Suggestion: Preserve line numbers

[streamich]

Currently, this TypeScript code

class PreserveLineNumbers {

    myMethod()  {

    }

}

is transpiled to this JavaScript code

var PreserveLineNumbers = (function () {
    function PreserveLineNumbers() {
    }
    PreserveLineNumbers.prototype.myMethod = function () {
    };
    return PreserveLineNumbers;
})();

However, you could reformat the code, so that the main code pieces keep the same lines, to something like this:

var PreserveLineNumbers = (function () { function PreserveLineNumbers() {}

    PreserveLineNumbers.prototype.myMethod = function () {

    };

return PreserveLineNumbers; })();

P.S. I know about the source maps.

[danquirk]

@CyrusNajmabadi can probably chime in with the specifics around how and why we do this

[CyrusNajmabadi]

The reason we do things in the current manner is that preservation of source code formatting was not a goal with the new compiler rewrite. So, the emitter acts as more of a pretty-printer than a true tree rewrite (i.e. one preserving as much of the original tree as possible).

We do attempt, in very specialized situations, to have our pretty-printing be informed by the original code. For example, we'll try to preserve certain single line constructs. Or we try to keep some newlines in complex expressions to help make the outputted code more readable.

Right now though, we don't do much additional work with keeping newline information around.

If you're interested in improving things here, feel free to contribute a PR. Thanks!

[mhegazy]

Though we can do this for simple transformations, with an added complexity bear in mind; we can not do that for all transformations. consider something like async/await. SourceMaps has been the standard for JS community to deal with these transformations. The TypeScript compiler supports soruce map generation with --soruceMap flag; the generated maps are supported by all major browsers and IDEs.

[namnm]

Sadly we can not have this feature. I implemented a log util to log the message together with the file name and line number. It's work great with babel since babel supports preserve line number. But not typescript.

[nicolasgarnier]

+1, there are many cases of develper tools that do not support courcemaps (for instance often hosted online logs of Functions As A Service providers)