TypeScript equality narrowing

retool-typescript-control-flow-analysis-2.ts

function example(x: string | number, y: string | boolean) {
  if (x === y) {
    x
    // `x` is of type `string`

    y
    // `y` is of type `string`
  } else {
    x
    // `x` is of type `string | number`

    y
    // `y` is of type `string | boolean`
  }
}