PreTSC

The aim of this project is

• To enable running any arbitrary pre-build step before invoking typescript compiler (tsc).
• The pre-build step should be able to modify/add the typescript files to be compiled without requiring to write those files to disk (and thus creating issues with version control).
• The pre-build step should also be able to modify any other file type, for e.g. an included JS file
• The project should be a drop-in replacement for the TSC compiler
• The project should take care not to introduce any hard dependency on a particular typescript version

By integrating this project in the build tool chain, a variety of use cases can become viable. For e.g.

• Conditional compilation (pre-build step can be written to remove conditional code on the basis of env the build is running on)
• Compile time decorators (pre-build step can be written to replace compile time decorator placeholders with some real typescript code dynamically generated on the basis of decorator position and parameters) -- 'nameOf' operator anyone? -- Logging framework (pre-build step can be written to inject required logger tied to current class name)
• Dependency injection framework (pre-build step can be written to parse typescript files and dump meta information about the interfaces to a separate json file. This json file can then be used by a decorator (e.g. @inject) to inject concrete implementations in place of the mentioned interface)
• And many more...Please feel free to add more use cases if you come across any

How to use

• Add npm devdependency on "pretsc"
• Add a dependency/devdependency on "typescript"
• Add in the working directory from where you invoke the build chain a file name ".tscPreBuildHook.js".

  •   module.exports = function (inMemoryFS) {
          //Use typescript compiler api or any other means to discover the files to be compiled and modified. 
          //For e.g.
          const ts = require("typescript");
          const tsConfig = ts.getParsedCommandLineOfConfigFile(path.resolve("tsconfig.json"), {}, ts.sys);
          const program = ts.createProgram(tsConfig.fileNames, tsConfig.options);
          program.getSourceFiles().forEach((sfNode) => {
              const modifiedContent = ...//process the file
              if(modifiedContent) {
                  //tell pretsc to use the modified content instead of the original file from disk
                  inMemoryFS.addFile(sfNode.fileName, modifiedContent);
              }
          }
      }

  `
• In your build tool chain instead of invoking tsc, invoke pretsc passing the exact same arguments as you would have passed to tsc

Note 1: In the first cut I will not support tsc watch mode

Note 2: I am open to suggestions on the api front and implementation in general