Feature request [compiler API]: expose project build logic to public API

[jsamr]

Scenario As a developer, I would like to use a very high level project compiler API for build purposes. It would be somehow symmetric to what tsc allows but in a programmatic way.
Although the compiler API is public, the project logic for file resolution isn't (i.e. : how the rootFileNames are resolved before passed to createProgram).
I would like something simpler that would allow me to intercept a serialized tsconfig and alter its properties (that would somehow provide a workaround for #20110). I could also chose to merge default excludes by importing a symbol (would fix #20108 )

Suggestion Perhaps abstracting the tsc logic would allow two interfaces to be implemented: API and CLI. This new layer could be labeled "build".

Speculative API sample

import { createProject, createProgramFromProject, Project, DEFAULT_EXCLUDE } from '@typescript/build'

function tsConfigInterceptor (tsconfig: any) {
  return {
    ...tsconfig,
    exclude: ['lib', DEFAULT_EXCLUDE].concat(tsconfig.exclude ? tsconfig.exclude : [])
  }
}

const project: Project = createProject(process.cwd(), tsConfigInterceptor)
const program = createProgramFromProject(project)
program.emit()

New definitions

export type TsConfigInterceptor = (tsconfig: any)  => any

export const DEFAULT_EXCLUDE = Symbol('DEFAULT_EXCLUDE')

/**
* @param tsconfigPath Root dir where tsconfig.json exists or tsconfig file name
* @param [interceptor] A callback that received the object deserialized from tsconfig 
* file and returns a new one
*/
export function createProject(tsconfigPath: string, interceptor?: TsConfigInterceptor): Project {
  // ...
}

/**
* @param project The project from which a program shall be created
*/
export function createProgramFromProject (project: Project): Program {
  // ...
}

[mhegazy]

Although the compiler API is public, the project logic for file resolution isn't (i.e. : how the rootFileNames are resolved before passed to createProgram).

All the tsconfig.json parsing function are public.. please see:

• findConfigFile to locate an applicable tsconfig.json file
• parseJsonText to parse the text into a JsonSourceFile
• parseJsonSourceFileConfigFileContent to parse a json file and pass a set of overrides

Here is the part of the compiler that use these functions for reference: https://github.com/Microsoft/TypeScript/blob/e934c305136ac3b8a980bfa089e18b301ce999e4/src/compiler/watch.ts#L65-L91

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.