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JavaScritp

Java is NOT JavaScript

for my working page go here

Notes

what makes our page dynamic: html is sctucture css details (sofa) js what makes it all work

JS can store data and change based on user input. It is HUGE and can do a lot!

• create file in VSCode called app.js (.js for javascript)
• Inspect a webpage (right click) and open the Console where we can write JS directly to test what we want to do
  • console.log() logs to the console
• best practice is to write in cammal case eg myNameIs for variables etc

Variable in Programming

What is a variable? In programming, a variable is a value that can change, depending on conditions or on information passed to the program. Typically, a program consists of instruction s that tell the computer what to do and data that the program uses when it is running. js_variables

• var. can be redeclaired for variable so try and not use it
• let does not need a veriable when created unlike var
• const the value will NOT change

The var keyword was used in all JavaScript code from 1995 > to 2015.
The let and const keywords were added to JavaScript in 2015.
The var keyword should only be used in code written for older browsers.

Errors

• VM555:1 Uncaught TypeError: Assignment to constant variable. at ‘anonymous’:1:9 (‘ replaces > icons in notes)
  • cannot change as its a constant variable cause we used const
• Uncaught SyntaxError: Identifier ‘herName’ has already been declared
  • tried to create a duplicate variable
• VM282:1 Uncaught ReferenceError: herN is not defined at ‘anonymous’:1:17
  • var NEEDS a value

Notes from app.js file

• console.log(“this is a test”);
• let myName = “Jessica”;
• console.log(myName);

// data types

• let myPet = “Dog”; // this data is a string
• let myPetsAge = 7; // this data is a number
• let petDetails = “My pet is “ + myPetsAge; // this will print as a string
• console.log(petDetails);
• console.log(typeof petDetails); // will tell us what type the data is
• let likesWalkies = true // this data type is a boolean true/false statement
• console.log(typeof likesWalkies);
  // other types are arrays and objects

// arethemetic operators

• let addition = 7 + 7 // + is for addition
• console.log(addition)
• let subtraction = 5 - 2 // - is for subtraction
• console.log(subtraction)
• let multiplication = 2 * 3 // * is for multiplication
• console.log(multiplication)
• let division = 15 / 3 // / this is for division
• console.log(division)
• let myNum = 5
• let thisNum = ++myNum //++ is to increment (will look different if ++ is put at the end)
• console.log(thisNum)
• let thatNum = –myNum // – this will decrement by one
• console.log(thatNum)

// Comparison operators

• // == equal to
• // === equal value and equal type “7” === 7
• // != not equal
• // !== not equal value and not equal type
• // > greater than
• // < less than
• // >= greater than or equal to
• // <= less than or equal to

// Methods
// consol.log()
// prompt()
// document.write() !!! DO NOT USE THIS AS IT WILL OVERWRITE HTML
// alert() will show a message to the user

let userName = prompt(“Hi user :) please tell me your prefered name!”)
console.log(userName)
// if (userName == “Je”){ // } else (alert(“Please enter your name”))

const welcomeMsg = alert(“Welcome to my page “ + userName)

document.write()

Q&A

1. What are variables in JavaScript?

   In programming, a variable is a value that can change, depending on conditions or on information passed to the program.

2. What does it mean to declare a variable?

   Declaration of a variable in a computer programming language is a statement used to specify the variable name and its data type. Declaration tells the compiler about the existence of an entity in the program and its location. When you declare a variable, you should also initialize it.

3. What is an “assignment” operator, and what does it do?

   The assignment operator = assigns the value of its right-hand operand to a variable, a property, or an indexer element given by its left-hand operand. The result of an assignment expression is the value assigned to the left-hand operand.

4. What is information received from the user called?

   we use the prompt() function to ask the user for input. As a parameter, we input the text we want to display to the user. Once the user presses “ok,” the input value is returned. We typically store user input in a variable so that we can use the information in our program.

Links

js Functions