Course Overview

Overview of history of the JavaScript language

History of JavaScript

Overview of the relationship between JavaScript and Java

JavaScript vs Java

Overview of how JavaScript is evolved as a programming language

How is it evolved?

Overview of how we can track upcoming features in the JavaScript language

How to track upcoming features?

Overview of how the ECMAScript is versioned

How is ECMAScript versioned?

Explanation of whether JavaScript is interpreted or a compiled language

Is it interpreted or compiled?

Explanation of how JavaScript is executed

How is it executed?

Execution Context

Explanation of what is a callstack and how it is used in JavaScript

Callstack

Overview of memory management in JavaScript

Memory

What is JavaScript

What is hoisting?

Explanation of common misconception about hoisting in JavaScript

Misconception about hoisting

Hoisting

What is scope?

Explanation of global scope in JavaScript

Global scope

Explanation of function scope in JavaScript

Function scope

Block scope

Explanation of module scope in JavaScript

Module scope

Scope chain

Scope

What is coercion?

Diving into inner details of  coercion in JavaScript

Deeper look at coercion

Explanation of what abstract operations are and common types of abstract operations in JavaScript

Abstract operations

Explanation of how abstract or loose equality works in JavaScript

Abstract equality operator

Explanation of how the addition operator works in JavaScript

Addition operator

Explanation of how relational operators work in JavaScript

Relational operators

Coercion exercise for testing and practicing the knowledge learned in this module

Coercion exercise

Coercion

What is a closure?

Explanation of a common misconception about closures in JavaScript

Misconception about closures

Explanation of the problem of closures inside loops and how to fix it

Closures inside loops

Explanation of closures can be used to hide data

Data hiding using closures

Closures

What is prototypal inheritance?

Prototype chain

__proto__ property

Explanation of how we can create objects with custom prototypes in JavaScript

Custom prototypes

Explanation of how prototype chain is set with ES6 classes

ES6 classes and prototypes

Prototypes

Explanation of this keyword in JavaScript

What is this?

Explanation of how "this" works inside arrow functions

Arrow functions and "this"

Explanation of how value of "this" can be set explicitly

Explicitly binding "this"

globalThis

Summarizing "this"

'this' keyword

What is a symbol?

Explanation of why symbols were added in JavaScript

Why symbols?

Symbols and privacy

Explanation of global symbols in JavaScript

Global symbols

Explanation of what well-known symbols are in JavaScript

Well-known symbols

Symbol

Overview of asynchronous behavior of JavaScript

Overview

Overview of how callbacks help us write asynchronous code in JavaScript

Callbacks

Event loop

Promise

Explanation of promise chaining in JavaScript

Promise chaining

Explanation of some static methods of promises and their use cases

Use cases for static Promise methods

Explanation of async-await syntax in JavaScript

async await

Explanation of microtasks, how they are processed, and how they are linked to promises

Microtasks

Explanation of common anti-patterns to avoid when using promises in JavaScript

Promise anti-patterns

Asynchronous JavaScript

Explanation of what is an iterator in JavaScript

Iterators

Explanation of what is a generator in JavaScript

Generators

Explanation of how we can create asynchronous iterators in JavaScript

Asynchronous iterators

Explanation of how we can create asynchronous generators in JavaScript

Asynchronous generators

Iterators and Generators

Overview of what is debugging and different ways to debug JavaScript

Debugging

Overview of using the "debugger" statement to debug JavaScript

Debugger statement

Overview of using the breakpoints in browser developers' tools to debug JavaScript

Breakpoints in browser

Overview of using the VS Code debugger to debug JavaScript

VS Code debugger

Debugging JavaScript

Final thoughts and advice regarding how to continue the exciting journey of learning JavaScript

Conclusion

Wrap up

Enter your name and email address below, then click the **"Send my free chapter"** button to get started

300+ page PDF

EPub

newline Access with Replits

All Code Samples

Discord Community Access

One-time price of $69

Become a confident JavaScript pro by unraveling the most challenging aspects of the language. With easy-to-understand explanations and code examples, you’ll gain a deeper and solid understanding of JavaScript's most complex features.

Advanced JavaScript Unleashed is for beginner and intermediate developers alike, who want to elevate their JavaScript skills and become senior engineers by gaining a deeper understanding of the core topics of the language.

Advanced JavaScript Unleashed

Generators are special functions in JavaScript that can suspend their execution at different points in their execution and then resume from the point at which they were paused. Just like iterators, generator functions can be used to produce a sequence of values that can be consumed by constructs like the 

 loop. In fact, a generator function returns a generator object, which is an iterator. So, we can use the return value of a generator function just like any other iterator.

Following is an example of a generator function that produces odd numbers from 0 to 10:

function* odds() {
  for (let i = 1; i <= 10; i += 2) {
    yield i;
  }
}

for (const num of odds()) {
  console.log(num);
}

/*
1
3
5
7
9
*/

Here's a Replit of the above code in action:

Note the following two things in the generator function above:

Generator functions can also be used to create an infinite sequence. as shown below:

function* randomNumberGenerator(max) {
  while (true) {
    yield Math.floor(Math.random() * max);
  }
}

const randomNumGen = randomNumberGenerator(10);

// log 10 random numbers
for (let i = 0; i < 10; i++) {
  console.log(randomNumGen.next().value);
}

The code example above logs only 10 random numbers, but it is possible to use the generator to generate an infinite number of random numbers. This is possible because generator functions are evaluated lazily; their execution is paused until a new value is requested.

In the above code example, we are consuming the generator using the 

 method. Note that calling the generator function doesn't execute it; instead, it returns a generator object. The generator object is an iterator as well as an iterable. So we can use it like an iterator by calling the next method on it. Calling the 

 method on a generator object returns the values that the generator yields. Multiple calls to the 

 method keep returning the value that the generator yields until the generator object stops yielding the values. At this point, we get an object with the 

. However, the generator function above can yield values infinitely.

As a generator function returns an iterator object, generator functions make it convenient to write iterators. We can rewrite the student iterator example in the previous lesson using a generator function, as shown below:

---
title: Generators
description: Explanation of what is a generator in JavaScript
isPublicLesson: false
useMdx: true
code: https://gitlab.com/fullstackio/books/newline-course-apps/advanced-javascript-unleashed-app/-/tree/main/module_10/lesson_10.02
---

Generators are special functions in JavaScript that can suspend their execution at different points in their execution and then resume from the point at which they were paused. Just like iterators, generator functions can be used to produce a sequence of values that can be consumed by constructs like the `for...of` loop. In fact, a generator function returns a generator object, which is an iterator. So, we can use the return value of a generator function just like any other iterator.

Following is an example of a generator function that produces odd numbers from 0 to 10:

```js
function* odds() {
  for (let i = 1; i <= 10; i += 2) {
    yield i;
  }
}

for (const num of odds()) {
  console.log(num);
}

/*
1
3
5
7
9
*/
```

Here's a Replit of the above code in action:

<ReplitEmbed src="https://replit.com/@newlineauthors/generators-example1" />

Note the following two things in the generator function above:

-   The `function*` syntax marks a function as a generator function. Space between `*` and `function` is also valid syntax: `function *`.
-   The `yield` keyword produces a value. The value on the right of the `yield` keyword is what we get when we call the `next` method on the iterator object returned from the generator function. The `yield` keyword also marks a place where a generator function is paused.

## Infinite sequence

Generator functions can also be used to create an infinite sequence. as shown below:

```js
function* randomNumberGenerator(max) {
  while (true) {
    yield Math.floor(Math.random() * max);
  }
}

const randomNumGen = randomNumberGenerator(10);

// log 10 random numbers
for (let i = 0; i < 10; i++) {
  console.log(randomNumGen.next().value);
}
```

Here's a Replit of the above code in action:

<ReplitEmbed src="https://replit.com/@newlineauthors/generators-example2" />

The code example above logs only 10 random numbers, but it is possible to use the generator to generate an infinite number of random numbers. This is possible because generator functions are evaluated lazily; their execution is paused until a new value is requested.

In the above code example, we are consuming the generator using the `next` method. Note that calling the generator function doesn't execute it; instead, it returns a generator object. The generator object is an iterator as well as an iterable. So we can use it like an iterator by calling the next method on it. Calling the `next` method on a generator object returns the values that the generator yields. Multiple calls to the `next` method keep returning the value that the generator yields until the generator object stops yielding the values. At this point, we get an object with the `done` property set to `true`. However, the generator function above can yield values infinitely.

## Implementing iterators

As a generator function returns an iterator object, generator functions make it convenient to write iterators. We can rewrite the student iterator example in the previous lesson using a generator function, as shown below:

```js
function Student(name, age, id, courses) {
  this.name = name;
  this.age = age;
  this.id = id;
  this.courses = courses;
}

Student.prototype[Symbol.iterator] = function* () {
  // "this" refers to the student object on which this method is called
  const currentStudent = this;
  const studentProps = Object.getOwnPropertyNames(currentStudent);