Method in JavaScript

Method is also called function. A method is a block of code designed to perform a particular task. Method can belong to an object and define behaviors for objects, allowing them to perform actions using their own data.

Example of some simple common built-in methods

// String methods
let str = "hello";
console.log(str.toUpperCase()); // "HELLO"

// Array methods
let arr = [1, 2, 3];
console.log(arr.push(4)); // 4 (new length)
console.log(arr); // [1, 2, 3, 4]

Define a Method

Method may have parameter, argument and return statement

• Parameter: a named variable in a function's definition that acts as a placeholder for a value.
• Argument: the actual value that's passed to the function when it's called.
• Return statement: used to end the execution of a function and send a value back to the code that called it. If a function doesn't have a return statement, or if the return statement is used without a value, the function will return undefined by default.

There are a few ways to define a method:

Function Declaration: This is the most common way. You use the function keyword, followed by a name, a list of parameters in parentheses, and the code to be executed in curly braces.

function greet(name) {
  return `Hello, ${name}!`;
}

greet('jeff');

Function Expression: This is when you define a function and assign it to a variable. The function can be anonymous (without a name).

const getProduct = function(a, b) {
  return a * b;
};

const product = getProduct(2, 4);

Arrow Function: Introduced in ES6, this is a shorter syntax, especially useful for simple functions.

const add = (a, b) => a + b;

const c = add(1, 3);

Function Constructor: This is generally not recommended because it can be a security risk and is slower than other methods.

const add = new Function('a', 'b', 'return a + b');
const c = add(2, 4);

Method in Object

A method can belong to an object. It can perform actions using the data inside the object (its properties).

const person = {
  name: "Alice",
  greet: function() {
    console.log("Hello, my name is " + this.name);
  }
};

person.greet(); // Output: Hello, my name is Alice

In modern JavaScript (ES6+), you can use shorthand method syntax define methods like this:

const person = {
  name: "Bob",
  greet() {
    console.log(`Hi, I am ${this.name}`);
  }
};

person.greet(); // Output: Hi, I am Bob

Method can be defined in class for instances

class Person {
  constructor(name) {
    this.name = name;
  }
  
  greet() {
    console.log(`Hello, I am ${this.name}`);
  }
}

const alice = new Person("Alice");
alice.greet(); // Output: Hello, I am Alice

Common build-in method

parseInt("42");       // 42, string to integer
parseFloat("3.14");   // 3.14, string to float
isNaN("abc");         // true, check if not a number
setTimeout(() => console.log("Hello"), 1000); // delay execution
setInterval(() => console.log("Hi"), 1000);  // repeat execution