Type Narrowing
Leverage TypeScript’s type system to gain insights into the runtime behavior of your code by narrowing down what union types.
Start
Key Concepts
Review core concepts you need to learn to master this subject
TypeScript Union Type Narrowing
TypeScript Type Guard
TypeScript Type Guard Accepted Types with typeof
TypeScript Type Guard with in operator
TypeScript Type Guard if-else Statement
TypeScript Type Guard if Statement Function Return
TypeScript Union Type Narrowing
TypeScript Union Type Narrowing
const choices: [string, string] = ['NO', 'YES'];
const processAnswer = (answer: number | boolean) => {
if (typeof answer === 'number') {
console.log(choices[answer]);
} else if (typeof answer === 'boolean') {
if (answer) {
console.log(choices[1]);
} else {
console.log(choices[0]);
}
}
}
processAnswer(true); // Prints "YES"
processAnswer(0); // Prints "NO"Since a variable of a union type can assume one of several different types, you can help TypeScript infer the correct variable type using type narrowing. To narrow a variable to a specific type, implement a type guard. Use the typeof operator with the variable name and compare it with the type you expect for the variable.
What you'll create
Portfolio projects that showcase your new skills
How you'll master it
Stress-test your knowledge with quizzes that help commit syntax to memory
Type Narrowing
Start