null and undefined in TypeScript

december 2019

At Yoast, we currently use NestJS for our server. This is a NodeJS server framework written in TypeScript. From the homepage of TypeScript: "TypeScript is a typed superset of JavaScript that compiles to plain JavaScript." The main advantage of using TypeScript instead of plain JavaScript is that TypeScript does type-checking at compile time. This leads to more robust code and faster development since you can be sure about the type of data you are working with.

That is what we thought, at least.

The casus

We were tasked with creating a function that takes an array of objects and executes a function on each object. For this example, lets call the type Cheese and the function that needs to be executed slice(), which turns a Cheese in a CheeseSlice.

We started with declaring the function.

function sliceCheeses( cheeses: Cheese[] ): CheeseSlice[] {}

So far, so good, so lets add some functionality to this function.

function sliceCheeses( cheeses: Cheese[] ): CheeseSlice[] { cheese => cheese.slice() );

Looks good right? The array of cheeses are now getting sliced into smaller pieces.


Since we are good developers, we decide to implement some unit tests for this function. We start with a basic test, that simply checks whether our function works as expected. We use jest for testing but the framework should not matter.

it( "slices one cheese correctly", () => {
        const cheese = new Cheese();
        const expected = [ cheese.slice() ];
        const result = sliceCheeses( [ cheese ] );
        expect( result ).toEqual( expected );
    } )

This test works as expected. We decide to add some more tests to cover some other cases.

it( "slices multiple cheeses", () => {
        const cheese1 = new Cheese();
        const cheese2 = new Cheese();
        const expected = [ cheese1.slice(), cheese2.slice() ];
        const result = sliceCheeses( [ cheese1, cheese2 ] );
        expect( result ).toEqual( expected );
    } );
    it( "handles an empty array", () => {
        const result = sliceCheeses( [] );
        expect( result ).toEqual( [] );
    } );

All our tests are still succeeding. We become so convinced of the correctness of our function that we decide to test some really weird cases.

it( "handles invalid input in the array", () => {
        const cheese1 = new Cheese();
        const expected = [ cheese1.slice() ];
        const result = sliceCheeses( [ null, null, cheese1 ] );
        expect( result ).toEqual( expected );
    } )
    it( "handles invalid input", () => {
        const result = sliceCheeses( null );
        expect( result ).toEqual( [] );
    } )

Now something odd happens. I expected that TypeScript would at least complain about the second call since null does not resemble Cheese[] in my opinion.

But TypeScript does not do any of that. It happily compiles the test and lets the test crash because map() does not exist on null.

Something similar happens in the other test. It compiles fine but crashes because slice() does not exist on null. This was not what we expected to happen.

So what happens here?

It turns out that TypeScript has two special types that do not adhere to the normal type-checking. These types are Null and Undefined, which have the values null and undefined respectively. In TypeScript, null and undefined are assignable to every type.


We do not want our application to crash when, for whatever reason, an incorrect input gets passed to this function. This might happen when our database cannot find any results and returns a null.

Of course, it is better to immediately check for those cases where they occur and do the error handling there, but we all know that we often forget about that while developing.

In the end, we decided to rewrite the function so that it can handle more cases. But doing this for every function where a null or undefined might show up, is tedious.

function sliceCheeses( cheeses: Cheese[] ): CheeseSlice[] {
        if ( ! cheeses ){
            return [];
            .filter( cheese => cheese )
            .map( cheese => cheese.slice() );

A compiler flag (--strictNullChecks) has been added to TypeScript that enforces stricter typing and that would have complained about this. But for some reason, this flag is not on by default. I suspect it has to do with backwards-compatibility but this lenient checking created some confusion on our end.

It would have been cool if TypeScript would have complained about it from the beginning. In their documentation, they write that turning on the flag helps to avoid many common errors. Three lines below, they again "encourage the use of --strictNullChecks when possible".

Maybe the time has come to turn on this flag by default?