Simon Willison’s Weblog

Promise Maps. Egbert Teeselink describes a neat JavaScript caching pattern: instead of caching key:value cache key:promise-that-resolves-to-value—doing this gives you dog piling prevention for free, because the first lookup of a value trigers the computation to fetch it while subsequent lookups wait on the same promise to resolve—or resolve instantly if the computation has completed. # 22nd July 2022, 3:52 pm

Bringing page transitions to the web (via) Jake Archibald’s 13 minute Google I/O talk demonstrating the page transitions API that’s now available in Chrome Canary. This is a fascinating piece of API design—it works by effectively creating a static image screenshot of the before and after states of the transition, then letting you define CSS animations that animate a transition between the two static images. By default the screenshot encompasses the full viewport, but you can instead define multiple elements within the page and apply separate transitions to them. It’s only available for SPAs right now but the final design should include support for multi-page applications as well—which means transitions with no JavaScript needed at all! # 13th July 2022, 4:26 pm

Helpful 404s for Jekyll (and GitHub Pages). Neat trick from Ben Balter: JavaScript that runs on your 404 page, fetches the sitemap.xml, parses all of the URLs out of it and then uses a levenshtein edit-distance comparison to find the closest URL to the one that you landed on and suggests that as a “Perhaps you’re looking for?”. # 6th July 2022, 5:31 pm

Bun. “Bun is a fast all-in-one JavaScript runtime”—this is very interesting. It’s the first project I’ve seen written using the Zig language, which I see as somewhat equivalent to Rust. Bun provides a full Node.js-style JavaScript environment plus a host of packaged tools—an npm install client, a TypeScript transpiler, bundling tools—all wrapped up in a single binary. The JavaScript engine itself extends JavaScriptCore. Bun also ships with its own wrapper for SQLite. # 6th July 2022, 5:24 pm

Temporal: getting started with JavaScript’s new date time API. Axel Rauschmayer explores the new proposed API for handling dates, times and timezones in JavaScript., which is under development by Ecma TC39 at the moment (and made available as a Polyfill which you are recommended not to run in production since the API is still being figured out). This is a notoriously difficult problem so it’s always interesting to see the latest thinking on how to best address it. # 7th July 2021, 10:29 pm

ES6 In Depth: Template strings. Useful guide to the advanced features of backtick template strings in ES6. # 17th July 2018, 4:08 pm

Feature comparison between hyperHTML and lit-html. Compiled by hyperHTML author Andrea Giammarchi. lit-html is a similar project maintained by Google’s Polymer team. # 17th July 2018, 3:58 pm

hyperHTML (via) I’ve been playing around with multiline backtick template literals in JavaScript recently, and Claudio Cicali on Twitter pointed me to hyperHTML. It’s a fascinating project: it implements an alternative to React’s virtual DOM in just 4.6KB of code, taking advantage of the way tagged template literals can call a custom function which gets to affect how the interpolated values from the template are treated. The documentation does a great job of describing the various clever tricks that are used under the hood, in particular the way it takes advantage of the way tagged literals “freeze” the incoming array of literal chunks allowing hyperHTML to compile each template once and reuse the complied version for different interpolations. # 17th July 2018, 3:39 pm

future-fstrings (via) Clever module that backports fstrings to versions of Python earlier than 3.6, by registering itself as a codec and abusing Python’s # -*- coding: future_fstrings -*- feature. Via a conversation on Twitter that pointed out that the JavaScript community have been using transpilation to successfully experiment with new language features for years now. # 13th July 2018, 4:39 am

react-from-zero (via) Interesting approach to teaching and understanding React: unlike most other tutorials this skips Webpack and ES6 entirely an focuses on things you can get running just using a browser and loading code via script tags. It does eventually load Babel to enable client-side JSX transforms, but before that it shows how React can be used by loading react.js and react-dom.js and then calling React.createElement() manually (or by using the 0xeac7 magic symbol and constructing JavaScript objects manually with $$typeof: magicValue). # 3rd July 2018, 5:27 pm

Hookbox (via) For most web projects, I believe implementing any real-time comet features on a separate stack from the rest of the application makes sense—keep using Rails, Django or PHP for the bulk of the application logic, and offload any WebSocket or Comet requests to a separate stack built on top of something like Node.js, Twisted, EventMachine or Jetty. Hookbox is the best example of that philosophy I’ve yet seen—it’s a Comet server that makes WebHook requests back to your regular application stack to check if a user has permission to publish or subscribe to a given channel. “The key insight is that all application development with hookbox happens either in JavaScript or in the native language of the web application itself”. # 29th July 2010, 9:48 am

canto.js: An Improved HTML5 Canvas API (via) Improved is an understatement: canto adds jQuery-style method chaining, the ability to multiple coordinates to e.g. lineTo at once, relative coordinate methods (regular Canvas does everything in terms of absolute coordinates), the ability to use degrees instead of radians, a rounded corner shortcut, a more convenient .revert() method and a simple parser that can understand SVG path expressions! The only catch: it uses getters and setters so won’t work in IE. # 29th July 2010, 9:39 am

nodejitsu’s node-http-proxy (via) Exactly what I’ve been waiting for—a robust HTTP proxy library for Node that makes it trivial to proxy requests to a backend with custom proxy behaviour added in JavaScript. The example app adds an artificial delay to every request to simulate a slow connection, but other exciting potential use cases could include rate limiting, API key restriction, logging, load balancing, lint testing and more besides. # 28th July 2010, 11:34 pm

Multi-node: Concurrent NodeJS HTTP Server. Kris Zyp’s library for spawning multiple Node child processes (one per core is suggested) for concurrent request handling, taking advantage of Node’s child_process module. This alleviates the need to run multiple Node instances behind an nginx load balancer in order to take advantage of multiple cores. # 15th July 2010, 8:22 am

DNode: Asynchronous Remote Method Invocation for Node.js and the Browser. Mind-bendingly clever. DNode lets you expose a JavaScript function so that it can be called from another machine using a simple JSON-based network protocol. That’s relatively straight-forward... but DNode is designed for asynchronous environments, and so also lets you pass callback functions which will be translated in to references and used to make remote method invocations back to your original client. And to top it off, there’s a browser client library so you can perform the same trick over a WebSocket between a browser and a server. # 11th July 2010, 2:27 pm

Diffable: only download the deltas. JavaScript library for detecting and serving diffs to JavaScript rather than downloading large scripts every time a few lines of code are changed. “Using Diffable has reduced page load times in Google Maps by more than 1200 milliseconds (~25%). Note that this benefit only affects users that have an older version of the script in cache. For Google Maps that’s 20-25% of users.” # 11th July 2010, 12:19 pm

getlatlon.com commit dae961a... I’ve finally added an OpenStreetMap tab to getlatlon.com—here’s the diff, it turns out adding a custom OpenStreetMap layer to an existing Google Maps application only takes a few lines of boilerplate code. # 10th July 2010, 12:22 pm

Escaping regular expression characters in JavaScript (updated). The JavaScript regular expression meta-character escaping code I posted back in 2006 has some serious flaws—I’ve just posted an update to the original post. # 4th July 2010, 6:23 pm

JSONP Memory Leak. Neil Fraser advocates iterating over and deleting every property on a JSONP script DOM node after you removeChild it from the DOM, to protect against memory leaks of “in excess of 15 MB per hour”. # 28th July 2009, 12:46 pm

EtherPad. Outstanding implementation of an online real-time collaborative text editor—basically SubEthaEdit in your browser. I can see myself using this a lot. # 24th July 2009, 12:35 am

Reverse HTTP Demo (via) This is a bit of a brain teaser—a web server running in JavaScript in your browser which uses long polling comet to respond to incoming HTTP requests channelled through a “Reverse HTTP” proxy. # 21st July 2009, 3:54 pm

Announcing Alice and Wonderland. Continuing the RabbitMQ “stuff to do with rabbits” naming convention, Alice is a RESTful interface to RabbitMQ which exposes information about vhosts/queues/users/exchanges/etc as JSON. Wonderland is a web UI for RabbitMQ implemented as a pure Ajax application which calls Alice. # 17th July 2009, 9:12 am

John Resig on Glow. John criticises Glow for reinventing the wheel—BBC insiders respond in the comments below. # 8th July 2009, 7:24 pm

BBC: Glow (via) The BBC have released Glow, their jQuery-like JavaScript library developed in house over the past few years. It’s open source under the Apache license. # 8th July 2009, 3:25 pm

Modernizr (via) Neat idea and an unobtrusive implementation: a JavaScript library that runs feature tests for various HTML5 features (canvas, box shadow, CSS transforms and so on) and adds classes to the HTML body element, allowing you to write CSS selectors that only apply if a feature is present. Detected features are exposed to JavaScript as boolean properties, e.g. Modernizer.multiplebgs. # 2nd July 2009, 10:56 am