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

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Advanced JavaScript Unleashed

Now that we have had a deeper look at coercion and how some of the common abstract operations work to make coercion work, let us test what we learned in this module. This following exercise will allow us to consolidate our understanding of the topic of coercion.

Following are some expressions that involve coercion. Try to guess the output based on the knowledge you gained in this module. Don't worry if you don't understand all of them. Their explanation is also given below. You can obviously refer to the specification and the earlier lessons in this module to understand and guess the output of the expressions below.

0 == false

"" == false

0 == []

[123] == 123

[1] < [2]

[] == ![]

!!"true" == !!"false"

[1, 2, 3] + [4, 5, 6]

[undefined] == 0

[[]] == ''

[] + {}

Below is an explanation of the output of each of the above-mentioned expressions.

Let's start with an easy one, and most people will probably get it right, even without reading this module. Now that we know how the abstract equality operator works let us understand the steps taken to evaluate this expression as 

---
title: Coercion exercise
description: Coercion exercise for testing and practicing the knowledge learned in this module
isPublicLesson: false
---

Now that we have had a deeper look at coercion and how some of the common abstract operations work to make coercion work, let us test what we learned in this module. This following exercise will allow us to consolidate our understanding of the topic of coercion.

Following are some expressions that involve coercion. Try to guess the output based on the knowledge you gained in this module. Don't worry if you don't understand all of them. Their explanation is also given below. You can obviously refer to the specification and the earlier lessons in this module to understand and guess the output of the expressions below.

```js
0 == false

"" == false

0 == []

[123] == 123

[1] < [2]

[] == ![]

!!"true" == !!"false"

[1, 2, 3] + [4, 5, 6]

[undefined] == 0

[[]] == ''

[] + {}
```

Below is an explanation of the output of each of the above-mentioned expressions.

### `0 == false`

Let's start with an easy one, and most people will probably get it right, even without reading this module. Now that we know how the abstract equality operator works let us understand the steps taken to evaluate this expression as `true`.

1.  As the types are not equal and one of the operands is a boolean, the boolean operand is converted into a number using the [ToNumber](https://tc39.es/ecma262/multipage/abstract-operations.html#sec-tonumber) abstract operation. So, the first coercive step is to convert `false` into a number, i.e., `0`. The expression becomes:

    ```js
    0 == 0
    ```
2.  Now the types are equal, so the strict equality comparison is performed, i.e., `0 === 0`, giving us `true` as an output.

### `"" == false`

1.  As the types are not equal and one of the operands is a boolean, the boolean operand is converted into a number using the `ToNumber` abstract operation. So, the first coercive step is to convert `false` into a number, i.e., `0`. The expression becomes:

    ```js
    "" == 0
    ```
2.  Now, we have a string and a number. Recall that the abstract equality operator prefers number comparison, so the string operand is converted into a number using the `ToNumber` abstract operation. An empty string, when converted into a number, outputs `0`. So the expression becomes:

    ```js
    0 == 0
    ```
3.  The types are equal, so the strict equality comparison is performed, i.e., `0 === 0`, giving us `true` as an output.

### `0 == []`

1.  The array is converted into a primitive value using the `ToPrimitive` abstract operation. As the abstract equality operator prefers number comparison, the array is converted into a primitive value with a number as the preferred type. An empty array, when converted into a primitive value, outputs an empty string. So the expression becomes:

    ```js
    0 == ""
    ```
2.  Next, the string will be converted into a number. An empty string converted into a number outputs `0`. So the expression becomes:

    ```js
    0 == 0
    ```
3.  The types are equal, so the strict equality comparison is performed, i.e., `0 === 0`, giving us `true` as an output.

### `[123] == 123`

1.  We have a comparison between an array and a number. So, the array is converted into a primitive value using the `ToPrimitive` abstract operation, with number as the preferred type. The `valueOf` method will be invoked first, as the preferred type is a number. But we know that the default implementation of the `valueOf` method simply returns the object on which it is called. So, the `toString` is invoked next. For Arrays, the `toString` method returns an empty string for empty arrays; for an array like `[1, 2, 3]`, it returns the contents of the array as a string, joined by commas, i.e., `"1,2,3"`. Each array element is coerced into a string and then joined by comma(s).

    In our case, we have a single element in an array, i.e., `[123]`, so it will be coerced into `"123"`. So the expression becomes:

    ```js
    "123" == 123
    ```
2.  Next, the string will be converted into a number. So the expression becomes:

    ```js
    123 == 123
    ```
3.  The types are equal, so the strict equality comparison is performed, i.e., `123 === 123`, giving us `true` as an output.

:::info
Weird fact about array conversion into a primitive value: an array containing `null` or `undefined` is coerced into an empty string, i.e., `[null]` ---> `""` and `[undefined]` ---> `""`. Similarly, an array containing both of them is coerced into a string containing a single comma, i.e., `[null, undefined]` ---> `","`. Why don't we get `"null"`, `"undefined"`, and `"null,undefined"` for such arrays, respectively? This is just one of the corner cases of coercion.
:::