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Notes on Javascript Readings

Intro to JavaScript

Javascript is "a lightweight, interpreted, or just-in-time compiled programming language with first-class functions".

Input Output in plain JavaScript

Some new type of code I haven't seen before:

<body>

First name: <input id="first_name">
Last name: <input id="last_name">
<button id="say">Say hi!</button>
 
<hr>
<div id="result"></div>
 
<script>
function say_hi() {
    var fname = document.getElementById('first_name').value;
    var lname = document.getElementById('last_name').value;
 
    var html = 'Hello <b>' + fname + '</b> ' + lname;
 
    document.getElementById('result').innerHTML = html;
}
 
document.getElementById('say').addEventListener('click', say_hi);
</script>

<body>

JavaScript Variables

4 Ways to Declare a JavaScript Variable:

  • Using var
  • Using let
  • Using const
  • Using nothing

For constant variables, use const.

They follow logic. It's all just math!

Note: JavaScript identifiers are case sensitive.

Assignment operator (assigning value) --> =

"equal to" operator --> ==

Two main data types are numbers and strings.

Declare a variable with var or let.

Assign a value using =

You can assign a value when you create a variable:

let age = 29

When declaring multiple variables in one statement, separate variables with a comma, and end with semicolon. You can span multiple lines if you choose.

A variable without a declared value will have value of undefined.

If re-declaring a JavaScript variable with var, it will not lose its value. But you cannot re-declare a variable declared with let or const. Won't work.