Nullish Coalescing

Nullish coalescing (??) uses a replacement value only when the left operand is null or undefined.

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

Unlike ||, it preserves useful falsy values including 0, false and the empty string. ?? is particularly appropriate for optional settings, counts and explicit absence values. Parenthesize expressions when mixing it with && or || according to JavaScript grammar.

Use the diagram to locate the checks, values or branches involved in the example.

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 savedCount: number | undefined = 0;
 const displayCount = savedCount ?? 10;
 const nickname: string | null = null;
 console.log(displayCount);
 console.log(nickname ?? "Guest");

Verified output

0
Guest

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.

Because 0 is present rather than nullish, displayCount stays 0. Because nickname is null, the fallback Guest is printed.

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 replace ?? with || if zero or an empty string must remain meaningful.

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 savedCount to undefined and then 5. Compare savedCount ?? 10 with savedCount || 10 for the value 0.

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

Nullish Coalescing: Unlike ||, it preserves useful falsy values including 0, false and the empty string.

?? tests specifically for null and undefined, not general falsiness.

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