Multiple `__extends` output across various files

[whitneyit]

Given the following files

// animal.ts
class Animal {}
export = Animal;

// mammal.ts
import Animal = require('./animal');
class Mammal extends Animal {}
export = Mammal;

// human.ts
import Mammal = require('./mammal');
class Human extends Mammal {}
export = Human;

And the following command, either --module flag will work for this example

$ tsc --module amd human.ts

The following files are output

define(["require", "exports"], function (require, exports) {
    var Animal = (function () {
        function Animal() {
        }
        return Animal;
    })();
    return Animal;
});

var __extends = this.__extends || function (d, b) {
    for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p];
    function __() { this.constructor = d; }
    __.prototype = b.prototype;
    d.prototype = new __();
};
define(["require", "exports", './animal'], function (require, exports, Animal) {
    var Mammal = (function (_super) {
        __extends(Mammal, _super);
        function Mammal() {
            _super.apply(this, arguments);
        }
        return Mammal;
    })(Animal);
    return Mammal;
});

var __extends = this.__extends || function (d, b) {
    for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p];
    function __() { this.constructor = d; }
    __.prototype = b.prototype;
    d.prototype = new __();
};
define(["require", "exports", './mammal'], function (require, exports, Mammal) {
    var Human = (function (_super) {
        __extends(Human, _super);
        function Human() {
            _super.apply(this, arguments);
        }
        return Human;
    })(Mammal);
    return Human;
});

As you can see, a lot of repetition of the __extends functions starts to appear when working with a larger codebase.

My question is this, would the TypeScript team be open to a flag that can be passed to tsc that will generate something lke the following

define(["require", "exports", "__extends", './mammal'], function (require, exports, __extends, Mammal) {
    var Human = (function (_super) {
        __extends(Human, _super);
        function Human() {
            _super.apply(this, arguments);
        }
        return Human;
    })(Mammal);
    return Human;
});

This will be an option that will need to be bought into by the user executing the tsc command. This is not a suggestion to refactor the existing --module flag, maybe --customExports.

The onus would then be on the develop to wire up an __extends module themselves. A module that could be reworked to suit there needs should they see fit. See #1193. Some boilerplate could be generated and printed to stdout similar to the way that behat squawks about missing test snippets.

// You can implement the __extends function with the following snippet.
define('__extends', function () {
  return function (d, b) {
    for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p];
    function __() { this.constructor = d; }
     __.prototype = b.prototype;
    d.prototype = new __();
  };
});

The advantage of this solution is the obvious reduction in repetitive code, as well allowing customisation of the __extends function, and helping teams hit that elusive 100% coverage.

Some references
https://typescript.codeplex.com/workitem/2002
http://stackoverflow.com/questions/22155106

[RyanCavanaugh]

This has been a common request and we already had the notion of not emitting __extends when an implementation was in scope.

[whitneyit]

Thanks for the feedback. Cheers

[whitneyit]

This has been covered in #2901