LanguageServerHost fileExists / readFile no longer optional?

[tomv564]

TypeScript Version: 2.4.2, also in master

This issue was encountered when upgrading javascript-typescript-langserver from typescript 2.3.4 (see sourcegraph/javascript-typescript-langserver#356)

Code

When implementing a LanguageServiceHost without implementing optional fileExists or readFile, the compilerHost instance silently skips import statements:

export class InMemoryLanguageServiceHost implements ts.LanguageServiceHost {
  // only required members implemented here..
}

The above implementation compiled without errors against 2.4.2, but all import statements in source files added to the language server failed to resolve, resulting in diagnostics, missing definitions, completions, etc.

Expected behavior:

Either:

• Compilation failure indicating that LanguageServiceHost is now incomplete
• Module resolver falls back to ModuleResolutionHost to access fileExists / readFile

Actual behavior:

Imports fail to resolve where they do exist, in nodeModuleResolverWorker, following the path:

nodeLoadModuleByRelativeName -> loadModuleFromFile
loadModuleFromFile ->  tryExtension -> tryFile -> state.host.fileExists

Debugger screenshot showing the issue at runtime at https://github.com/Microsoft/TypeScript/blob/21bbdd33c51f07a46663e9b50fff62e9c007c806/src/services/services.ts#L1166

The typings published at https://github.com/Microsoft/TypeScript/blob/master/lib/typescript.d.ts#L3764-L3765 show that the two members are still optional.

This dependency appears to have been introduced at: 2ec92b9

[mhegazy]

@andy-ms can you take a look.

[DanielRosenwasser]

Looks like a dupe of #16616

[tomv564]

Thanks for the quick update. A comment about why those members are optional in the published typescript.d.ts might be nice for future implementers?

I'm satisfied with the explanation, feel free to close!