facebook/flow · error · Error

Unexpected function parameter ${param.type}

Error message

Unexpected function parameter ${param.type}

What it means

Thrown by flow-api-translator while converting a TypeScript declaration AST into a Flow definition. When mapping a function signature's parameter list it handles only Identifier, ArrayPattern, and ObjectPattern parameters (plus a leading `this` Identifier and a trailing RestElement); any other parameter node type reaches the else branch and throws, naming the node type.

Source

Thrown at packages/flow-api-translator/src/TSDefToFlowDef.js:1023

          } else if (
            param.type === 'ArrayPattern' ||
            param.type === 'ObjectPattern'
          ) {
            return constructFlowNode<FlowESTree.FunctionTypeParam>({
              type: 'FunctionTypeParam',
              name: constructFlowNode<FlowESTree.Identifier>({
                type: 'Identifier',
                name: `$$param${i}$`,
                optional: false,
                typeAnnotation: null,
              }),
              optional: Boolean(param.optional),
              typeAnnotation: Transform.TSTypeAnnotationOpt(
                param.typeAnnotation?.typeAnnotation,
              ),
            });
          } else {
            throw new Error(`Unexpected function parameter ${param.type}`);
          }
        }),
      };
    }

    static TSImportType(
      node: TSESTree.TSImportType,
    ): FlowESTree.TypeAnnotationType {
      const source =
        node.argument ??
        (node.source == null
          ? null
          : ({
              type: 'TSLiteralType',
              loc: node.source.loc,
              literal: node.source,
            } as TSESTree.TSLiteralType));
      if (source == null) {

View on GitHub (pinned to d1341dac89)

Solutions

1. Update flow-api-translator to the latest version — newer versions handle more parameter node types
2. Grep the input .d.ts for constructor parameter properties (`constructor(private|protected|public ...)`) and rewrite them as an explicit property plus a plain parameter, since the message names the offending node type
3. If you build the TS AST yourself, normalize parameters to Identifier/ArrayPattern/ObjectPattern before calling the transform
4. If the input is valid TypeScript, file an issue with the minimal .d.ts that reproduces it

Example fix

// before (input .d.ts)
declare class Foo {
  constructor(private name: string): void;
}

// after (input .d.ts)
declare class Foo {
  name: string;
  constructor(name: string): void;
}

const SUPPORTED_PARAMS = new Set(['Identifier', 'ArrayPattern', 'ObjectPattern']);

function functionParamsSupported(fn: {
  params: ReadonlyArray<{ type: string }>;
}): boolean {
  return fn.params.every(
    (p) =>
      SUPPORTED_PARAMS.has(p.type) ||
      // leading `this` and trailing rest params are handled specially
      (p.type === 'Identifier') ,
  );
}
// walk the TS AST and run functionParamsSupported on every function-ish
// node before handing the file to TSDefToFlowDef

function isSupportedParamType(
  type: string,
): type is 'Identifier' | 'ArrayPattern' | 'ObjectPattern' {
  return type === 'Identifier' || type === 'ArrayPattern' || type === 'ObjectPattern';
}

try {
  const flowDef = translateTSDefToFlowDef(tsAst);
} catch (err) {
  if (err instanceof Error && err.message.startsWith('Unexpected function parameter')) {
    // report the .d.ts path + the param type from the message, skip the file
    report.skipped(file, err.message);
  } else {
    throw err;
  }
}

When it happens

Trigger: Calling the TSDefToFlowDef transform (e.g. translating a .d.ts file to Flow defs) on a TS AST containing a function-type parameter that is not Identifier/ArrayPattern/ObjectPattern — for example a TSParameterProperty from `constructor(private x: string)` style signatures, an AssignmentPattern, or an unexpected node shape produced by a different parser or TypeScript version.

Common situations: Translating .d.ts files that use constructor parameter properties; declarations generated by TypeScript versions whose AST shapes the translator does not know; hand-built or post-processed ASTs passed directly to the transform; translator and TypeScript versions out of sync.

Related errors

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• updatePendingStatements: Variable parent of "${dep}" not fou
• Flow check failed!
• Flow version ${version} doesn't support 'flow lsp'. Please u
• Unsupported .flowconfig option `log.file`. The VS Code exten

AI-assisted analysis of facebook/flow@d1341dac89 (2026-08-17). Data as JSON: /api/errors/0f695d497dcff05c. Report an issue: GitHub.