Awesome
Learning JavaScript
Miscellaneous practice code in JS.
Just one of the things I'm learning. https://github.com/hchiam/learning
https://random-code-tips.surge.sh
Broaden JavaScript knowledge
e.g.:
- https://javascript.info/weakref-finalizationregistry
- https://javascript.info/proxy
- https://javascript.info/currying-partials
ReqBin - test API endpoints by making API requests
https://reqbin.com/ - also shows a bunch of example requests like GET with bearer token auth header, or Curl/JS/Python/PHP/REST/POST/JSON/POST/PATCH/PUT/DELETE
https://reqbin.com/json-formatter
https://reqbin.com/json-formatter
How to Run .js Files Using Terminal/CommandLine:
Make sure to include console.log("output text here");
node filename.js
Learn about Modern ES6 JS Features
https://stackoverflow.blog/2019/09/12/practical-ways-to-write-better-javascript
Also note the stuff around the default values for function arguments here: https://devhints.io/es6?ref=devawesome.io
Support Both Legacy JS and Modern JS Without Slowing All Browsers
Bonus
Automatically format your code upon save
In VSCode: use the Prettier extension, and then in VSCode > Code > Preferences > Settings > search for "Format on save" > turn it on with the checkbox
Now it'll automatically format your code, just like the built-in stuff for Golang.
Get immediate feedback on code errors and style
To automatically re-run a .js file and the test cases in it whenever you edit that file, copy the .js code into index.js
in my ESLint repo and do the setup so you can run this command just once:
nodemon -x 'npm run lint; node index.js'
Or nodemon -x 'npm run lint; npm run test; node index.js'
Or nodemon -x 'jest lowestIndexSameAsValue.test.js'
for example, to re-run the tests just for one file.
This works just like rerun
for Python.
Alternatively:
To set up eslint and jest locally inside this learning-js folder:
npm install jest --global
npm install # eslint is already listed in package.json for you
npm test # runs scripts test command listed in package.json
Or just run this: jest
.
Or to run just a-specific.test.js, run this: jest a-specific.test.js
.
You can also automatically include code style fixes in your commits with lint-staged
set up with husky
.
Get code Maintainability Index (MI score)
The MI combines lines of code, cyclomatic complexity, and the Halstead volume metric (i.e. number of variables, operations, decision paths, and lines of code). After you npm install -g plato
or yarn global add plato
, you can get the MI score of your code:
plato -r -d report index.js
Similar to how I use radon
for Python code.
Minify code
Install minify
:
npm i minify -g # or: yarn global add minify
minify -v
Use minify
:
minify minify-this-script.js > minified-script.js
stuff you can do without JS-heavy web frameworks
https://codepen.io/hchiam/pen/ExbmjEP
Using yarn
instead of npm
yarn # instead of npm install or npm run install
yarn test # instead of npm test
Instead of nodemon -x 'npm run test; node index.js'
, you can do:
nodemon -x 'yarn test; node index.js'
Service Workers
Learning about them: https://github.com/hchiam/learning-service-workers
A list of useful one-liner utilities
Interesting a11y-related JS code
https://github.com/hchiam/keyboard-focus-trap
https://github.com/hchiam/flying-focus
More data structures/algorithms
https://github.com/hchiam/learning-splay-tree
https://github.com/hchiam/learning-b-tree
https://github.com/hchiam/learning-skip-list
https://github.com/hchiam/learning-bloom-filter
https://github.com/hchiam/learning-union-find
https://github.com/hchiam/learning-suffix-tree
https://github.com/hchiam/learning-lzw
Chrome dev tools tricks for more productive debugging
https://www.youtube.com/watch?v=hdRDTj6ObiE
https://blog.bitsrc.io/10-tips-to-improve-productivity-using-chrome-dev-tools-7918fc8203f3
and tips like this: https://umaar.com/dev-tips/15-dollar-zero
$_
= previous value
$0
, $1
, $2
, $3
, $4
= the last 5 DOM elements you clicked on in the Elements tab
$('some_selector')
= shortform for document.querySelector('some_selector')
$$('some_selector')
= shortform for document.querySelectorAll('some_selector')
$(
some_selector, ancestorElement)
or $('some_selector', $0)
$x('some_x_path')
= XPath
inspect($('some_selector')[0]);
jumps to Elements panel (jQuery not required for that $
). Works in Firefox too.
queryObjects(YourConstructor)
= all objects created by YourConstructor
getEventListeners($0)
= event listeners on the element you last clicked on in Elements tab
monitorEvents(element, eventName)
= prints captured events
unmonitorEvents(element)
monitor(functionName)
= prints function call with arguments, and also output
unmonitor(functionName)
table()
(shortcut in Chrome) = console.table()
-
console.table([ { a: 1, c: "hi" }, { a: 3, b: 2 }, ]); /** * (index) a c b * 0 1 'hi' * 1 3 2 */
clear()
= console.clear()
keys()
= Object.keys()
values()
= Object.values()
copy()
= copies to clipboard any value/object/element inside it.
More: https://developers.google.com/web/tools/chrome-devtools/console/utilities#geteventlisteners
Edit any website (temporarily on the client side)
Enter this into the console log in dev tools: document.designMode='on'
Read a file
In the browser:
- https://github.com/hchiam/learning-js/blob/master/read-text-file.js or
- https://github.com/hchiam/learning-js/blob/master/read-json-file.js
In the console terminal CLI:
Compare JSON files
https://codepen.io/hchiam/pen/RwXqxwZ
- edited
- deleted
- added
Get a lot of the functions and jQuery event listeners in a script string:
https://github.com/hchiam/getFunctions
Support modern browsers and older browsers (like IE11) at the same time
<!-- No need for special server setup or user agent sniffing! -->
<script type="module" src="modern.js"></script>
<script nomodule src="legacy.js"></script>
<!-- https://www.youtube.com/watch?v=cLxNdLK--yI -->
Bookmarklets
Only use them after you read and understand the contents of each bookmarklet!
https://en.wikipedia.org/wiki/Bookmarklet
https://github.com/hchiam/learning-js/tree/master/bookmarklets#bookmarklets
this
keyword in old-school functions vs the newer arrow functions
this
in function()
= caller. Useful if you want this
to change to whatever calls your one function.
this
in () => {}
= owner. Useful if you want this
to always be the creator of the function. I think nested arrow functions also pass along this
, which you might like.
// https://www.freecodecamp.org/news/the-difference-between-arrow-functions-and-normal-functions/
const obj = {
a: () => {
console.log(this);
console.log(
`"this" will be whatever "this" was before the function was created`
);
},
f: function () {
console.log(this);
console.log(`"this" will be the current object that contains the function`);
},
};
obj.a();
obj.f();
CJS vs MJS/ESM/ES6M vs all the other types of JavaScript modules syntax
- ESM =
import
andexport
. Simplest, async, tree-shakeable, but not universally compatible. - UMD = works everywhere, is more of a pattern of fallbacks, usually the fallback when ESM fails.
- CJS =
require
andmodule.exports
, sync, good for BE, commonly seen for node stuff. - AMD =
define
, async, good for FE, confusing compared to CJS.
Read later:
https://www.sitepoint.com/understanding-es6-modules
https://stackoverflow.com/a/46677972
https://dev.to/iggredible/what-the-heck-are-cjs-amd-umd-and-esm-ikm
https://hacks.mozilla.org/2018/03/es-modules-a-cartoon-deep-dive/
Priority queue which can be used as a min heap or max heap
// in CJS syntax:
const {
MinPriorityQueue,
MaxPriorityQueue,
} = require("@datastructures-js/priority-queue");
or
// in ESM syntax:
import {
MinPriorityQueue,
MaxPriorityQueue,
PriorityQueueOptions, // queue options interface
PriorityQueueItem, // queue item interface
} from "@datastructures-js/priority-queue";
API usage example:
const pqAsHeap = new MinPriorityQueue();
pqAsHeap.enqueue(num);
pqAsHeap.dequeue().element;
pqAsHeap.size();
pqAsHeap.front().element;
add type checking even in JS files, no config necessary (in VSCode):
Just add this comment to the top of your JS file:
// @ts-check
Scope console log scripts in DevTools to an iframe or other environment:
Chrome:
Firefox:
Regex and ReDoS security
- avoid evil RegEx, avoid exponential backtracking, avoid ReDoS: https://owasp.org/www-community/attacks/Regular_expression_Denial_of_Service_-_ReDoS
Regex cheatsheet
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions/Cheatsheet
x(?=y)
= lookahead (AKA positive lookahead, in contrast to negative lookahead) = "match x if followed by y"x(?!y)
= negative lookahead = "match x if NOT followed by y"(?<=y)x
= lookbehind (AKA positive lookbehind, in contrast to negative lookbehind) = "match x if preceded by y"(?<!y)x
= negative lookbehind = "match x if NOT preceded by y"(?:x)
= noncapturing group = "match x but don't remember the group"(?<Name>x)
= capture group with nameName
\n
= reference to the nth capture group match (count the left brackets)\k<Name>
= reference to the capture group with nameName
(note:\k
is not some variable, it's literally like replacing the "?
")^
= start$
= endx*
= 0 or more times (mnemonic:*
looks more like a 0 than+
does, and*0
changes a number, while*1
doesn't)x+
= 1 or more times (mnemonic:+
looks more like a 1 than*
does, and+1
changes a number, while+0
doesn't)x?
= 0 or 1 time = "optionally exists"x{n}
= n timesx{n,}
= n or more timesx{n,m}
= n to m times, inclusivex*?
,x+?
,x??
,x{n}?
,x{n,}?
,x{n,m}?
= match non-greedily = "match minimally", e.g./<.*?>/
only matches<a>
instead of<a></a>
entirely.\b
= "word boundary" (note:/\w\b\w/
can't ever matche anything, but/\w\b\W\w/
can)- likely useful as:
/\bword\b/g
(replaceword
with your word or word pattern)
- likely useful as:
[\b]
= backspace\xhh
= character with 2 hexadecimal digits\uhhhh
= UTF-16 character with 4 hexadecimal digits\u{hhhh}
,\u{hhhhh}
(withu
flag) = Unicode character with 4/5 hexadecimal digits
My applied example: regex to automatically group digits with spaces in input boxes:
- https://codepen.io/hchiam/pen/yLrjgrV (4 digits from left to right, or from right to left)
.split(/(\d{4})/g)
-->.replace(/\D/g,'').split(/(\d{4})/g).filter(x=>x).join(' ')
1234567890
-->1234 5678 90
Web Locks
// multiple browser tabs can try to access a lock named my_resource, but only one will be processed at a time (queued)
// also scoped to origins (https://example.com is different from https://example.org:8080)
navigator.locks.request("my_resource", async (lock) => {
// lock acquired
// do stuff
await do_something();
await do_something_else();
// lock released
});
Note: deadlocks can still happen if, say, multiple locks are requested out-of-order.
More details/options: https://developer.mozilla.org/en-US/docs/Web/API/Web_Locks_API
import
vs import()
import { something as aliasedName } from "some-module.js";
import("/some-module.js").then((aliasedName) => {});
// or:
const { default: myDefault, foo, bar } = await import("/some-module.js");
// or just:
await import("/some-module.js"); // like if you just want its side-effects
D3 <path>
/svg .click()
note
/** Because simply using d3Element.click() or jQuery $(d3Element).click() doesn't work: https://stackoverflow.com/questions/9063383/how-to-invoke-click-event-programmatically-in-d3 */
function triggerD3PathClick(d3Element) {
const event = new MouseEvent("click");
d3Element.dispatchEvent(event);
}
xpath
Here's an xpath to get only the English entry part of a Wiktionary page:
$x("//h2[span[text()='English']] | //h2[span[text()='English']]/following-sibling::*[preceding-sibling::h2[1][span[text()='English']] and not(self::h2)]")
// use | so you can include the English h2 in the selection
// you need the [1] so you stop selecting elements after the next h2
// you need self:: in not(self::h2) to avoid including the next h2
Building on that, here's an xpath to get just the etymology and definition(s) (<p>
or <ol>
) of the English part of a Wiktionary page:
$x("//h2[span[text()='English']]/following-sibling::*[self::p or self::ol][preceding-sibling::h2[1][span[text()='English']]]")
Event listener event.target
vs event.currentTarget
vs event.relatedTarget
https://stackoverflow.com/a/10086501
currentTarget
= listening element (e.g. the individual button that has the click event listener fired on it)target
= triggering element (i.e. maybe the button, or maybe the i or span you actually clicked on inside of the button)
https://developer.mozilla.org/en-US/docs/Web/API/FocusEvent/relatedTarget
relatedTarget
= element focused to forblur
, focused from forfocus
; similar idea forfocusin
andfocusout
andmouse
(...)/drag
(...) events, but note thatrelatedTarget
may benull
for non-focusable elements or for security reasons like tabbing out of a page.
operator precedence reference
for example, &
is evaluated before &&
before ||
: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Operator_precedence#table
example of debugging with chrome dev tools and fixing code base
"Optimizing INP: A deep dive": https://youtu.be/cmtfM4emG5k
weird timing behaviour with try
catch
finally
https://jakearchibald.com/2021/last-return-wins/
The finally
console log prints 'two'
"before" the return 'three'
:
// this code prints out 'one', 'two', 'three', 'four':
function finallyTest() {
try {
console.log('one');
return 'three';
} catch (err) {
console.log('error');
} finally {
console.log('two');
}
}
console.log(finallyTest());
console.log('four');
This might cause unexpected timing issues if you're not aware of this. (Also, .finally
behaves differently for Promise
s.)
More notes on Promise
s and async
/await
: https://www.joshwcomeau.com/javascript/promises/
Example use of JS animation ScrollTimeline
that goes dynamically beyond what CSS can do
https://web.dev/articles/building/a-tabs-component#animation
tag function of template literals string, raw
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/raw
var thisisTrue = String.raw`C:\folder\path\file.js` === `C:\\folder\\path\\file.js`;
// using new RegExp with String.raw and variable:
var x = 'Street';
console.log(new RegExp(`(\\d+) ${x}`).exec('123 Street')[1]);
// '123'
console.log(new RegExp(String.raw`(\d+) ${x}`).exec('123 Street')[1]);
// '123'
space characters
You might know about ' '
and
, but did you know about  
('\u2008'
) which takes up space but is able to wrap? there's even more Unicode characters: https://stackoverflow.com/questions/8515365/are-there-other-whitespace-codes-like-nbsp-for-half-spaces-em-spaces-en-space You can also do this in JS:
'a'.padEnd(10, '\u2008'); // U+2008 is Unicode for  
using JS to set CSS styles
element.style.color = 'red'; // this does something
element.style.color = 'red !important'; // WARNING: this won't do anything! it won't even change the color!
element.style.setProperty('color', 'red', 'important'); // this works if you want to include !important
width and height of HTML elements in CSS/JS
- offsetWidth, offsetHeight (includes padding and border)
- clientWidth, clientHeight (like offsetWidth and offsetHeight but without the padding or border)
- CSS width, CSS height
- scrollWidth, scrollHeight (doesn't include padding or border, but does include overflow content)
remove initial "1." text node without replacing children HTML:
// if first child (likely textNode) starts with "#.", then remove that child:
if (/^\d+\.$/.test(element.childNodes[0].nodeValue)) {
element.childNodes[0].remove();
}