.cacheable && this.cacheable();

    var options = loaderUtils.getOptions(this) || {};
    var relativeTo = getAndInterpolateOption.call(this, 'relativeTo', '');
    var prefix = getAndInterpolateOption.call(this, 'prefix', '');
    var absolute = false;
    var pathSep = options.pathSep || '/';
    var resource = this.resource;
    var pathSepRegex = new RegExp(escapeRegExp(path.sep), 'g');

    // if a unix path starts with // we treat is as an absolute path e.g. //Users/wearymonkey
    // if we're on windows, then we ignore the / prefix as windows absolute paths are unique anyway e.g. C:\Users\wearymonkey
    if (relativeTo[0] === '/') {
        if (path.sep === '\\') { // we're on windows
            relativeTo = relativeTo.substring(1);
        } else if (relativeTo[1] === '/') {
            absolute = true;
            relativeTo = relativeTo.substring(1);
        }
    }

    // normalise the path separators
    if (path.sep !== pathSep) {
        relativeTo = relativeTo.replace(pathSepRegex, pathSep);
        prefix = prefix.replace(pathSepRegex, pathSep);
        resource = resource.replace(pathSepRegex, pathSep)
    }

    var relativeToIndex = resource.indexOf(relativeTo);
    if (relativeToIndex === -1 || (absolute && relativeToIndex !== 0)) {
        throw new Error('The path for file doesn\'t contain relativeTo param');
    }

    // a custom join of prefix using the custom path sep
    var filePath = [prefix, resource.slice(relativeToIndex + relativeTo.length)]
        .filter(Boolean)
        .join(pathSep)
        .replace(new RegExp(escapeRegExp(pathSep) + '+', 'g'), pathSep);

    return "module.exports = '"+jsesc(filePath)+"';\n";

    function getAndInterpolateOption(optionKey, def) {
        return options[optionKey]
            ? loaderUtils.interpolateName(this, options[optionKey], {
                context: options.context,
                content: content,
                regExp: options[optionKey + 'RegExp'] || options['regExp']
            })
            : def
    }

    // source: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions#Using_Special_Characters
    function escapeRegExp(string) {
        return string.replace(/([.*+?^=!:${}()|\[\]\/\\])/g, "\\$1");
    }
};