[Feature]: Make it easier to extend an existing tsconfig

🚀 Feature Proposal

I tried extending an existing tsconfig.json file by parsing it and then extending it like so:

const ts = require('typescript');
const loaded = ts.readConfigFile('./tsconfig.json', ts.sys.readFile);
const basePath = path.dirname(configPath); // equal to "getDirectoryPath" from ts, at least in our case.
const { options } = ts.parseJsonConfigFileContent(loaded.config, ts.sys, basePath);


module.exports = {
	// […]

	// A set of global variables that need to be available in all test environments
	globals: {
		'ts-jest': {
			tsconfig: {
				...options,
				importHelpers: false,
			}
		},
	},

	// […]
};

However, the parseJsonConfigFileContent transforms some of the strings in compilerOptions to their enum values, like target, module, jsx, and moduleResolution, which will yield errors such as:

    error TS5024: Compiler option 'target' requires a value of type string.
    error TS5024: Compiler option 'module' requires a value of type string.

I propose adding a ts-jest config option compilerOptions and deprecate setting config options directly on tsconfig. Instead tsconfig can either point to a path, in which case the compilerOptions passed (if any) extend the config from that path or to false, in which case the default config is used (as before) and compilerOptions (if any) can be used to override it.

Motivation

Extending an existing tsconfig.json requires either:

• creating a custom tsconfig file that extends from the other (bad because it needlessly requires a file, even when I only want to override a single config)
• or: copy-pasting the entire compilerOptions into jest.config.js (obviously bad)
• or: reading a tsconfig.json using ts.readConfigFile only (bad because this doesn’t follow inheritance defined using extends)

It would be nice if this was a built-in feature of ts-jest.

Example

module.exports = {
	// […]

	// A set of global variables that need to be available in all test environments
	globals: {
		'ts-jest': {
			tsconfig: './tsconfig.json',
			compilerOptions: {
				importHelpers: false
			},
		},
	},

	// […]
};

[viT-1]

@PhiLhoSoft You also can try use paths configuration for jest moduleNameMapper with jest-resolver-tsconfig-paths like this.

[dzearing]

Just hit this. I have a monorepo full of packages (about 150) I want to use each package’s default tsconfig but override a few things for jest runs. There isn’t a great way to do this without creating test-only configs for each package.

In my ts-jest config:

{
  // use the project config where jest is run.
  tsconfig: './tsconfig.json'
}

So… how can I turn on cjs and turn off isolatedModules for jest runs but still use everything else from the config?

I was hoping tsconfig was array, but it’s not. All I can think of is to drop a temporary config on disk which extends the original and reference that. Which is undesirable.

If we had what the OP suggested which was separate compilerOptions, that’d work great, but yeah it requires merging.