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

Recall & Review

beginner

What is a default parameter in TypeScript?

A default parameter is a function parameter that has a default value assigned. If the caller does not provide a value, the default is used.

beginner

How do you specify a type for a default parameter in TypeScript?

You write the parameter name, then a colon, then the type, followed by an equals sign and the default value. Example: <code>function greet(name: string = "Guest")</code>

intermediate

What happens if you call a function with a default parameter but pass undefined explicitly?

The default value is used because undefined triggers the default parameter.

beginner

Can default parameters be of any type in TypeScript?

Yes, default parameters can be any valid TypeScript type, including primitives, objects, arrays, or even functions.

beginner

Show a simple function with a typed default parameter and explain its parts.

Example: <code>function multiply(x: number, y: number = 2): number { return x * y; }</code><br>Here, <code>y</code> is a number with a default value 2. If <code>y</code> is not passed, it uses 2. The function returns a number.

What is the correct way to write a default parameter with type in TypeScript?

A function foo(bar = string "hello")

B function foo(bar: "hello" = string)

C function foo(bar: string = "hello")

D function foo(bar string = "hello")

Option C correctly declares a parameter 'bar' of type string with a default value 'hello'.

If a function parameter has a default value, what happens when you call the function without that argument?

A The parameter is undefined.

B The default value is used.

C An error is thrown.

D The function ignores the parameter.

When a parameter has a default value, calling the function without that argument uses the default.

What type of value triggers the use of a default parameter when passed explicitly?

A null

B false

C 0

D undefined

Passing undefined explicitly triggers the default parameter value to be used.

Can you have multiple default parameters in a TypeScript function?

A Yes, any number of parameters can have defaults.

B No, only one default parameter is allowed.

C Yes, but only if they are at the end.

D No, default parameters are not supported.

TypeScript allows multiple parameters to have default values.

What is the return type of this function? function add(a: number, b: number = 5) { return a + b; }

A number

B string

C void

D any

The function returns the sum of two numbers, so the return type is number.

Explain how to declare a function parameter with a default value and a type in TypeScript.

Think about the order: name, colon, type, equals, value.

Describe what happens when a function with default parameters is called with missing or undefined arguments.

Consider both missing and explicitly undefined arguments.