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

https://javascript.info/

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

https://1loc.dev/

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:

• https://github.com/hchiam/learning-nodejs/blob/master/README.md#read-a-local-file-via-console-cli

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 and export. 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 and module.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 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 name Name
• \n = reference to the nth capture group match (count the left brackets)
• \k<Name> = reference to the capture group with name Name (note: \k is not some variable, it's literally like replacing the "?")
• ^ = start
• $ = end
• x* = 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 times
• x{n,} = n or more times
• x{n,m} = n to m times, inclusive
• x*?, 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 (replace word with your word or word pattern)
• [\b] = backspace
• \xhh = character with 2 hexadecimal digits
• \uhhhh = UTF-16 character with 4 hexadecimal digits
• \u{hhhh}, \u{hhhhh} (with u 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()

Static import:

import { something as aliasedName } from "some-module.js";

Dynamic import():

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

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)

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 Promises.)