Custom type guard functions in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a custom type guard function in TypeScript?

A custom type guard function is a special function that checks if a value matches a specific type and tells TypeScript about this check, helping it understand the type better during code execution.

beginner

How do you define a custom type guard function signature?

You define it with a return type using the syntax: <code>paramName is Type</code>. For example, <code>function isString(value: any): value is string</code> means the function returns true if <code>value</code> is a string.

intermediate

Why use custom type guard functions instead of simple type checks?

Custom type guards help TypeScript narrow down types in complex situations, improving code safety and enabling better autocompletion and error checking.

beginner

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

This function checks if the input value is a number. If yes, it returns true and tells TypeScript that value is a number from that point on.

intermediate

How does TypeScript behave after a custom type guard returns true?

TypeScript narrows the type of the checked variable to the type specified in the guard, allowing safer access to properties or methods of that type.

What is the correct return type for a custom type guard function checking if a value is a string?

A boolean

B string

C value is string

D value: string

The return type for a custom type guard uses the syntax 'value is string' to tell TypeScript about the type narrowing.

Which keyword is used inside a custom type guard function to check the type of a variable?

A typeof or instanceof depending on the type

B instanceof

C typeof and instanceof together

D typeof

You can use 'typeof' for primitive types and 'instanceof' for class instances inside custom type guards.

What benefit does a custom type guard provide in TypeScript?

A Removes the need for type annotations

B Runs faster code

C Automatically converts types

D Improves type safety by narrowing types

Custom type guards help TypeScript narrow types, improving safety and developer experience.

Which of these is a valid custom type guard function signature?

A function isArray(value: any): boolean

B function isArray(value: any): value is any[]

C function isArray(value: any): any[]

D function isArray(value: any): void

The signature 'value is any[]' tells TypeScript this function narrows the type to an array.

After a custom type guard returns true, what can you safely do with the variable?

A Access properties or methods specific to the narrowed type

B Assign it to any type without error

C Ignore type checking

D Use it only as 'any' type

TypeScript knows the variable has the narrowed type, so you can safely use its specific properties or methods.

Explain what a custom type guard function is and how it helps TypeScript.

Think about how TypeScript understands variable types better after the guard.

Write a simple custom type guard function that checks if a value is an array.

Use Array.isArray to check the value inside the function.