You can use an `in` check for this, since TypeScript version 2.7 [0]. Here's an example (Playground link: [1]): import * as React from 'react' type P

You can use an `in` check for this, since TypeScript version 2.7 [0]. Here's an example (Playground link: [1]): import * as React from 'react' type Props = | {a: number} | {b: string} function MyComponent(props: Props): JSX.Element { if ('a' in props) { return <div>a is {props.a}</div> } return <div>b is {props.b}</div> } let x = <MyComponent a={3} /> let y = <MyComponent b="foo" /> [0] https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-7.html#type-guards-inferred-from--in-operator https://www.typescriptlang.org/docs/handbook/release-notes/t... [1] https://www.typescriptlang.org/play/?jsx=2#code/JYWwDg9gTgLgBAKjgQwM5wEoFNkGN4BmUEIcA5FDvmQFA0wCeYWcACsWOgLw1x9wAfOAG9kALjgA7AK4gARligBfXvyHC5E1DCjBJAcxU0C0yfmARJcALIMAwiUiSskmAAowHVBPYROASgkAKQBlAA0AOgBRABssEBd4YVVgAjg3MmQyOD04Tz9UfxFVPkoYaSgrAB4AE2AANwA+ZBz0YXzOCOQlKoB6OqbVFVKscsq4WobGuVaRDtQIuR7+qZojOPgADzguCdsHcEtElC5hAGYlOF7Gmg24Bh29+0cj1zg5LgAiAggIT6ubkA https://www.typescriptlang.org/play/?jsx=2#code/JYWwDg9gTgLg...