Javascript anatomy: Difference between revisions
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Javascript is used in a variety of places.<br> | |||
Here are some common entrypoints, and some sample code to get a feel for it. | |||
__TOC__ | __TOC__ | ||
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<script src="https://foo.com/js/foo.js"></script> | <script src="https://foo.com/js/foo.js"></script> | ||
</source> | </source> | ||
You can use script tags anywhere, but this is discouraged in the [[Http content security policy]]. | |||
<syntaxhighlight lang="html5"> | |||
<div> | |||
<script> | |||
document.getElementById("foo").innerHTMLL = "BAR"; | |||
</script> | |||
</div> | |||
</syntaxhighlight> | |||
</blockquote><!-- Script Tags --> | </blockquote><!-- Script Tags --> | ||
Latest revision as of 12:54, 5 February 2023
Javascript is used in a variety of places.
Here are some common entrypoints, and some sample code to get a feel for it.
NodeJS CLI
${PROJECT}/main.mjs
#!/usr/bin/env node import * as Process from 'node:process' import * as Path from 'node:path' const EXECUTABLE = Path.basename(Process.argv[1]); class CliCommand { help = false; verbose = false; static fromArgs() { var cmd = new CliCommand(); var argv = Process.argv.slice(2); while (argv.length) { var item = argv.shift(); switch (item) { case "-h": case "--help": cmd.help = true; break; case "-v": case "--verbose": cmd.verbose = true; break; default: console.error(`Invalid Param: '${item}'. See --help`); Process.exit(1); } } return cmd; } exec() { if (this.help) { showHelp(); Process.exit(0); } if (this.verbose) { console.debug("loggers set to stun"); } console.log(".. your program starts here"); } } function showHelp() { var msg = `${EXECUTABLE} [-h] [-v] ARGS: -h --help show this help message. EXAMPLE: ${EXECUTABLE} -h # show help `.trim() console.log(msg); } function main() { var cmd = CliCommand.fromArgs(); cmd.exec(); } main();./main.mjs -h # run
Web Browser
Script Tags
One way that javascript can added to HTML within a
<script>
tag.
You can enter it directly, or set thesrc=
parameter.
All HTML elements defined above the<script
is accessible within it (and not items below).<script> function myFunction() { document.getElementById("demo").innerHTML = "Paragraph changed."; } </script><script src="myscript.js"></script> <script src="https://foo.com/js/foo.js"></script>You can use script tags anywhere, but this is discouraged in the Http content security policy.
<div> <script> document.getElementById("foo").innerHTMLL = "BAR"; </script> </div>ES6 Modules
ES6 modules let you import your code from other files (ex
import * as Math from './modules/math.js'
).Note for testing: ES6 modules) do not work when loading webpages directly from the filesystem.
You can use a tiny toy web-server like python http or golang simple-go-web-server to load your javascript instead.${PROJECT}/index.html
<!doctype html public "-//W3C//DTD HTML 4.01 Frameset//EN" "http://www.w3.org/TR/html4/frameset.dtd"> <html lang="en"> <head> <title>My Website</title> <script type="module" src="main.js"></script> <!-- <link rel="stylesheet" href="stylesheet.css" /> --> </head> <body> <h1>My Title</h1> <p>Some paragraph</p> <div id="sum-div"> <button id="sum-btn">Add</button> <input class="addend" type="text" placeholder="1"> <input class="addend" type="text" placeholder="2"> <span>=</span> <input class="result" type="text" readonly="1"> </div> </body> </html>${PROJECT}/main.js
import * as Math from './modules/math.js' function sumClicked() { var sum_btn = document.getElementById("sum-div"); var addend_inputs = Array.from(sum_btn.querySelectorAll(".addend")); var addend_values = addend_inputs.map((input) => { return Number(input.value) }); var sum = Math.sum(...addend_values); var result_input = sum_btn.querySelector(".result"); result_input.setAttribute("value", String(sum)); } document.getElementById("sum-btn").addEventListener("click", sumClicked);${PROJECT}/modules/math.js
export function sum(a, b) { return a + b; } export function multiply(a, b) { return a * b; }
open 'http://localhost:3000' simple-go-web-server --hot # reload to get changesOn Event Hooks
You can register callbacks to events on matching items.
document .getElementById('button') .addEventListener('click', () => { /* do something when clicked */ })You also can trigger javascript from HTML DOM events, with the
on${Event}
syntax.
This is now discouraged for security reasons (See Http content security policy)<button onClick="console.log('hello');">Log Hi</button>Javascript Console
Most browsers have a javascript console where you can execute arbitrary javascript commands on a loaded webpage.
While you cannot evaluate files directly from your filesystem, but you can use a toy webserver like golang simple-go-web-server
to host your files, and then source them within the JS console.// dir-with-js/foo.js console.log("HI");cd dir-with-js open 'http://localhost:3000' simple-go-web-server --hot # reload to get changes// javascript console $.getScript("http://127.0.0.1:3000/foo.js")