index.js

'use strict';

var loaderUtils = require('loader-utils');
var SourceMap = require('source-map');
var esprima = require('esprima');

var prefix = '\'use strict\';\n\n';

// used https://github.com/webpack/imports-loader/ as an example
module.exports = function(source, map) {
  if (this.cacheable) {
    this.cacheable();
  }

  var SourceMapGenerator = SourceMap.SourceMapGenerator;
  var defaultOptions = { generateSourceMapFromTokens: false };
  var loaderOptions = Object.assign(defaultOptions, loaderUtils.getOptions(this) || {});
  var currentRequest = loaderUtils.getCurrentRequest(this);

  if (loaderOptions.generateSourceMapFromTokens) {
    var prefixLineLength = 2;
    var output = prefix + source;
    var tokens = esprima.tokenize(output, { loc: true });
    var generator = new SourceMapGenerator({ name: currentRequest });

    generator.setSourceContent(currentRequest, source);

    for (var i = prefixLineLength; i < tokens.length; i++) {
      var token = tokens[i];

      generator.addMapping({
        source: currentRequest,
        original: { line: token.loc.start.line - prefixLineLength, column: token.loc.start.column },
        generated: { line: token.loc.start.line, column: token.loc.start.column }
      });
    }

    this.callback(null, output, generator.toJSON());

    return;
  } else if (map) {
    var SourceNode = SourceMap.SourceNode;
    var SourceMapConsumer = SourceMap.SourceMapConsumer;
    var node = SourceNode.fromStringWithSourceMap(source, new SourceMapConsumer(map));

    node.prepend(prefix);

    var result = node.toStringWithSourceMap({ file: currentRequest });

    this.callback(null, result.code, result.map.toJSON());

    return;
  }

  return prefix + source;
};