# E0255: missing parentheses around parameter; TypeScript type annotation requires parentheses

Arrow functions with one parameter can omit the parentheses around that parameter. However, if the parameter has a type annotation, it is a syntax error to omit the parentheses:

    let friendNames =
      friends.map(f: Friend => f.name);

    const toString = thing: string => {
      if (thing === true)  return 'TRUE';
      if (thing === false) return 'FALSE';
      return thing.toString();
    };

To fix this error, put a pair of parentheses around the parameter name and its type:

    let friendNames =
      friends.map((f: Friend) => f.name);

Alternatively, put a pair of parentheses around the parameter name, and leave the `:` and the return type outside:

    const toString = (thing): string => {
      if (thing === true)  return 'TRUE';
      if (thing === false) return 'FALSE';
      return thing.toString();
    };

Introduced in quick-lint-js version 2.7.0.

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