Can an optional parameter be null in TypeScript?

EDIT: Important Note As Quentin C pointed out in the below comment, the behavior listed here is only when strict null checking is enabled: "strictNullChecks": true in tsconfig.json.

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The types null and undefined are handled as separate types. The optional type is special, also allowing arguments to be left out of function calls.

1. Without a union or optional, nothing except the type itself is allowed.

function foo(bar: string) {
    console.info(bar);
}

foo("Hello World!"); // OK
foo(null); // Error
foo(undefined); // Error
foo() // Error

2. To additionally allow null, a union with null can be made.

function foo(bar: string | null) {
    console.info(bar);
}

foo("Hello World!"); // OK
foo(null); // OK
foo(undefined); // Error
foo() // Error

3. Allowing undefined works similarly. Note that the argument cannot be left out or null.

function foo(bar: string | undefined) {
    console.info(bar);
}

foo("Hello World!"); // OK
foo(null); // Error
foo(undefined); // OK
foo() // Error

4. You can also allow both, but the argument MUST still be given.

function foo(bar: string | null | undefined) {
    console.info(bar);
}

foo("Hello World!"); // OK
foo(null); // OK
foo(undefined); // OK
foo() // Error

5. With optional you can leave the argument out, or pass undefined, BUT NOT null.

function foo(bar?: string) {
    console.info(bar);
}

foo("Hello World!"); // OK
foo(null); // Error
foo(undefined); // OK
foo() // OK

6. To allow all three special cases, optional and null can be combined.

function foo(bar?: string | null) {
    console.info(bar);
}

foo("Hello World!"); // OK
foo(null); // OK
foo(undefined); // OK
foo() // OK

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Also, optional is only usable in parameter or other type declarations, such as interfaces, not on regular variables. As it would make no sense to leave out the value of a variable when assigning to it.

Thus,

let d?: string;

would make no sense and results in a compilation error.