TypeScript Functions – REST, Optional and Default Parameters

Learn to create functions, function type declaration, optional parameters, default parameters and rest parameters with example.

Table of Contents

Create function
Function Types
Optional Parameters
Default Parameters
Rest Parameters

Create function

In TypeScript, you can create functions in two ways.

1. Function Declarations

   These are named functions written in traditional JavaScript style.

   console.log( showMyName("Lokesh") ); // Hi! Lokesh
   
   function showMyName (name: string): string { 
       return `Hi! ${name}`; 
   } 
   

2. Function Expressions

   These functions don’t have names. They are assigned to

   console.log(showMyName("Lokesh"));	//Error - Define functional expression first.
   
   let showMyName = function(name: string): string { 
       return `Hi! ${name}`; 
   };
   
   console.log(showMyName("Lokesh"));	//Hi! Lokesh 
   

Function Types

In TypeScript, everything is type, functions are also types. You can declare a variable’s type to be Function as shown.

let showMyName: Function = function(name: string): string { 
    return `Hi! ${name}`; 
};

In above example, showMyName is a variable which can point to a Function type only.

If you don’t specify a type of variable, TypeScript infers the type automatically.

Optional Parameters

Unlike JavaScript, the TypeScript compiler will throw an error if we attempt to invoke a function without providing the exact number and types of parameters that its signature declares.

To solve this problem, you can use optional parameters using question mark sign ('?'). In below example, message is marked as optional parameter.

let showMyName = function(name: string, message?: string): string { 
    return `Hi! ${name} {message}`; 
};

showMyName();							//Error
showMyName('Lokesh');					//Hi! Lokesh 
showMyName('Lokesh', 'How are you?');	//Hi! Lokesh How are you?

Default Parameters

Optional parameters are great feature but leave us with the logic to implement the scenarios where it’s values can be undefined. It require lots of null checks and if-else blocks.

A better approach would be to have default values of these parameters, rather than to declare them optional. If will lead to more cleaner code and easy maintenance of code.

let showMyName = function(name: string, message: string = 'How are you?'): string { 
    return `Hi! ${name} {message}`; 
};

showMyName('Lokesh');						//Hi! Lokesh How are you?
showMyName('Lokesh', 'How are you buddy?');	//Hi! Lokesh How are you buddy?	

REST Parameters

Sometimes, you may want to create functions which can have undetermined number of parameters. For one or two parameters, you can use optional parameters or default parameters. But what if number of parameters are not known, or can vary at runtime.

e.g. You are reading input field values and that can vary due to dynamic UI. Here rest parameters will help you. The REST parameter syntax allows us to represent an indefinite number of arguments as an array.

1. To create REST parameter, use ellipsis i.e. three dots ('...') before the variable name.
2. A REST parameter must be of an array type or we will get a compilation error.
3. Theoretically, there is no specific limit to the maximum number of arguments.

let addInputValues = function( ...values: number[] ): number { 
    let result = 0; 
    for (let val of values) { 
        result += val; 
    } 
    return result; 
};

addInputValues();					//OK - You can choose not to pass anything as well
addInputValues(1, 1);				//OK
addInputValues(1, 2, 3);			//OK
addInputValues(1, 2, 3, 4, 5, 6);	//OK

Drop me your questions in comments section.

Happy Learning !!