Default Parameters

In JavaScript, default parameters allow you to initialize named parameters with default values if no value or `undefined` is passed. This feature was introduced in ES6 (ECMAScript 2015) and helps in making your functions more robust and readable

Syntax

The syntax for default parameters is straightforward. You simply assign a default value to the parameter directly in the function signature.

default-parameters-basic.js 
function greet(name = "Guest") {
  return `Hello, ${name}!`;
}

console.log(greet()); // "Hello, Guest!"
console.log(greet("Hunter")); // "Hello, Hunter!"

Multiple Default Parameters

You can also have multiple default parameters in a single function. Each parameter can be assigned a default value.

default-parameters.js 
function createUser(name = "Anonymous", age = 18, country = "Unknown") {
  return `Name: ${name}, Age: ${age}, Country: ${country}`;
}

console.log(createUser()); // "Name: Anonymous, Age: 18, Country: Unknown"
console.log(createUser("Amy", 25, "USA")); // "Name: Amy, Age: 25, Country: USA"

Using Expressions as Default Values

Default values can also be the result of an expression, including function calls.

default-values.js 
function getDefaultAge() {
  return 21;
}

function createUser(name = "Anonymous", age = getDefaultAge()) {
  return `Name: ${name}, Age: ${age}`;
}

console.log(createUser()); // "Name: Anonymous, Age: 21"
console.log(createUser("Bob", 30)); // "Name: Bob, Age: 30"

Interdependent Default Parameters

Default parameters can reference parameters that appear earlier in the parameter list.

interdependent-params.js 
function createMessage(name, greeting = `Hello, ${name}`) {
  return greeting;
}

console.log(createMessage("Alice")); // "Hello, Alice"
console.log(createMessage("Bob", "Hi there!")); // "Hi there!"

Practical Examples

API Request Function

Consider an API request function where certain parameters have sensible defaults.

api-ex.js 
function fetchData(url, method = "GET", timeout = 5000) {
  console.log(`Fetching data from ${url} with method ${method} and timeout ${timeout}`);
  // Imagine an actual fetch request here
}

fetchData("https://api.example.com/data"); 
// Fetching data from https://api.example.com/data with method GET and timeout 5000

fetchData("https://api.example.com/data", "POST", 10000); 
// Fetching data from https://api.example.com/data with method POST and timeout 10000

Event Handler

A function to add an event listener with default options.

event-handler.js 
function addClickListener(element, callback, useCapture = false) {
  element.addEventListener("click", callback, useCapture);
}

const button = document.querySelector("button");
addClickListener(button, () => alert("Button clicked!"));

Common Errors and Best Practices

Avoid Overwriting Undefined Values

Be cautious of cases where undefined might be intentionally passed.

common-errors.js 
function process(value = 10) {
  return value;
}

console.log(process()); // 10
console.log(process(undefined)); // 10
console.log(process(null)); // null (null is a valid value, not undefined)

Avoid Side Effects in Default Parameters

Default parameters should be free of side effects to avoid unexpected behaviors.

common-errors.js 
let counter = 0;

function increment(count = ++counter) {
  return count;
}

console.log(increment()); // 1
console.log(increment()); // 2
console.log(increment(10)); // 10
console.log(increment()); // 3 (side effect, counter is incremented even if count is provided)

Notes

  • Default Parameters were introduced in ES6 and allow for more flexible and robust function definitions.
  • Undefined Values are treated as missing parameters and default values are applied.
  • Expressions as Defaults can be useful but be cautious of side effects.
  • Interdependent Parameters can reference earlier parameters, allowing for more dynamic defaults.

FAQ

Q: Can default parameters be objects or arrays?

A: Yes, default parameters can be any valid JavaScript value, including objects and arrays.

Q: What happens if I pass undefined to a function with a default parameter?

A: If undefined is explicitly passed, the default value is used. If null is passed, it is treated as a valid value and the default is not used.

Q: Are default parameters evaluated every time the function is called?

A: Yes, default parameters are evaluated at the time the function is called, not when the function is defined.

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