pr-7836.md

Avoid trailing commas on index signatures with only one parameter (#7836 by @bakkot)

TypeScript index signatures technically allow multiple parameters and trailing commas, but it's an error to have multiple parameters there, and Babel's TypeScript parser does not accept them. So Prettier now avoids putting a trailing comma there when you have only one parameter.

// Input
export type A = {
  a?: {
    [
      x: string
    ]: typeof SomeLongLongLongTypeName[keyof typeof SomeLongLongLongTypeName];
  } | null;
};

// Prettier stable
export type A = {
  a?: {
    [
      x: string,
    ]: typeof SomeLongLongLongTypeName[keyof typeof SomeLongLongLongTypeName];
  } | null;
};

// Prettier master
export type A = {
  a?: {
    [
      x: string
    ]: typeof SomeLongLongLongTypeName[keyof typeof SomeLongLongLongTypeName];
  } | null;
};