TypeScript — Fixing Null Type Ignored in Union Type

One of the benefits of using TypeScript is the type system which requires you to provide the correct values between all interactions in your application. For values that can be null or undefined, you may need to handle different code paths, for example by using default values.

TypeScript comes with strict null support by providing a null type. This is helpful when working with browser APIs. For example, document.querySelector(<selector>) returns an Element or null if the selector doesn’t match any element. But if you’re seeing TypeScript removing the null type from type definitions and returning the other possible types, this tutorial is for you.

TypeScript Series Overview

Fix the Missing Null Type in a Union Type

Here’s a basic example describing the problem: you have a method that expects an argument to be either of type string or a null. When you’re hovering over the method name inside of Visual Studio Code, the type of the argument is string. TypeScript removed the null type. That’s wrong and possibly not intended.

Here’s a sample function outlining the issue:

function sayHelloTo (name: string | null) {  
  // in VS Code: hover over "sayHelloTo" and the arguments are -> "name: string"
  console.log(`Hello ${name}`);

Solving the Missing Null Type

Fix the removed null type in TypeScript by making sure either your tsconfig.json contains at least one of the following compiler options:

Here’s a sample tsconfig.json file containing both of the checks:

  "compilerOptions": {
    "target": "ES2021",
    // …

    // one of these settings is required for the `null` type
    "strict": true,
    "strictNullChecks": true,

If you’re referencing a second tsconfig.json, ensure it’s not overwriting the settings from the first one. And if you’re not seeing the null types showing up in Popovers of Visual Studio Code, you may restart VS Code itself or the Extension Host.

Enjoy the null type in TypeScript!

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