wap-notes

JavaScript Functions

1. Introduction to Functions

• Definition: Functions are reusable blocks of code designed to perform a specific task. They help in reducing code repetition.
• Syntax:

  function functionName(parameters) {
    // Code to be executed
  }
  

• Example:

  function greet(name) {
    console.log("Hello, " + name + "!");
  }
  greet("Alice");
  

2. Function Expressions

• Definition: Functions can also be defined as expressions and assigned to variables.
• Syntax:

  const functionName = function(parameters) {
    // Code to be executed
  };
  

• Example:

  const greet = function(name) {
    console.log("Hello, " + name + "!");
  };
  greet("Bob");
  

3. Arrow Functions

• Definition: A shorter syntax for writing function expressions, introduced in ES6.
• Syntax:

  const functionName = (parameters) => {
    // Code to be executed
  };
  

• Example:

  const greet = (name) => {
    console.log("Hello, " + name + "!");
  };
  greet("Charlie");
  

4. Function Parameters and Arguments

• Parameters: Variables listed as part of the function definition.
• Arguments: Values passed to the function when it is invoked.
• Default Parameters: Functions can have default parameter values if no arguments are provided.

  function greet(name = "Stranger") {
    console.log("Hello, " + name + "!");
  }
  greet(); // Output: Hello, Stranger!
  

5. Return Statement

• Definition: Functions can return a value using the return statement. The function execution stops when the return statement is reached.
• Example:

  function add(a, b) {
    return a + b;
  }
  const result = add(5, 3); // result = 8
  

Callback Functions

1. Definition

• A callback function is a function passed as an argument to another function and is executed after the completion of that function.
• Example:

  function greet(name, callback) {
    console.log("Hello, " + name);
    callback();
  }
  
  function sayGoodbye() {
    console.log("Goodbye!");
  }
  
  greet("Alice", sayGoodbye); // Output: Hello, Alice \n Goodbye!
  

2. Asynchronous Callback

• Callbacks are often used in asynchronous operations, such as reading files, making network requests, or setting timers.
• Example:

  setTimeout(() => {
    console.log("This runs after 2 seconds");
  }, 2000);
  

Array Functions

1. map()

• Definition: The map() method creates a new array by applying a function to each element of an array.
• Syntax:

  const newArray = array.map(function(currentValue, index, arr), thisValue);
  

• Example:

  const numbers = [1, 2, 3, 4];
  const squares = numbers.map(number => number * number);
  console.log(squares); // [1, 4, 9, 16]
  

2. filter()

• Definition: The filter() method creates a new array containing elements that pass a test provided by a function.
• Syntax:

  const newArray = array.filter(function(currentValue, index, arr), thisValue);
  

• Example:

  const numbers = [1, 2, 3, 4, 5];
  const evenNumbers = numbers.filter(number => number % 2 === 0);
  console.log(evenNumbers); // [2, 4]
  

3. reduce()

• Definition: The reduce() method executes a reducer function on each element of the array, resulting in a single output value.
• Syntax:

  const result = array.reduce(function(accumulator, currentValue, index, arr), initialValue);
  

• Example:

  const numbers = [1, 2, 3, 4];
  const sum = numbers.reduce((total, number) => total + number, 0);
  console.log(sum); // 10
  

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