Type predicates in practice in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a type predicate in TypeScript?

A type predicate is a special return type in a function that tells TypeScript the type of a variable when the function returns true. It helps the compiler narrow down types in conditional checks.

beginner

How do you write a type predicate function signature?

You write it as <code>function isType(value: any): value is Type</code>. The <code>value is Type</code> part is the type predicate telling TypeScript what type <code>value</code> has if the function returns true.

intermediate

Why use type predicates instead of simple boolean functions?

Type predicates inform TypeScript about the type of a variable after a check, enabling better type safety and autocompletion. Simple boolean functions do not provide this type information.

beginner

Example: What does this function do?<br><pre>function isString(value: unknown): value is string { return typeof value === 'string'; }</pre>

This function checks if value is a string. If it returns true, TypeScript knows value is a string from that point on.

intermediate

Can type predicates be used with custom types or interfaces?

Yes! You can write type predicates to check if a value matches a custom interface or type, helping TypeScript narrow complex types in your code.

What does the syntax value is Type in a function return type mean?

A It tells TypeScript that if the function returns true, <code>value</code> has type <code>Type</code>.

B It means the function always returns a value of type <code>Type</code>.

C It declares a new type called <code>value is Type</code>.

D It restricts the function to accept only <code>Type</code> values.

The value is Type syntax is a type predicate telling TypeScript that when the function returns true, value is of type Type.

Which of these is a valid type predicate function signature?

A function isNumber(value: number): boolean

B function isNumber(value: any): boolean

C function isNumber(value: any): value is number

D function isNumber(value: any): number

Only option C uses the type predicate syntax value is number to inform TypeScript about the type.

What benefit do type predicates provide in TypeScript?

A They automatically convert types.

B They speed up program execution.

C They allow functions to return multiple types.

D They improve type safety by narrowing types after checks.

Type predicates help TypeScript understand the type of a variable after a check, improving type safety and developer experience.

Can type predicates be used to check if an object matches a custom interface?

A Yes, they can check any type including custom interfaces.

B No, only primitive types can be checked.

C Only arrays can be checked with type predicates.

D Type predicates only work with strings.

Type predicates can be written to check any type, including custom interfaces or complex types.

What happens if a type predicate function returns false?

A TypeScript throws a compile error.

B TypeScript assumes the variable is NOT of the specified type.

C The variable is automatically converted to the specified type.

D The function must be called again.

If the type predicate returns false, TypeScript knows the variable does not have the specified type.

Explain what a type predicate is and how it helps TypeScript with type checking.

Think about how TypeScript knows the type of a variable after a check.

Write a simple type predicate function that checks if a value is an array of numbers.

Use 'value is number[]' as the return type and check elements inside the array.