The Tor Browser / file revision

 /* This Source Code Form is subject to the terms of the Mozilla Public

  * License, v. 2.0. If a copy of the MPL was not distributed with this

  * file, You can obtain one at http://mozilla.org/MPL/2.0/.

  */

 "use strict";

 module.metadata = {

   "stability": "experimental"

 };

 const method = require("method/core");

 // Utility function that is just an enhancement over `method` to

 // allow predicate based dispatch in addition to polymorphic

 // dispatch. Unfortunately polymorphic dispatch does not quite

 // cuts it in the world of XPCOM where no types / classes exist

 // and all the XUL nodes share same type / prototype.

 // Probably this is more generic and belongs some place else, but

 // we can move it later once this will be relevant.

 let dispatcher = hint => {

   const base = method(hint);

   // Make a map for storing predicate, implementation mappings.

   let implementations = new Map();

   // Dispatcher function goes through `predicate, implementation`

   // pairs to find predicate that matches first argument and

   // returns application of arguments on the associated

   // `implementation`. If no matching predicate is found delegates

   // to a `base` polymorphic function.

   let dispatch = (value, ...rest) => {

     for (let [predicate, implementation] of implementations) {

       if (predicate(value))

         return implementation(value, ...rest);

     }

     return base(value, ...rest);

   };

   // Expose base API.

   dispatch.define = base.define;

   dispatch.implement = base.implement;

   dispatch.toString = base.toString;

   // Add a `when` function to allow extending function via

   // predicates.

   dispatch.when = (predicate, implementation) => {

     if (implementations.has(predicate))

       throw TypeError("Already implemented for the given predicate");

     implementations.set(predicate, implementation);

   };

   return dispatch;

 };

 exports.dispatcher = dispatcher;