JavaScript Variables and Types Syntax Quiz

Variables

• In JavaScript, variables are declared using the var keyword.
• Variable types are not specified, but JavaScript has types including Number, Boolean, String, Array, Object, Function, Null, and Undefined.
• The typeof operator can be used to find out a variable's type.

Number Type

• Integers and real numbers are the same type in JavaScript.
• Same operators are used for numbers: +, -, *, /, %, ++, --, =, +=, -=, *=, /=, %=.
• Many operators auto-convert types, e.g., "2" * 3 is 6.

Comments

• JavaScript comments are the same as Java comments.
• There are four comment syntaxes: HTML, CSS/JS/PHP, Java/JS/PHP, and PHP.

Math Object

• The Math object has methods: abs, ceil, cos, floor, log, max, min, pow, random, round, sin, sqrt, tan.
• The Math object has properties: E and PI.
• Examples: var rand1to10 = Math.floor(Math.random() * 10 + 1);, var three = Math.floor(Math.PI);.

Null and Undefined

• null exists, but was specifically assigned an empty or null value.
• undefined has not been declared, does not exist.

Document Object Model (DOM)

• Most JavaScript code manipulates elements on an HTML page.
• We can examine elements' state and change state.
• Examples: see whether a box is checked, insert new text into a div, change styles.

Accessing Elements

• document.getElementById returns the DOM object for an element with a given id.
• We can change the text inside most elements by setting the innerHTML property.
• We can change the text in form controls by setting the value property.

Functions

• A function is a block of code that can be called multiple times from different parts of the script.
• Example: function changeText() { ... }.
• Functions can be used as event handlers.

Event-Driven Programming

• JavaScript programs wait for user actions called events and respond to them.
• This is called event-driven programming.
• Example: a clickable button that pops up a "Hello, World" window.