What Is a Parameter in Programming?
A parameter specifies the name you wish to call your function’s argument.
A parameter is an optional component of a function. In other words, you do not need to specify a parameter if your function does not accept any argument.
For instance, JavaScript’s pop()
method is a function without any parameter because it does not accept arguments.
On the other hand, forEach()
has two parameters that accept two arguments.
The Syntax of a Parameter
function nameOfFunction(parameter1, parameter2, parameter3) { // function's body}
The function in the snippet above has three parameters.
Example of a JavaScript Parameter
// Define a function with two parameters:function myName(firstName, lastName) { console.log(`My full name is ${firstName} ${lastName}.`);}
// Invoke myName function while passing two arguments to its parameters:myName("Oluwatobi", "Sofela");
// The invocation above will return: "My full name is Oluwatobi Sofela."
The myName()
function in the snippet above has two parameters: firstName
and lastName
.
Suppose you wish to pre-define values for your parameters that browsers can use if users do not invoke the function with the required arguments. In that case, you can create default parameters.
What Is a Default Parameter?
Default parameters allow you to initialize your function’s parameters with default values.
For instance, suppose a user invokes your function without providing a required argument. In such a case, browsers will set the parameter’s value to undefined
.
However, default parameters allow you to define the values browsers should use instead of undefined
.
Examples of Default Parameters
Below are examples of how default parameters work in JavaScript.
How to define a function with no default parameters
// Define a function with two parameters:function myName(firstName, lastName) { console.log(`My full name is ${firstName} ${lastName}.`);}
// Invoke myName function while passing one argument to its parameters:myName("Oluwatobi");
// The invocation above will return: "My full name is Oluwatobi undefined."
The computer automatically set the lastName
parameter to undefined
because we did not provide a default value.
How to define a function with an undefined
argument and no default parameter
// Define a function with two parameters:function myName(firstName, lastName) { console.log(`My full name is ${firstName} ${lastName}.`);}
// Invoke myName function while passing two arguments to its parameters:myName("Oluwatobi", undefined);
// The invocation above will return: "My full name is Oluwatobi undefined."
The computer set the lastName
parameter to undefined
because we provided undefined
as myName()
’s second argument.
How to define a function with a default parameter
// Define a function with two parameters:function myName(firstName, lastName = "Sofela") { console.log(`My full name is ${firstName} ${lastName}.`);}
// Invoke myName function while passing one argument to its parameters:myName("Oluwatobi");
// The invocation above will return: "My full name is Oluwatobi Sofela."
Instead of undefined
, JavaScript used "Sofela"
as the lastName
parameter’s default argument.
How to define a function with an undefined
argument and a default parameter
// Define a function with two parameters:function myName(firstName, lastName = "Sofela") { console.log(`My full name is ${firstName} ${lastName}.`);}
// Invoke myName function while passing two arguments to its parameters:myName("Oluwatobi", undefined);
// The invocation above will return: "My full name is Oluwatobi Sofela."
Instead of undefined
, JavaScript used "Sofela"
as the lastName
parameter’s default argument.