Rest parameters with types in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What are rest parameters in TypeScript?

Rest parameters allow a function to accept an indefinite number of arguments as an array. They are declared using three dots (...) before the parameter name.

beginner

How do you type rest parameters in TypeScript?

You specify the type of the rest parameter as an array type. For example, <code>function sum(...numbers: number[]) {}</code> means <code>numbers</code> is an array of numbers.

intermediate

Can rest parameters have different types in TypeScript?

Yes, you can use union types or tuples to type rest parameters with different types. For example, <code>function example(...args: (string | number)[]) {}</code> accepts strings or numbers.

intermediate

What is the difference between rest parameters and regular array parameters?

Rest parameters gather all remaining arguments into an array automatically, while regular array parameters expect a single array argument. Rest parameters make calling functions easier when passing multiple values.

beginner

Show a simple example of a function using typed rest parameters.

Example:<br><pre>function greetAll(...names: string[]) {<br>  names.forEach(name => console.log(`Hello, ${name}!`));<br>}<br>greetAll('Alice', 'Bob', 'Charlie');</pre>

How do you declare a rest parameter named items that accepts numbers in TypeScript?

A function example(...items: number[]) {}

B function example(items: number...) {}

C function example(items: ...number[]) {}

D function example(...items: number) {}

Rest parameters use three dots before the parameter name and the type is an array type, so ...items: number[] is correct.

What type does a rest parameter have inside the function?

A A union of all argument types

B A single value of the specified type

C A tuple of fixed length

D An array of the specified type

Rest parameters collect all extra arguments into an array of the specified type.

Which of these is a valid rest parameter type for accepting strings or numbers?

A ...args: (string | number)[]

B ...args: string | number

C ...args: string[] | number[]

D ...args: string & number[]

The rest parameter must be typed as an array of union types: (string | number)[].

Can a function have more than one rest parameter?

A Yes, any number of rest parameters

B Only if they are optional

C No, only one rest parameter is allowed

D Yes, but only if they have different types

Functions can only have one rest parameter and it must be the last parameter.

What happens if you call a function with rest parameters without any arguments?

A The rest parameter is undefined

B The rest parameter is an empty array

C The function throws an error

D The rest parameter is null

If no arguments are passed, the rest parameter is an empty array.

Explain how to use rest parameters with types in a TypeScript function.

Think about how to accept many arguments as one array with a type.

Describe the rules and limitations of rest parameters in TypeScript.

Consider function parameter order and typing.