type alias does not work with classes

[ghost]

I defined a class:

    export class TCommand<TSender, TParam, TThis> 
    {
    }

and created an alias for the closed type:

 export type Command = TCommand<any, any, any>;

now when i try:

 export class NextCommand extends Command {
 }

the compiler says: Error 1 Cannot find name 'Command'

but i can write

  var cmd: Command = null;

without errors,

Expected: to allow using this type alias for inheritance purposes.

[ghost]

  var cmd = new MOD.mvvm.Command();

Does not work too. The same Error!

[mhegazy]

The problem here is extends needs a value to use at run time. The generated code will look something like:

var NextCommand = (function (_super) {
    __extends(NextCommand, _super);
    function NextCommand() {
        _super.apply(this, arguments);
    }
    return NextCommand;
})(Command);  /// notice Command is not defined.

So you can only extend from value. the type alias Command is just an alias, another name you can use in the TS code to refer to type NextCommand and does not exist in the generated code, this is why you can use it in a type position, e.g. var cmd: Command, as this has no implication at run-time.

you can achieve what you want by creating a new class, something like:

class Command extends TCommand<any, any, any> { }

// then this works
export class NextCommand extends Command {
}

// and this:
var cmd = new MOD.mvvm.Command();

for more information about what is value vs. what is type see: https://github.com/Microsoft/TypeScript-Handbook/blob/master/pages/Declaration%20Merging.md#basic-concepts

[ghost]

@mhegazy
Thanks for yor reply. The typescript should create a true alias.
That means instead of "Command" put in that place TCommand - because "Command" is only an alias for TCommand. We need replacements, not a new definition "Command" in the compiler's output.
It should be:
var NextCommand = (function (_super) {
__extends(NextCommand, _super);
function NextCommand() {
_super.apply(this, arguments);
}
return NextCommand;
})(TCommand);

and

 var cmd = new MOD.mvvm.Command();

should be in JavaScript

   var cmd = new MOD.mvvm.TCommand();

P.S. - please, reopen the bug and fix this in new version

[mhegazy]

if you want an alias for both value and type use import:

export import Command = MOD.mvvm.TCommand;

for your request, see #2559

[ghost]

@mhegazy
Is this would work too?

       export import Command = MOD.mvvm.TCommand <>

[mhegazy]

yes.

[ghost]

@mhegazy
No it does not work with generic parameters in TCommand

Better to make the alias work properly

[mhegazy]

No it does not work with generic parameters in TCommand

Please see #2559 .