It is a scripting language used to develop web pages
It adds interactivity in the websites
Is a prototype-based, multi-paradigm, single-threaded, dynamic language, supporting object-oriented, imperative, and declarative (e.g. functional programming) styles

NOTE:

The let keyword tells JavaScript you are declaring a variable.
JavaScript interacts with the HTML using the Document Object Model, or DOM. The DOM is a tree of objects that represents the HTML. You can access the HTML using the document object, which represents your entire HTML document.
One method for finding specific elements in your HTML is using the querySelector() method. The querySelector() method takes a CSS selector as an argument and returns the first element that matches that selector. For example, to find the <h1> element in your HTML, you would write:

let h1 = document.querySelector("h1");

Scope is the term used to describe where a variable can be accessed. If a variable is declared inside a block of code, it is only accessible to the code inside that block. This is called block scope.

let num = 1;
if (num === 1) {
let num = 2; // this num is scoped to the if statement
console.log(num); // expected output: 2
}
console.log(num); // expected output: 1 (the global variable)

The shift() method on an array removes the first element in the array and returns it.

const numbers = [1, 2, 3];
const firstNumber = numbers.shift(); // returns 1

The Math object in JavaScript contains static properties and methods for mathematical constants and functions. One of those is Math.random(), which generates a random number from 0 (inclusive) to 1 (exclusive). Another is Math.floor(), which rounds a given number down to the nearest integer. Using these, you can generate a random number within a range. For example, this generates a random number between 1 and 5: Math.floor(Math.random() * 5) + 1;.
The innerHTML property allows you to access or modify the content inside an HTML element using JavaScript.

Here is an example of updating the content for this paragraph element using the innerHTML property.

<p id="demo">This is a paragraph.</p>

document.querySelector("#demo").innerHTML ="Hello, innerHTML!"; 

Functions run specific blocks of code when they are called, but they can also return a value. This value can be assigned to a variable and used elsewhere in your code.
The ternary operator is a conditional operator and can be used as a one-line if-else statement. The syntax is: condition ? expressionIfTrue : expressionIfFalse.

Here is an example of returning a value using an if-else statement and a refactored example using a ternary operator:

// if-else statement
if (score > 0) {
  return score
} else {
  return default_score
}

// ternary operator
score > 0 ? score : default_score

The g flag, which stands for “global”, will tell the pattern to continue looking after it has found a match. Here is an example:

const helloRegex = /hello/g;

JavaScript has a feature called template literals, which allow you to interpolate variables directly within a string. Template literals are denoted with backticks , as opposed to single or double quotes. Variables can be passed in to a template literal by surrounding the variable with ${} – the value of the variable will be inserted into the string.

For example:

const name = "Naomi";
const templateLiteral = `Hello, my name is ${name}~!`;
console.log(templateLiteral);

The console will show the string “Hello, my name is Naomi~!”.

${example}
behaves like a placeholder
A for...of loop is used to iterate over elements in an iterable object like an array. The variable declared in the loop represents the current element being iterated over.

for (const element of elementArray) {
  console.log(element);
}

The Number constructor is a function that converts a value to a number. If the value cannot be converted, it returns NaN which stands for “Not a Number”.

Here is an example:

Number('10'); // returns the number 10
Number('abc'); // returns NaN

[!IMP] Templates and literals