JavaScript Notes

Tecnate | Last Updated: 2024.02.12

JavaScript Notes

Before You Begin Coding

There are a few things that you will pick up as you learn how to write more and more complex code. Save yourself the time that would otherwise be spent learning by trial and error by internalizing these key ideas:

Outline Structure

Explicitly identify what you are trying to do.
1. “The purpose of my program/script/code is __.”
List out the main steps that you anticipate.
1. Flowcharts are great for visualizing information flow and logic control.
2. Focus on the destination at this stage more than every step along the way.
  1. Don’t worry if you don’t know every step ahead of time.
  2. It’s normal (and often necessary) to change your strategy once you actually begin.

Remember: Pseudocode Is Your Friend

It is a professional tool: use it to outline requirements for contracts or clients.
1. Everyone involved will have a better understanding of the scope, complexity, and timeline if you take a moment to think through the steps of your development.
It should clearly outline tasks and include:
1. Input
  1. read, get, input, find, etc.
2. Processing
  1. add, transform, concatenate, sort, calculate, etc.
3. Output
  1. post, output, print, return, log, etc.
Make it language independent
1. Write statements that any programmer could utilize in their programming environment of choice.
2. In general, try to begin new statements with a verb that makes it clear what your code needs to do.
  1. Nest loops under actionable statements when necessary.
3. Name your variables within the statement.

Pseudocode Example: “Print the product of two numbers.”

Read two numbers, save as num1 and num2.
Multiply num1 and num2, save as product.
Print product.

Remember: Work Small

Refer back to what you’re trying to accomplish and choose one specific item to tackle.
1. Tunnel vision is usually better than thinking about everything all at once.
2. Break up your work into tasks, not projects.
Assume that your future code will work so you can focus on the present code.
1. Test your code often.
  1. Especially before you integrate it into any other code!
2. Squash bugs as soon as you find them.
Assume that you’ll figure out how to integrate all the parts together.
1. Get small parts of your code to work.
2. Once you know the discrete parts are functional, you can focus on combining everything together into a cohesive script.

Start ASAP

Don’t wait for perfect knowledge or understanding to begin a project.
1. You’ll never know everything you need to know.
2. Even if you did know everything at one point in time:
  1. Everything is subject to change: requirements, resources, funding, timelines, etc.
  2. Obsolescence is always around the corner.
Don’t wait for the perfect development environment or text editor customization.
1. Create your environment as you go and discover what you need.

JavaScript Paradigm

High-level Language
- Designed for humans rather than machines.
- High amounts of abstraction to resemble natural human language & math notation.
Just-in-time Compilation
- JavaScript is interpreted (not compiled) during program execution rather than before.

Dynamic Typing

The same variable can hold different data types.

let x; // x is undefined
x = 5; // x is now a number
x = "John"; // x is now a string

Prototype-based Object Orientation
- Objects inherit properties from parent object templates (prototypes).
First Class Functions
- Functions can be:
  - Passed as arguments to other functions.
  - Returned by another function.
  - Assigned as a variable value.
Supports Multiple Programming Paradigms
- Event-driven
  - The flow of programming is determined by events.
    - E.g. a mouse click, button click, key press, time interval, window resizing, etc.
  - Most JavaScript is written to react to events.
  - Event handlers can run at any time and in any order (i.e. they are asynchronous).
- Functional
  - Declarative programming in which functions are trees of expressions that map values to other values.
- Imperative
  - A sequence of imperative statements update the program's state.

Data Types

There are two basic data types in JavaScript: Primitives & Objects.

Source: “Head First JavaScript Programming - Chapter 7”

Objects keep state and behavior, i.e. they have properties and methods.
Primitives simply represent data without state and behavior.
- The major exception is Strings. The JavaScript interpreter will temporarily create string objects as needed to support methods available to strings.
```
var password = "open$esame";
var index = password.indexOf("$");
console.log(index); // 4
```
- The minor exceptions are Numbers and Booleans which can sometimes act like objects, though not nearly as often as Strings.

Primitives

Primitives are immutable (hardcoded and unchangeable) values that represent the lowest level of language.

// Immutability
var greeting = "hello"; // "hello"
greeting.toUpperCase(); // still "hello"

// Assignment gives the primitive a new value, not a mutated one.
greeting = greeting.toUpperCase(); // "HELLO"

There are seven primitive types in JavaScript:

String: 'single', "double", literal.

Number: integers, floats, Infinity, -Infinity, NaN.

NaN (Not a Number): an illegal number value not equal to any value (including itself) and is treated falsy for booleans.

console.log(NaN == NaN); // false
console.log(NaN != NaN); // true

// WRONG
let x = "banana";
if (x == NaN) {
    console.log(x); // (Nothing happens)
}

// RIGHT
let y = "orange";
if (isNaN(y)) {
    console.log(y); // orange
}

Used for numeric results that can’t be represented.

NaN is still a number type!

var a = 0 / 0;
var b = "food" * 8;
var c = Math.sqrt(-9); // JS can't natively represent complex numbers.

console.log(a, b, c); // NaN NaN NaN
console.log(typeof a, typeof b, typeof c); // number number number

Infinity or -Infinity: a property of the global object (i.e. a variable in global scope). Its value is greater than any other number.

console.log(typeof Infinity); // number
console.log(Infinity == "infinity"); // false ("infinity" ==> NaN)
console.log(Infinity - 16); // Infinity
console.log(-Infinity < Infinity + 16); // true

Bigint: numbers bigger than 2^53 -1
Symbol: an always-unique, "hidden" identifier that no other code can accidentally access.
Boolean: true, false
Undefined: no value.
- It’s assigned to variables, properties, or array items that don’t yet have a value (i.e. they haven’t been initialized) or functions that don’t return anything:
  1. Variables without a value assignment.
```
var x;
console.log(x); // undefined (a value, not the string "undefined")
```
  2. Value of a property that doesn’t exist or has been deleted.
```
const person = {
    name: "John",
};

console.log(person.age); // undefined
```
  3. An item that doesn’t exist within a sparse array.
```
var sparseArray = [];
sparseArray[0] = "peach";
sparseArray[5] = "apple";
console.log(sparseArray[3]); // undefined
```
  4. Functions without a return statement.
- Can be an identifier for a property of a global object.
- Treated falsey (or falsy) for booleans.
- Undefined is a primitive type but can also be a value.
  - If undefined is a value, its type is undefined (because it isn’t a string or number or object or anything else).
Null: no object.
- The intentional absence of any object value indicates that the variable points to no object.
  - Assigning null to a variable means “I’m going to assign an object to this variable at some point, just not yet.”
- Treated falsy for booleans.

undefined	null
Variables that haven’t been initialized.	The object can't be created/found
Objects with a missing property.	`document.getElementById` for an object that doesn’t exist in the DOM.
Arrays with a missing value	-

Objects

Objects are a collection of properties for state & behavior. They can be of any type, including other objects.

Objects are written as {key: value} pairs.
Any value that isn’t a primitive type is an object.

✅ **Scripting Best Practice**

Declare objects with const.

const myCar = {
	name: "Batmobile",
	make: "Custom"
	navigation: true,
	passengers: 2
};

When you pass an object into a function call, you’re passing in the object’s reference, not the object itself.

Changing object properties inside a function changes the original object’s properties!

const snoopy = {
    weight: 40,
};
console.log(snoopy.weight); // 40

// Function calls with objects will update the original object
function loseWeight(dog, amount) {
    dog.weight = dog.weight - amount;
}

loseWeight(snoopy, 5);
console.log(snoopy.weight); // 35 (Original object has been changed)

Data Variables

Primitive variables represent the value of the variable.
Object variables represent a way to get to the object.
- Their variables hold a reference that points to the object, which is accessed by the interpreter.

Note: unlike object variables, primitive variable values are shadowed in functions.

Data Type Comparison, Conversion, & Coercion

Type Coercion: the process of automatic conversion of values from one data type to another.

Primitives Comparison

String + Number == String.
1. Numbers will be converted to strings using coercion and then concatenated with the string.
2. + is always interpreted as concatenation when one of the operands is a string.
```
console.log(typeof ("apple" + 7)); // string ("apple7")
```

String * - / % Number == Number.

All non-number values are converted into the number data type.

console.log(typeof ("apple" * 7)); // number (NaN)
console.log(typeof ("apple" - 7)); // number (NaN)
console.log(typeof ("apple" / 7)); // number (NaN)
console.log(typeof ("apple" % 7)); // number (NaN)

Boolean + * - / % Number == Number:

Boolean values are converted to a number type for comparison: 0 for false, 1 for true.

console.log(typeof (false + 2)); // number
console.log(false + 3); // 3 (false == 0)
console.log(20 / true == 5 * 4); // true (true == 1)

The Equality Operator == can be used to compare values irrespective of type. Likewise with comparison operators <, >, <=, or >=.

String & String

console.log("a" == "A"); // false (different Unicode values)
console.log("a" > "b"); // false (first < second)
console.log("a" > "B"); // true (lowercase > UPPERCASE)

String & Number

console.log("apple" == 5); // false ("apple" ==> apple ==> NaN ≠ 5)
console.log("5" == 5); // true ("5" ==> 5 == 5)
console.log(99 <= "100"); // true ("100" ==> 100)

Boolean & String

console.log(true == "apple"); // false
console.log(true == "true"); // false (true ==> 1; "true" ==> NaN)
console.log(false == "0"); // true (false ==> 0; "0" ==> 0)
console.log(false != "apple"); // true ("apple" ==> apple ==> NaN))

Boolean & Number

console.log(true == 1); // true
console.log(true == NaN); // false (NaN not equal to anything)
console.log(NaN == NaN); // false (NaN not even equal to itself)
console.log(false == NaN); // false
console.log(true != NaN); // true
console.log(19 < false); // false (false ==> 0)

Boolean & Undefined/Null

console.log(null == undefined); // true (null & undefined are always equal)
console.log(true == null); // false
console.log(true != null); // true
console.log(false == undefined); // false
console.log(false == ""); // true (false ==> 0; "" ==> 0)
console.log(44 >= ""); // true ("" ==> 0)

Object Comparison

OBJECTS: Person1, Person2
REFERENCES: Aragorn, Strider, Samwise

const person1 = {
    type: "Human",
};

const person2 = {
    type: "Hobbit",
};

let aragorn = person1;
let strider = person1;
let samwise = person2;

if (aragorn == strider) {
    console.log("Same person.");
} else {
    console.log("Different people.");
}

// Same person.

State Management

The web is stateless, meaning that webpages are recreated at the time they are requested. Because HTTP is a stateless protocol, it can’t hold client information on the webpage.

ViewState is specific to a page in a session. It is a server-side class object that stores data on the client side in an encrypted (hashed) format and can’t be read by plain JavaScript. - ViewState variable can be accessed inside JavaScript code with the help of Server Side Code Blocks in ASP.Net with C# and VB.Net.

SessionState is specific to user-specific data that can be accessed across all web application pages.

ViewState	SessionState
Maintained at page level.	Maintained at session level.
Values retained in the event of a postback operation.	Data is available to user until the browser is closed or there is session expiration.
Information is stored on the client’s end only (e.g. browser).	Information is stored on the server.
Allows persistence of user-specific data at the page level.	Allows persistence of user-specific data throughout the entire web app.
Values are cleared when a new page is loaded.	Can be cleared by developer, user, or timeout.

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JavaScript Notes

Table of Contents

Before You Begin Coding

Outline Structure

Remember: Pseudocode Is Your Friend

Remember: Work Small

Start ASAP

JavaScript Paradigm

Data Types

Primitives

Objects

Data Variables

Data Type Comparison, Conversion, & Coercion

Primitives Comparison

Object Comparison

State Management

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JavaScript Notes

Table of Contents

Before You Begin Coding

Outline Structure

Remember: Pseudocode Is Your Friend

Remember: Work Small

Start ASAP

JavaScript Paradigm

Data Types

Primitives

Objects

Data Variables

Data Type Comparison, Conversion, & Coercion

Primitives Comparison

Object Comparison

State Management