Sometimes you need to include undefined to defined variable/object property. Optional type is a solution. It is not some separated type. It's special syntax for type that adds to any type additional value - undefined. You can create them by adding a question mark in front of the type name.
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const givenAge = -5;const userAge: ?number = givenAge > 0 ? givenAge : undefined;
Warning: Unlike Flow.js, Hegel will throw an error if you try to assign null as value of an optional type
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// Error: Type "null" is incompatible with type "number | undefined"const someValue: ?number = null;
Optional type is not only about undefined variables or object properties. You can use it inside function argument definition to annotate optional argument.
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function doSomething(optionalArg: ?number) {}doSomething(); // 👌!doSomething(42); // 👌!doSomething(undefined); // 👌!// Error: Type "null" is incompatible with type "number | undefined"doSomething(null);
Also you can use it inside object type definition to annotate optional property.
Playground
let user: { name: ?string } = { name: "Arya Stark" };user.name = undefined; // 👌!user.name = "Arya Stark"; // 👌!// Error: Type "null" is incompatible with type "number | undefined"user.name = null;user = {}; // 👌!user = { name: "Arya Stark" }; // 👌!user = { name: undefined }; // 👌!// Error: Type "{ name: null }" is incompatible with type "{ name: string | undefined }"user = { name: null };