We use JS to program the behavior of web pages. It can change..
- HTML Content
- HTML Attributes
- HTML Styles
We can also use it to hide and show elements on a page. The code must go either in a .js file or in script tags.
<body>
<p id="message">Default Message</p>
<button onclick="change()">Change Message</button>
//or <script src="external.js">
<script>
function changeMessage() {
document.getElementById("message").innerHTML = "Hello World";
}
<script>
</body>
Normally we place the script tag in the bottom of the body to improve display speed. It is better to use an external JS file to separate code for easier read. You can also display data with
window.alert(5 + 6)
document.write() //dont use this, it will delete all HTML
console.log() //debugging
A JS statement has values, operators, keywords, expressions, or comments. Keywords include for, if, else, function, let, switch, etc. An expression is what eventually computes to a value.
Variables
Variable identifiers can start with letters, _, or $, but no numbers. Adding a number and a string the result will always be a string.
let _price = 45;
let myName; //value is undefined
const theStr = "myString";
let fullName = "john" + "doe"; //johndoe
const combine = "5" + 2 + 3; //523 as a string
const 2 + 4 + "8" + 2 + 3; //6823 as a string
Operators
let sum = 0;
let value = 5;
sum += value; // sum = sum + value
=== means equal value AND type
!== means not equal value AND type
? : ternary
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