Skip to Content
Codecademy Logo

Type Narrowing

Print Cheatsheet

TypeScript Union Type Narrowing

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.

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"

TypeScript Type Guard

A TypeScript type guard is a conditional statement that evaluates the type of a variable. It can be implemented with the typeof operator followed by the variable name and compare it with the type you expect for the variable.

// A type guard implemented with the typeof operator if (typeof age === 'number') { age.toFixed(); }

TypeScript Type Guard Accepted Types with typeof

The typeof operator may be used to implement a TypeScript type guard to evaluate the type of a variable including number, string and boolean.

TypeScript Type Guard with in operator

If a variable is a union type, TypeScript offers another form of type guard using the in operator to check if the variable has a particular property.

/* In this example, 'swim' in pet uses the 'in' operator to check if the property .swim is present on pet. TypeScript recognizes this as a type guard and can successfully type narrow this function parameter. */ function move(pet: Fish | Bird) { if ('swim' in pet) { return pet.swim(); } return pet.fly(); }

TypeScript Type Guard if-else Statement

If a variable is of a union type, TypeScript can narrow the type of a variable using a type guard. A type guard can be implemented as a conditional expression in an if statement. If an else statement accompanies the if statement, TypeScript will infer that the else block serves as the type guard for the remaining member type(s) of the union.

function roughAge(age: number | string) { if (typeof age === 'number') { // In this block, age is known to be a number console.log(Math.round(age)); } else { // In this block, age is known to be a string console.log(age.split(".")[0]); } } roughAge('3.5'); // Prints "3" roughAge(3.5); // Prints 4

TypeScript Type Guard if Statement Function Return

If a variable is of a union type, TypeScript can narrow the type of a variable using a type guard. A type guard can be implemented as a conditional expression in an if statement. If the if block contains a return statement and is not followed by an else block, TypeScript will infer the rest of the code block outside the if statement block as a type guard for the remaining member type(s) of the union.

function formatAge(age: number | string) { if (typeof age === 'number') { return age.toFixed(); // age must be a number } return age; // age must not be a number } console.log(formatAge(3.5)); // Prints "4" console.log(formatAge('3.5')); // Prints "3.5"