microsoft/TypeScript · error · Error

Only declaration files should be generated when emitDeclarat

Error message

Only declaration files should be generated when emitDeclarationOnly:true

What it means

Thrown by prepareDeclarationCompilationContext when options.declaration and options.emitDeclarationOnly are both set and compilation produced no errors, yet the result violates emitDeclarationOnly semantics: either JS files were emitted (result.js.size > 0) or, when noEmit is false, no declaration files were produced (result.dts.size === 0). The invariant is that with emitDeclarationOnly the compiler must emit only .d.ts files.

Source

Thrown at src/harness/harnessIO.ts:403

        declOtherFiles: TestFile[];
        harnessSettings: TestCaseParser.CompilerSettings & HarnessOptions | undefined;
        options: ts.CompilerOptions;
        currentDirectory: string;
    }

    export function prepareDeclarationCompilationContext(
        inputFiles: readonly TestFile[],
        otherFiles: readonly TestFile[],
        result: compiler.CompilationResult,
        harnessSettings: TestCaseParser.CompilerSettings & HarnessOptions,
        options: ts.CompilerOptions,
        // Current directory is needed for rwcRunner to be able to use currentDirectory defined in json file
        currentDirectory: string | undefined,
    ): DeclarationCompilationContext | undefined {
        if (options.declaration && result.diagnostics.length === 0) {
            if (options.emitDeclarationOnly) {
                if (result.js.size > 0 || (result.dts.size === 0 && !options.noEmit)) {
                    throw new Error("Only declaration files should be generated when emitDeclarationOnly:true");
                }
            }
            else if (result.dts.size !== result.getNumberOfJsFiles(/*includeJson*/ false)) {
                throw new Error("There were no errors and declFiles generated did not match number of js files generated");
            }
        }

        const declInputFiles: TestFile[] = [];
        const declOtherFiles: TestFile[] = [];

        // if the .d.ts is non-empty, confirm it compiles correctly as well
        if (options.declaration && result.diagnostics.length === 0 && result.dts.size > 0) {
            ts.forEach(inputFiles, file => addDtsFile(file, declInputFiles));
            ts.forEach(otherFiles, file => addDtsFile(file, declOtherFiles));
            return { declInputFiles, declOtherFiles, harnessSettings, options, currentDirectory: currentDirectory || harnessSettings.currentDirectory };
        }

        function addDtsFile(file: TestFile, dtsFiles: TestFile[]) {

View on GitHub (pinned to b465fdbfe1)

Solutions

1. Verify the compiler actually respects emitDeclarationOnly for the given input — if JS is being emitted, this may be a real compiler bug to file.
2. Check that noEmit is not set; if noEmit is intended, no .d.ts will be produced and the second clause triggers.
3. Inspect result.js and result.dts contents to see which files were unexpectedly emitted (or missing).

Example fix

// before — test options conflict
// @declaration: true
// @emitDeclarationOnly: true
// @noEmit: true
// after — remove noEmit so declarations are emitted
// @declaration: true
// @emitDeclarationOnly: true

// Sanity-check emit invariants before baselining declarations
if (options.declaration && options.emitDeclarationOnly) {
    if (options.noEmit) throw new Error("emitDeclarationOnly requires noEmit:false");
    if (result.js.size > 0) throw new Error("Compiler emitted JS under emitDeclarationOnly");
}

When it happens

Trigger: A compiler test sets declaration:true and emitDeclarationOnly:true but the compilation result contains emitted JS, or produces zero .d.ts files while noEmit is false. This indicates the compiler output diverges from the declared intent.

Common situations: A test harness bug, a compiler regression that emits JS despite emitDeclarationOnly, or a test configuration where emitDeclarationOnly was set but noEmit was also unexpectedly true. Also seen when a transform/plugin interferes with emit filtering.

Related errors

• There were no errors and declFiles generated did not match n
• No sourcemap files should be generated if inlineSourceMaps w
• Number of sourcemap files should be same as js files.
• Cannot have undefined value for compiler option '${name}'.
• Unknown value '${value}' for compiler option '${name}'.

AI-assisted analysis of microsoft/TypeScript@b465fdbfe1 (2026-08-12). Data as JSON: /api/errors/5460f7922332fc64. Report an issue: GitHub.