• var: Function-scoped and can be re-declared and updated. It does not respect block scope.
• let: Block-scoped and can be updated but not re-declared within the same scope.
• const: Block-scoped and cannot be updated or re-declared. It is used for variables that should not change.

A closure is a function that has access to its own scope, the scope of its parent function, and the global scope, even after the parent function has finished executing.

Example:

function outer() {

  let counter = 0;

  return function inner() {

    counter++;

    return counter;

  };

}

const increment = outer();

console.log(increment()); // 1

console.log(increment()); // 2

Promises are objects used to handle asynchronous operations. They represent the eventual completion (or failure) of an asynchronous task and its resulting value.

States of a Promise:

• Pending: Initial state.
• Resolved/Fulfilled: Operation completed successfully.
• Rejected: Operation failed.

Example:

const promise = new Promise((resolve, reject) => {

  setTimeout(() => resolve(“Success”), 1000);

});

promise.then((value) => console.log(value)).catch((error) => console.error(error));

• ==: Compares values after type coercion.
• ===: Compares both value and type, making it a stricter comparison.

Example:

console.log(5 == “5”);  // true (type coercion)

console.log(5 === “5”); // false (strict comparison)

This keyword refers to the object it belongs to:

• In a method, this refers to the owner object.
• Alone, this refers to the global object (window in browsers).
• In a function, this refers to the global object in non-strict mode or undefined in strict mode.

In an arrow function, this is lexically inherited from the parent scope.

async/await simplifies writing asynchronous code by making it look synchronous.

• async marks a function as asynchronous, returning a promise.
• await pauses the execution of an async function until the promise is resolved or rejected.

Example:

async function fetch data() {

  try {

    const response = await fetch(“https://api.example.com/data”);

    const data = await response.json();

    console.log(data);

  } catch (error) {

    console.error(“Error fetching data:”, error);

  }

}

fetchData();