Conditional Types

A conditional type uses the form T extends U ? X : Y. TypeScript selects a branch while analyzing types. This is not the JavaScript ternary operator; it creates neither a runtime test nor an emitted branch.

Reading the code: Because string extends string, Description<string> resolves to the literal type "text". Number does not extend string, so the second type is "other". JavaScript prints the two strings that were assigned.

Keep the boundary clear: Do not assume conditional types change a value dynamically. Use a runtime typeof check when the actual value determines behavior.

Save the code as conditional-types.ts. The result below was captured after compiling this exact example with strict TypeScript checking and executing the emitted JavaScript in Node.js.

TypeScript code

type Description<T> = T extends string ? "text" : "other";
const a: Description<string> = "text";
const b: Description<number> = "other";
console.log(a);
console.log(b);

Verified output

text
other

Build: npx tsc --strict --target es2022 --module commonjs --lib es2022,dom,esnext.disposable conditional-types.ts; run: node conditional-types.js. The exact filename may be changed freely. When a multi-file project exists, prefer its tsconfig.json and build scripts.

Because string extends string, Description<string> resolves to the literal type "text". Number does not extend string, so the second type is "other". JavaScript prints the two strings that were assigned.

The output shown above comes from the initialized values in this program. Replacing a type annotation alone does not transform a JavaScript value.

Do not assume conditional types change a value dynamically. Use a runtime typeof check when the actual value determines behavior.

Check both the TypeScript diagnostic and the emitted JavaScript behavior. A program can compile correctly and still need runtime validation or error handling.

Define IsArray<T> using T extends readonly unknown[] and test it with string[] and string.

Before running your change, write down the type you expect TypeScript to infer and the output you expect JavaScript to print. Then check both predictions.

A conditional type uses the form T extends U ? X : Y.

Try the practice change and re-run the exact program to test your understanding.