A user-defined type guard function that narrows the type of its argument.
Syntax
function isT(v: unknown): v is T { ... }Example
Enter values below to update the example in real time.
function→isString→unknown→string→return→typeof→const→val→if→console→toUpperCase→HELLO→function isString(v: unknown): v is string {
return typeof v === "string";
}
const val: unknown = "hello";
if (isString(val)) {
console.log(val.toUpperCase()); // HELLO
}