Live code reviews make life better

I’ve just got off a call with a colleague. During that call I:

  • learned a lot about how the work he is doing fits in with what my team is working on
  • understood the specific code we were discussing much better than if I’d looked at it alone
  • helped him find a whole bit of code we didn’t actually need (the best kind of code)!
  • helped him figure out something else he has been musing about, that was mostly unrelated
  • had a nice chat and felt much better about life

Asynchronous code reviews can feel like a duty I must perform, and one that distracts me from what I’m doing. Meanwhile, when receiving such reviews, if we’re not careful it can feel like someone criticising for criticism’s sake, or just making your work take longer. As an offline reviewer I don’t often remember to say what I like as well as what I’d want to change, and I normally feel a push to suggest at least one change just to prove I read the code.

Live reviews are so much better! They:

  • are full of opportunities to learn, for both people
  • help you understand the code so you can do a better review
  • often lead the person who wrote the code to spot improvements that the reviewer had no chance of seeing
  • don’t have to be the whole review: you can still take your time to think carefully after the discussion, before clicking Submit
  • can be fun, and give you a chance to interact with your colleagues

I much prefer them. I’m going to do more.

Setting the text selection in a browser: just use setBaseAndExtent

The Selection API is confusing and weird.

But, here’s what I’ve discovered: just use setBaseAndExtend, and when (rarely) needed, extend.


Every selection in a browser consists of:

  • an “anchor” – the beginning, where you started dragging, and
  • a “focus” – the end, where your stopped dragging, and where your cursor is

To select some text in a browser, find the from and to DOM nodes you want, and how far through them you want to be, and set the selection like this:

const sel = document.getSelection();
sel.setBaseAndExtent(from_node, from_offset, to_node, to_offset);

If you want just a cursor with nothing selected, use the same node and offset for both from and to.

The Internet has a lot of recipes that involve creating Ranges and adding them to the Selection, but there is no need to do that.

Finding the from and to nodes

When the locations are inside a text node in the DOM, it’s easy to find the from and to nodes – they are just the text nodes, and the offsets are how many UTF-16 code units through the text you want to be. [Not sure what a code unit is? See my video Interesting Characters].

“But isn’t your cursor always in a text node?”

No – for example, when I have two <br>s next to each other, and I want the cursor to be in between them (so it’s on the blank line). In this case there is no text node, and browsers handle it weirdly.

In Firefox, when my cursor is between two br nodes, the node is set to the tag (e.g. a div) that contains the brs, and the offset is the index of the second br node in the list.

Other browsers may well do it differently.

So do expect that the nodes may not be text nodes, and where that is the case, the offset will be the index of one of their children.

To select the blank line (between two brs) you can actually specify the br node directly, and give an index number of 0, and it works too. But, it does not match exactly what the browser sets when you click on the blank line, so I can’t guarantee it works exactly the same.

Backwards selections

The fly in the ointment is backwards selections: when you click and drag the mouse from right to left, the selection that is created in the browser is backwards: the anchor is on the right and the focus is on the left.

However, if you call setBaseAndExtent attempting to set up a selection like that, the focus and anchor will be swapped so that the anchor is on the left, and the focus is on the right.

But never fear! We can use extend to force the selection to be what we wanted:

const sel = document.getSelection();
sel.setBaseAndExtent(from_node, from_offset, from_node, from_offset);
sel.extend(to_node, to_offset);

Job done.

Go deeper

For more info, check out my demo page: Browser Selections Inventory.

Tips for contenteditables

I’ve been working a bit with contenteditable tags in my HTML, and learnt a couple of things, so here they are.

Update: See also my demo of how to select text in various ways in a contenteditable.

Why can’t I see the cursor inside an empty contenteditable?

If you make an editable div like this:

<div contenteditable="true">

and then try to focus it, then sometimes, in some browsers, you won’t see a cursor.

You can fix it by adding a <br /> tag:

<div contenteditable="true">
<br />

Now you should get a cursor and be able to edit text inside.

Programmatically selecting text inside a contenteditable

It’s quite tricky to get the browser to select anything. Here’s a quick recipe for that:

<div id="ce" contenteditable="true">
Some text here
const ce = document.getElementById("ce");
const sel = document.getSelection();
self.setBaseAndExtent(ce.firstChild, 6, ce.lastChild, 10);

This selects characters 6 to before-10, i.e. the word “text”. To select more complicated stuff inside tags etc. you need to find the actual DOM nodes to pass in to setStart and setEnd, which is quite tricky.

Whenever you setHTML on a contenteditable, add a BR tag

If you use setHTML on a contenteditable you should always append a <br /> on the end. It doesn’t appear in any way, and it prevents weird problems.

Most notably, if you want to have an empty line at the end of your text, you need two <br /> tags, like this:

<div id="ce" contenteditable="true">
Some text here
const ce = document.getElementById("ce");
ce.innerHTML = "a<br /><br />"

If you only include one br tag, there will be no empty line at the end.

Selecting the end of a contenteditable

It’s surprisingly tricky to put the cursor at the end of a contenteditable div but here is a recipe that works:

const range = document.createRange();
const sel = document.getSelection();

(Where ce is our contenteditable div.)

NOTE: this won’t work if you don’t add a br tag at the end, as described above!

Update: beware of newlines in your source HTML

Note: not <br> tags, but actual newlines in your HTML source, can cause weird behaviour, especially around selections.

For example, if your div ends with a newline, you can’t select the end of it by doing this:

const sel = document.getSelection();

The above will work (put your cursor at the end of the text) if there is no newline in the source, but not work (make your cursor disappear or jump to the right of the div) if there is a newline.

So if you’re programmatically generating the HTML, I recommend removing this “extraneous” whitespace if you can.

More tips?

Any more tips? Drop them in the comments and I’ll include them.

Matrix is a Distributed Real-time Database Video

Curious to know a bit more about Matrix? This video goes into the details of what kinds of requests you need to send to write a Matrix client, and why it’s interesting to write a Matrix server.

Slides: Matrix is a Distributed Real-time Database Slides

Really excited that since I started my job working on Matrix, I have become more enthusiastic about it, rather than less.

Building cross-platform Rust for Web, Android and iOS – a minimal example

One of the advantages of writing code in Rust is that it can be re-used in other places. Both iOS and Android allow using native libraries within your apps, and Rust compiles to native. Web pages can now use WebAssembly (WASM), and Rust can compile to WASM.

So, it should be easy, right?

Well, in practice it seems a little tricky, so I created a small example project to explain it to myself, so maybe it’s helpful to you too.

The full code is at, but here is the general idea:

  • crates/example-rust-bindings – the real Rust code
  • bindings/ffi – uniffi code to build shared objects for Android and iOS
  • bindings/wasm – wasm_bingen code to build WASM for Web
  • examples/example-android – an Android app that generates a Kotlin wrapper, and runs the code in the shared object
  • examples/example-ios – an iOS XCode project where we generate Swift bindings, so we can call the code in the shared object
  • examples/example-web – a web page that imports the WASM and runs it

Steps for WASM

Proof that I did this on Web - Firefox showing "This string is from Rust!"

Variation: if you modify the build script in package.json to call wasm-pack with --target node instead of --target web you can generate code suitable for using from a NodeJS module.

Steps for Android

Proof that I did this on Android: Android emulator showing a label "This string is from Rust!"

Steps for iOS