null and undefined

JavaScript has two common absence values: null and undefined. TypeScript can make you handle them deliberately.

In this lesson, you will study the rule, run a complete TypeScript example, explain its output, and test a small change.

undefined often means a value was not supplied or a property is missing. null is commonly an explicit no-value choice. With strictNullChecks enabled, a string is not automatically interchangeable with null or undefined; use union types when an absent value is possible.

Save this code as main.ts or paste it into the TypeScript Playground. The example uses fixed values so it runs without prompting for input.

TypeScript code

const middleName: string | undefined = undefined;
 const selected: number | null = null;
 console.log(middleName ?? "(not provided)");
 console.log(selected ?? "no selection");

Verified output

(not provided)
no selection

How to run locally: npx tsc --strict --target es2022 --module commonjs main.ts, then node main.js. When using a project configuration, run the matching project build instead.

The ?? operator uses the fallback because each left operand is nullish. The output contains two readable fallback messages.

Predict the output before you run the sample. Compare each line with the source expression that produced it, and distinguish the type checker’s work from what happens in JavaScript at runtime.

Do not use a truthiness check when 0 or an empty string is a legitimate value; ?? preserves those values.

Debugging habit: when a change is rejected, read the exact compiler diagnostic and inspect the operand types. When it compiles but behaves unexpectedly, follow the values through the JavaScript expressions.

Set middleName to an empty string and selected to zero, then check that neither fallback is selected.

Challenge: write down your prediction first, run the changed source, and explain any difference. Return to the verified version before continuing.

null and undefined: undefined often means a value was not supplied or a property is missing.

A union explicitly includes allowed absence states.

Before continuing, be able to explain what is checked before execution and what actually changes at runtime in the example above.