JavaScript Function Parameters

Parameters (Function Input)

Parameters allow you to pass (send) values to a function.

Parameters are listed inside the parentheses in the function definition.

Example

function multiply(a, b) {

  return a * b;

}

let result = multiply(4, 5);

Functions with One Parameter

A function can have one parameter.

Examples

function sayHello(name) {

  return "Hello " + name;

}

let greeting = sayHello("John");

Functions with Multiple Parameters

You can add as many parameters as you want, separated by commas.

Example

function fullName(firstName, lastName) {

  return firstName + " " + lastName;

}

let name = fullName("John", "Doe");

Parameters vs. Arguments

In JavaScript, function parameters and arguments are distinct concepts:

Parameters are the names listed in the function definition.

Arguments are the real values passed to, and received by the function.

Parameter Rules

JavaScript function definitions do not specify data types for parameters.

JavaScript functions do not perform type checking on the arguments.

JavaScript functions do not check the number of arguments received.

Incorrect Parameters

Accessing a function with an incorrect parameter can return an incorrect answer:

Example

function toCelsius(fahrenheit) {

    return (5/9) * (fahrenheit-32);

}

let value = toCelsius();

Default Parameter Values

**ECMAScript 2015** allows function parameters to have default values.

The default value is used if no argument is provided.

Example

function myFunction(x, y = 10) {

  return x + y;
}
myFunction(5);