Josh Goldberg
Screenshot of TypeScript's --pretty --watch output showing 2 errors in a file, with colors in file names, error locations, and red squigglies.

TypeScript Contribution Diary: Pretty Error Counts

Apr 6, 201815 minute read

How I helped add an 'errors summary' to TypeScript's command-line --pretty --watch mode.

I’ve started making contributions to TypeScript, working my way up from small error reporting nitpicks to (hopefully!) real syntactic and control flow improvements. These blog posts are documentation of the steps I took to figure out what to do for these contributes & then do them. If you’re thinking of contributing to TypeScript or other large open source libraries, I hope seeing how to delve into a giant foreign monolith is of some help!

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Next post: Identifiers after Numeric Literals

Problem Statement

TypeScript Issue 22124: Provide summary of errors when in — pretty mode? When refactoring large projects, the CLI output from compile runs (both from solo compiles and --watch) can build up pretty quickly. Every time I save a file it feels like the terminal is filling up with a giant sea of angry errors. It would be convenient to have an error around the lines of ”Found 43 errors in 21 files.” at the bottom of each compile to help track progress.

Here’s what a --pretty --watch run with errors looks like now:

Time to debug!

Step 1: Where

Where should TypeScript print the list of error messages?

Presumably, there must be something that prints out each of the error messages in order. The messages only have some of this fancy formatting in --pretty mode, so I figured it’d be easier to look for whatever prints that stuff and see who calls it. Two unique, search-friendly features of --pretty printing are:

I ran a search on ”~” and found a few places in src/compiler/program.ts, all within a function named formatDiagnosticsWithColorAndContext. Perfect!

I put a console.log("Found", diagnostics.length, "errors"); at the bottom of formatDiagnosticsWithColorAndContext and ran the compiler on a project with two errors across two files to see the message being printed twice:

So it looks like formatDiagnosticsWithColorAndContext gets run on every file. Not on every run. Not so perfect. 😦

A full-text search showed it only ever being called in createDiagnosticReporter in src/compiler/watch.ts. That is called by createWatchCompilerHostOfConfigFile, which passes it as a reportDiagnostic parameter to createWatchCompilerHost. Within that function it’s only ever used within a new function named emitFilesAndReportErrorUsingBuilder, which passes it to emitFilesAndReportErrors.

I put the same console message in the new location and again ran the compiler on the same project. This time there was a single message at the end about two complaints:

Hooray! Found the place.

Step 2: How?

How to print all the errors?

I wanted to print both the count of errors and count of files. The first part seemed to already be solved by the count of diagnostics. Remaining to do was to count the unique file names from the diagnostics, create some kind of diagnostic message about the results, and print that message.

2.1: Counting Unique File Names

The Diagnostic type has a field file: SourceFile | undefined and the SourceFile type has a field fileName: string. I figured that was a reasonable source of information on file names. But counting them?

Counting unique objects is generally optimally efficient in JavaScript with a Set. I ran full-text searches on new Set and Set.from but didn’t find anything… which makes sense, as TypeScript works in environments that don’t natively have Set. But surely there must be something in TypeScript for counting unique objects? What about the slightly-older Map?

I tried again searching for new Map and found two generic, useful-looking functions at the top of src/compiler/core.ts:

Maps are useful when you want a dictionary-like object but can’t guarantee you won’t have have to check edge case key names such as ”hasOwnProperty”. That tends to happen when you’re coding a superset of JavaScript.

createMap uses createDictionaryObject internally to satisfy the Map interface. Looks like TypeScript polyfills Map. I haven’t personally executed reliable performance tests to evaluate which is faster, but if TypeScript has a function to choose between the two options, I figured it would make sense to go along with whatever TypeScript used.

Conclusion: use createMap to keep track of unique file names with errors.

2.2: Localizing Error and File Counts

The diagnostic messages stored by TypeScript in diagnosticInformationMap.generated.ts are string templates without any logic for pluralization logic. That’s a little inconvenient as I wanted to print a different message in each of three singular/plural possibilities:

Three new strings it was.

The contribution guide’s section on localization mentions that those strings come from src/compiler/diagnosticMessages.json and are generated by the jake generate-diagnostics command. For example:

"Compilation complete. Watching for file changes.": {
    "category": "Message",
    "code": 6042

The ”code” field is interesting. There are codes around that compilation message from 6001 to 6190, then gaps around. I assumed this was some kind of organization strategy and added the three new messages in 6191-6193:

Later, I thought to click the Diagnostics guidelines link in…

2.3 Diagnostics Printing

Other diagnostics in emitFilesAndReportErrors are printed via a function named reportDiagnostic. I tried using that to print the new summary message but got a different-looking message prefix:

Elsewhere, such as with the nice ”Compilation complete. ...” message, the reportWatchDiagnostic function not available in emitFilesAndReportErrors was used. It was being created locally within createWatchCompilerHost but not passed through to (or visible within) emitFilesAndReportErrors. I changed emitFilesAndReportErrors to take it as a parameter (in addition to system.newLine to pass to the reporter) and use it:

Great that it printed! But— that screenshot of just the two messages was everything. Why no "Starting compilation in watch mode..."?

I went up the call chain for printing messages through createWatchStatusReporter and found a clearScreenIfNotWatchingForFileChanges function clearing the screen on printing each diagnostic message if the message isn’t the specific ”Starting compilation in watch mode...” one. Spooky! 👻

I didn’t understand why this function was made that way (why not only clear the screen when starting a new compilation run?) and didn’t feel comfortable changing it. Spooky behavior like that felt best left alone and asked about in the pull request. I added an array of allowed message codes to not clear on (not TypeScript’s Map shim because it only allows strings as keys).

Step 3: Cleanup

emitFilesAndReportErrors was also being used by performCompilation in src/compiler/tsc.ts. I discovered this when I tried to compile and it yelled at me. That was a little surprising: why is the core tsc compiler using watch-specific logic? More spookiness… 🤔

I couldn’t pass the added reporter in from performCompilation because that’s of type WatchStatusReporter, which is only available in the code that deals with watching logic. That meant I would have to find a way to only report the new diagnostics in watch mode. After hackily fumbling around with optional parameters for a while I ended up cutting & pasting emitFilesAndReportErrors up into three functions:

tsc calls the first two; watch calls all three.

Step 4: Pull Request

Normally before sending out a PR I would write unit tests validating the change. Always test your changes! However, because I didn’t understand the screen clearing behavior, I decided to put up what I had at this point for code review and ask there.

Added an errors summary for —pretty —watch results by JoshuaKGoldberg · Pull Request #22254

The first commit in that PR has everything I mentioned above: e0d067b48f71e545c9bef49ad5fceb0a2a17a0ea.

Step 5: Feedback

Someone asked me to change things! This didn’t happen in the last contribution diary article’s pull request…! 😥!

There were a few big pieces of initial feedback:

5.1: Removing File Counts

Not needing to report file counts actually made my change easier. I removed the function that counted unique files with a Map. It was a little upsetting at first to not have needed knowledge from the createMap investigation… but on the bright side, I now knew more about TypeScript’s internals than I would have otherwise. That’s a good thing! 😇

5.2: Filtering Errors

I had to ask for confirmation on how to filter out non-error diagnostic messages. The method was pretty simple:

diagnostics.filter(diagnostic.category === DiagnosticCategory.Error);

👉 Protip: never be scared to ask for clarification on pull requests! 👈

5.3 Optional Error Summarizer

Specifically, the feedback was:

I would rather modify the existing emitFilesAndReportErrors to take additional parameter to summaryReporter which could either be undefined | reporter and if provided you write the summary using that reporter.

An excuse to undo all the changes in tsc.ts! Perfect.

5.4 Miscellaneous Feedback

There were a couple small points left, but nothing major. One was a preemptive bug fix (fixed 😌) and the other was a unit test comment (unable to do for a technical reason).

A merge from master, a couple of updates, and the PR was accepted! 🙌


Thanks for reading! I hope you got something out of it! 💖

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