Repository: microsoft/TypeScript
File: src/services/stringCompletions.ts (TypeScript)

Code before:

function getSupportedExtensionsForModuleResolution(compilerOptions: CompilerOptions, typeChecker?: TypeChecker): readonly string[][] {
    /** file extensions from ambient modules declarations e.g. *.css */
    const ambientModulesExtensions = !typeChecker ? [] : mapDefined(typeChecker.getAmbientModules(), module => {
        const name = module.name.slice(1, -1);
        if (!name.startsWith("*.") || name.includes("/")) return;
        return name.slice(1);
    });

    const extensions = [...getSupportedExtensions(compilerOptions), ambientModulesExtensions];
    const moduleResolution = getEmitModuleResolutionKind(compilerOptions);
    return moduleResolutionUsesNodeModules(moduleResolution) ?
        getSupportedExtensionsWithJsonIfResolveJsonModule(compilerOptions, extensions) :
        extensions;
}

/**
 * Takes a script path and returns paths for all potential folders that could be merged with its
 * containing folder via the "rootDirs" compiler option
 */
function getBaseDirectoriesFromRootDirs(rootDirs: string[], basePath: string, scriptDirectory: string, ignoreCase: boolean): readonly string[] {
    // Make all paths absolute/normalized if they are not already
    rootDirs = rootDirs.map(rootDirectory => normalizePath(isRootedDiskPath(rootDirectory) ? rootDirectory : combinePaths(basePath, rootDirectory)));

    // Determine the path to the directory containing the script relative to the root directory it is contained within
    const relativeDirectory = firstDefined(rootDirs, rootDirectory => {
        const rootDirectoryWithSlash = rootDirectory.endsWith("/") ? rootDirectory : rootDirectory + "/";
        return containsPath(rootDirectoryWithSlash, scriptDirectory, basePath, ignoreCase) ? scriptDirectory.substr(rootDirectoryWithSlash.length) : undefined;
    })!; // TODO: GH#18217

    // Now find a path for each potential directory that is to be merged with the one containing the script
    return deduplicate<string>(
        [...rootDirs.map(rootDirectory => combinePaths(rootDirectory, relativeDirectory)), scriptDirectory],
        equateStringsCaseSensitive,
        compareStringsCaseSensitive,
    );
}

function getCompletionEntriesForDirectoryFragmentWithRootDirs(rootDirs: string[], fragment: string, scriptDirectory: string, extensionOptions: ExtensionOptions, compilerOptions: CompilerOptions, host: LanguageServiceHost, exclude: string): readonly NameAndKind[] {
    const basePath = compilerOptions.project || host.getCurrentDirectory();
    const ignoreCase = !(host.useCaseSensitiveFileNames && host.useCaseSensitiveFileNames());
    const baseDirectories = getBaseDirectoriesFromRootDirs(rootDirs, basePath, scriptDirectory, ignoreCase);
    return flatMap(baseDirectories, baseDirectory => arrayFrom(getCompletionEntriesForDirectoryFragment(fragment, baseDirectory, extensionOptions, host, /*moduleSpecifierIsRelative*/ true, exclude).values()));
}

const enum ReferenceKind {
    Filename,
    ModuleSpecifier,
}
/**
 * Given a path ending at a directory, gets the completions for the path, and filters for those entries containing the basename.

Review comment (suggestion):
```suggestion
    const relativeDirectory = firstDefined(rootDirs, rootDirectory => containsPath(rootDirectory, scriptDirectory, basePath, ignoreCase) ? scriptDirectory.substr(rootDirectory.length) : undefined)!; // TODO: GH#18217
```

Code after:

function getSupportedExtensionsForModuleResolution(compilerOptions: CompilerOptions, typeChecker?: TypeChecker): readonly string[][] {
    /** file extensions from ambient modules declarations e.g. *.css */
    const ambientModulesExtensions = !typeChecker ? [] : mapDefined(typeChecker.getAmbientModules(), module => {
        const name = module.name.slice(1, -1);
        if (!name.startsWith("*.") || name.includes("/")) return;
        return name.slice(1);
    });

    const extensions = [...getSupportedExtensions(compilerOptions), ambientModulesExtensions];
    const moduleResolution = getEmitModuleResolutionKind(compilerOptions);
    return moduleResolutionUsesNodeModules(moduleResolution) ?
        getSupportedExtensionsWithJsonIfResolveJsonModule(compilerOptions, extensions) :
        extensions;
}

/**
 * Takes a script path and returns paths for all potential folders that could be merged with its
 * containing folder via the "rootDirs" compiler option
 */
function getBaseDirectoriesFromRootDirs(rootDirs: string[], basePath: string, scriptDirectory: string, ignoreCase: boolean): readonly string[] {
    // Make all paths absolute/normalized if they are not already
    rootDirs = rootDirs.map(rootDirectory => ensureTrailingDirectorySeparator(normalizePath(isRootedDiskPath(rootDirectory) ? rootDirectory : combinePaths(basePath, rootDirectory))));

    // Determine the path to the directory containing the script relative to the root directory it is contained within
    const relativeDirectory = firstDefined(rootDirs, rootDirectory => containsPath(rootDirectory, scriptDirectory, basePath, ignoreCase) ? scriptDirectory.substr(rootDirectory.length) : undefined)!; // TODO: GH#18217

    // Now find a path for each potential directory that is to be merged with the one containing the script
    return deduplicate<string>(
        [...rootDirs.map(rootDirectory => combinePaths(rootDirectory, relativeDirectory)), scriptDirectory].map(baseDir => removeTrailingDirectorySeparator(baseDir)),
        equateStringsCaseSensitive,
        compareStringsCaseSensitive,
    );
}

function getCompletionEntriesForDirectoryFragmentWithRootDirs(rootDirs: string[], fragment: string, scriptDirectory: string, extensionOptions: ExtensionOptions, compilerOptions: CompilerOptions, host: LanguageServiceHost, exclude: string): readonly NameAndKind[] {
    const basePath = compilerOptions.project || host.getCurrentDirectory();
    const ignoreCase = !(host.useCaseSensitiveFileNames && host.useCaseSensitiveFileNames());
    const baseDirectories = getBaseDirectoriesFromRootDirs(rootDirs, basePath, scriptDirectory, ignoreCase);
    return flatMap(baseDirectories, baseDirectory => arrayFrom(getCompletionEntriesForDirectoryFragment(fragment, baseDirectory, extensionOptions, host, /*moduleSpecifierIsRelative*/ true, exclude).values()));
}

const enum ReferenceKind {
    Filename,
    ModuleSpecifier,
}
/**
 * Given a path ending at a directory, gets the completions for the path, and filters for those entries containing the basename.
 */
function getCompletionEntriesForDirectoryFragment(
    fragment: string,