Bones/node_modules/systemjs/lib/conditionals.js

/*
 * Conditions Extension
 *
 *   Allows a condition module to alter the resolution of an import via syntax:
 *
 *     import $ from 'jquery/#{browser}';
 *
 *   Will first load the module 'browser' via `SystemJS.import('browser')` and 
 *   take the default export of that module.
 *   If the default export is not a string, an error is thrown.
 * 
 *   We then substitute the string into the require to get the conditional resolution
 *   enabling environment-specific variations like:
 * 
 *     import $ from 'jquery/ie'
 *     import $ from 'jquery/firefox'
 *     import $ from 'jquery/chrome'
 *     import $ from 'jquery/safari'
 *
 *   It can be useful for a condition module to define multiple conditions.
 *   This can be done via the `|` modifier to specify an export member expression:
 *
 *     import 'jquery/#{./browser.js|grade.version}'
 *
 *   Where the `grade` export `version` member in the `browser.js` module  is substituted.
 *
 *
 * Boolean Conditionals
 *
 *   For polyfill modules, that are used as imports but have no module value,
 *   a binary conditional allows a module not to be loaded at all if not needed:
 *
 *     import 'es5-shim#?./conditions.js|needs-es5shim'
 *
 *   These conditions can also be negated via:
 *     
 *     import 'es5-shim#?~./conditions.js|es6'
 *
 */

  function parseCondition(condition) {
    var conditionExport, conditionModule, negation;

    var negation = condition[0] == '~';
    var conditionExportIndex = condition.lastIndexOf('|');
    if (conditionExportIndex != -1) {
      conditionExport = condition.substr(conditionExportIndex + 1);
      conditionModule = condition.substr(negation, conditionExportIndex - negation) || '@system-env';
    }
    else {
      conditionExport = null;
      conditionModule = condition.substr(negation);
    }

    return {
      module: conditionModule,
      prop: conditionExport,
      negate: negation
    };
  }

  function serializeCondition(conditionObj) {
    return (conditionObj.negate ? '~' : '') + conditionObj.module + (conditionObj.prop ? '|' + conditionObj.prop : '');
  }

  function resolveCondition(conditionObj, parentName, bool) {
    return this['import'](conditionObj.module, parentName)
    .then(function(m) {
      if (conditionObj.prop)
        m = readMemberExpression(conditionObj.prop, m);
      else if (typeof m == 'object' && m + '' == 'Module')
        m = m['default'];

      if (bool && typeof m != 'boolean')
        throw new TypeError('Condition ' + serializeCondition(conditionObj) + ' did not resolve to a boolean.');

      return conditionObj.negate ? !m : m;
    });
  }

  var interpolationRegEx = /#\{[^\}]+\}/;
  function interpolateConditional(name, parentName) {
    // first we normalize the conditional
    var conditionalMatch = name.match(interpolationRegEx);

    if (!conditionalMatch)
      return Promise.resolve(name);

    var conditionObj = parseCondition(conditionalMatch[0].substr(2, conditionalMatch[0].length - 3));

    // in builds, return normalized conditional
    if (this.builder)
      return this['normalize'](conditionObj.module, parentName)
      .then(function(conditionModule) {
        conditionObj.module = conditionModule;
        return name.replace(interpolationRegEx, '#{' + serializeCondition(conditionObj) + '}');
      });

    return resolveCondition.call(this, conditionObj, parentName, false)
    .then(function(conditionValue) {
      if (typeof conditionValue !== 'string')
        throw new TypeError('The condition value for ' + name + ' doesn\'t resolve to a string.');

      if (conditionValue.indexOf('/') != -1)
        throw new TypeError('Unabled to interpolate conditional ' + name + (parentName ? ' in ' + parentName : '') + '\n\tThe condition value ' + conditionValue + ' cannot contain a "/" separator.');

      return name.replace(interpolationRegEx, conditionValue);
    });
  }

  function booleanConditional(name, parentName) {
    // first we normalize the conditional
    var booleanIndex = name.lastIndexOf('#?');

    if (booleanIndex == -1)
      return Promise.resolve(name);

    var conditionObj = parseCondition(name.substr(booleanIndex + 2));

    // in builds, return normalized conditional
    if (this.builder)
      return this['normalize'](conditionObj.module, parentName)
      .then(function(conditionModule) {
        conditionObj.module = conditionModule;
        return name.substr(0, booleanIndex) + '#?' + serializeCondition(conditionObj);
      });

    return resolveCondition.call(this, conditionObj, parentName, true)
    .then(function(conditionValue) {
      return conditionValue ? name.substr(0, booleanIndex) : '@empty';
    });
  }

  // normalizeSync does not parse conditionals at all although it could
  hook('normalize', function(normalize) {
    return function(name, parentName, parentAddress) {
      var loader = this;
      return booleanConditional.call(loader, name, parentName)
      .then(function(name) {
        return normalize.call(loader, name, parentName, parentAddress);
      })
      .then(function(normalized) {
        return interpolateConditional.call(loader, normalized, parentName);
      });
    };
  });