--outFile is BAD

Its a bad idea for you to use because of the following reasons:

  • Runtime Errors
  • Fast compile
  • Global scope
  • Hard to analyze
  • Hard to scale
  • _references
  • Code reuse
  • Multiple Targets
  • Isolated Compile

Runtime Errors

If your code depends on any form of js ordering you will get random errors at runtime.

  • class inheritance can break at runtime.

Consider foo.ts: 

class Foo {

}

and a bar.ts:

class Bar extends Foo {

}

If you fail to compile it in correct order e.g. perhaps alphabetically tsc bar.ts foo.ts the code will compile fine but error at runtime with ReferenceError.

  • module splitting can fail at runtime.

Consider foo.ts: 

module App {
    export var foo = 123;
}

And bar.ts: 

module App {
    export var bar = foo + 456;
}

If you fail to compile it in correct order e.g. perhaps alphabetically tsc bar.ts foo.ts the code will compile fine but silently fail at runtime with bar set to NaN.

Fast compile

If you use --out then single .ts files cannot be codegened into single .js files in isolation without unnecessary hacks. --out essentially forces a slower incremental build.

Also source maps are positionally sensitive and run-length encoded so most of the map has to be rebuilt on a recompile if you use source maps (which you should!). At high-10s to 100s kloc combined it’s going to get slow.

Global Scope

Sure you can use name spaces but its still on window if you run it in the browser. Namespaces are just an unnecessary workaround. Also /// <reference comments introduce an global context in your code that can get hard to maintain.

Also if your company has several teams working independently and then someone decides to try integrating two independently written apps there is a high likelihood of a name conflict.

Hard to analyze

We wish to provide more code analysis tools. These will be easier if you provide us with the dependency chain (implicitly there on a silver platter using external modules).

Also its not just the dev tools that have a hard time making sense of the code. The next human needs to understand a lot of the code base before they start to understand where stuff is actually imported from. Using internal modules also makes code difficult to review in isolation e.g. on github.

Hard to scale

Really just a result of random runtime errors + slower and slower compile times + difficulty in understanding someone else's code.

_references.ts

Isn't supported by tsconfig.json : https://github.com/Microsoft/TypeScript/issues/2472#issuecomment-85330803 You'll have to manually sort the files array.

Code reuse

If you want to reuse a portion of your code in another project, with all that implicit dependency management, it will be difficult to port it over without potential runtime errors.

Multiple Targets

Also if you decide to reuse your browser code in something like nodejs (e.g. for testing APIs) you are going to need to port it over to a module system or come up with ugly hacks to make the nodejs global your new global scope (i.e. window).

Isolated Compile

Files cannot be compiled in isolation. E.g. consider a.ts: 

module M {
  var s = t;
}

Will have different output depending upon whether there is a b.ts of the form: 

module M {
  export var t = 5;
}

or 

var t = 5;

So a.ts cannot be compiled in isolation.

Summary

--out is really the job of some build tool. And even such a build tool can benefit from the dependency mentions provided by external modules. So we recommend you use external modules and then let the build tool create a single .js for you if you so desire.

https://twitter.com/nycdotnet/status/613705850574778368

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