Can I narrow this down in TypeScript?

Question

I have a utility function to check whether a variable is not null or undefined, and I want TypeScript to narrow down the input variable if it passes the check, for example:

public init(input?: string): void {
    function isSpecified(input: any): boolean {
        return (typeof input !== "undefined") && (input !== null);
    }

    if (isSpecified(input)) {
        let copiedString: string = input; // <-- Error; input is still 'string | undefined'
    }
}

As you can see TS is not removing the possibility of the string being undefined even though the function makes that logically impossible. Is there a way I can get this function call to narrow down input inside the if block?

Answer 1

You can use a generic type guard function:

public init(input?: string): void {
    function isSpecified<T>(input: null | undefined | T): input is T {
        return (typeof input !== "undefined") && (input !== null);
    }

    if (isSpecified(input)) {
        let copiedString: string = input; // OK
    }
}

Note that if TypeScript narrows down a variable's union type through narrowing during an assignment, isSpecified will still not act as a type guard. For example, in the following code, TS will give an error:

someMethod = () => {
    const foo: undefined | null | number = undefined;
    if (isSpecified(foo)) {
        this.nonNullValue = foo; // Error
    }
};

In the above, foo being assigned as undefined narrows the variable type and the T in isSpecified is undefined, so TS still doesn't treat foo as definitely being specified. This can be worked around by using a type assertion (val as ...) to prevent narrowing:

someMethod = () => {
    //const foo = 123 as
    // ... or:
    const foo = undefined as
        undefined | null | number;
    if (isSpecified(foo)) {
        this.nonNullValue = foo;
    }
};