refactor: simplify hosts to directly assign tsModule.sys where possible

- no need to duplicate types this way, which can and have changed over
  time -- it's always the same typings this way

- also reorganize `host.ts` to have similar categories of functions near
  each other, instead of a mix of functions wherever
  - similar to how I organized the tests for `host` as well
- shrink the code a bit this way too

- add a comment about `getDefaultLibFileName`'s confusing naming
  pointing to the TS issues about how this is an old mistake but
  changing it now would be breaking

- this is also how the TS Wiki recommends setting up hosts: https://github.com/microsoft/TypeScript/wiki/Using-the-Compiler-API#incremental-build-support-using-the-language-services
  - NOTE: because of how `tsproxy` works to support alternate TS
    implementations, this does require that `tsModule` _exists_ at the
    time of instantiation, i.e. that `setTypescriptModule` has already
    been called
    - for `host.ts`, `LanguageServiceHost` is only instantiated after
      `setTypescriptModule`, but for `diagnostics-format-host.ts`, it is
      immediately instantiated (at the bottom of the file), hence why
      `getCurrentDirectory` can't just be assigned to `tsModule.sys`
      - there is a way to fix this, but the refactoring is more complex
        as it would require creating in `index.ts` and then passing it
        as an argument -- would want to refactor more at that point too,
        so leaving that out for now in this otherwise small, isolated
        refactor
  - for a different, but similar reason, the `host.trace` tests have to
    mock `console` instead of just `console.log`, since `trace` would
    just be set to the old, unmocked `console.log` otherwise
    - as it's assigned directly to `console.log` now

__tests__/host.spec.ts

@@ -9,6 +9,11 @@ import { LanguageServiceHost } from "../src/host";
 
 setTypescriptModule(ts);
 
+// mock for host.trace
+(global as any).console = {
+	log: jest.fn(),
+};
+
 const defaultConfig = { fileNames: [], errors: [], options: {} };
 
 const unaryFunc = "const unary = (x: string): string => x.reverse()";
@@ -72,7 +77,6 @@ test("LanguageServiceHost", async () => {
 	expect(host.getTypeRootsVersion()).toEqual(0);
 
 	// mock out trace
-	console.log = jest.fn();
 	host.trace('test log');
 	expect(console.log).toHaveBeenCalledWith('test log');
 });

src/diagnostics-format-host.ts

@@ -10,15 +10,8 @@ export class FormatHost implements tsTypes.FormatDiagnosticsHost
 		return tsModule.sys.getCurrentDirectory();
 	}
 
-	public getCanonicalFileName(fileName: string): string
-	{
-		return path.normalize(fileName);
-	}
-
-	public getNewLine(): string
-	{
-		return tsModule.sys.newLine;
-	}
+	public getCanonicalFileName = path.normalize;
+	public getNewLine = () => tsModule.sys.newLine;
 }
 
 export const formatHost = new FormatHost();

src/host.ts

@@ -6,16 +6,14 @@ import { TransformerFactoryCreator } from "./ioptions";
 
 export class LanguageServiceHost implements tsTypes.LanguageServiceHost
 {
-	private cwd: string;
 	private snapshots: { [fileName: string]: tsTypes.IScriptSnapshot } = {};
 	private versions: { [fileName: string]: number } = {};
 	private service?: tsTypes.LanguageService;
 	private fileNames: Set<string>;
 
-	constructor(private parsedConfig: tsTypes.ParsedCommandLine, private transformers: TransformerFactoryCreator[], cwd: string)
+	constructor(private parsedConfig: tsTypes.ParsedCommandLine, private transformers: TransformerFactoryCreator[], private cwd: string)
 	{
 		this.fileNames = new Set(parsedConfig.fileNames);
-		this.cwd = cwd;
 	}
 
 	public reset()
@@ -58,10 +56,7 @@ export class LanguageServiceHost implements tsTypes.LanguageServiceHost
 		return undefined;
 	}
 
-	public getCurrentDirectory()
-	{
-		return this.cwd;
-	}
+	public getScriptFileNames = () => Array.from(this.fileNames.values());
 
 	public getScriptVersion(fileName: string)
 	{
@@ -70,61 +65,6 @@ export class LanguageServiceHost implements tsTypes.LanguageServiceHost
 		return (this.versions[fileName] || 0).toString();
 	}
 
-	public getScriptFileNames()
-	{
-		return Array.from(this.fileNames.values());
-	}
-
-	public getCompilationSettings(): tsTypes.CompilerOptions
-	{
-		return this.parsedConfig.options;
-	}
-
-	public getDefaultLibFileName(opts: tsTypes.CompilerOptions)
-	{
-		return tsModule.getDefaultLibFilePath(opts);
-	}
-
-	public useCaseSensitiveFileNames(): boolean
-	{
-		return tsModule.sys.useCaseSensitiveFileNames;
-	}
-
-	public readDirectory(path: string, extensions?: string[], exclude?: string[], include?: string[]): string[]
-	{
-		return tsModule.sys.readDirectory(path, extensions, exclude, include);
-	}
-
-	public readFile(path: string, encoding?: string): string | undefined
-	{
-		return tsModule.sys.readFile(path, encoding);
-	}
-
-	public fileExists(path: string): boolean
-	{
-		return tsModule.sys.fileExists(path);
-	}
-
-	public realpath(path: string): string
-	{
-		return tsModule.sys.realpath!(path); // this exists in the default implementation: https://github.com/microsoft/TypeScript/blob/ab2523bbe0352d4486f67b73473d2143ad64d03d/src/compiler/sys.ts#L1288
-	}
-
-	public getTypeRootsVersion(): number
-	{
-		return 0;
-	}
-
-	public directoryExists(directoryName: string): boolean
-	{
-		return tsModule.sys.directoryExists(directoryName);
-	}
-
-	public getDirectories(directoryName: string): string[]
-	{
-		return tsModule.sys.getDirectories(directoryName);
-	}
-
 	public getCustomTransformers(): tsTypes.CustomTransformers | undefined
 	{
 		if (this.service === undefined || this.transformers === undefined || this.transformers.length === 0)
@@ -151,7 +91,19 @@ export class LanguageServiceHost implements tsTypes.LanguageServiceHost
 		return transformer;
 	}
 
-	public trace(line: string) {
-		console.log(line)
-	}
+	public getCompilationSettings = () => this.parsedConfig.options;
+	public getTypeRootsVersion = () => 0;
+	public getCurrentDirectory = () => this.cwd;
+
+	public useCaseSensitiveFileNames = () => tsModule.sys.useCaseSensitiveFileNames;
+	public getDefaultLibFileName = tsModule.getDefaultLibFilePath; // confusing naming: https://github.com/microsoft/TypeScript/issues/35318
+
+	public readDirectory = tsModule.sys.readDirectory;
+	public readFile = tsModule.sys.readFile;
+	public fileExists = tsModule.sys.fileExists;
+	public directoryExists = tsModule.sys.directoryExists;
+	public getDirectories = tsModule.sys.getDirectories;
+	public realpath = tsModule.sys.realpath!; // this exists in the default implementation: https://github.com/microsoft/TypeScript/blob/ab2523bbe0352d4486f67b73473d2143ad64d03d/src/compiler/sys.ts#L1288
+
+	public trace = console.log;
 }