JS - Dave's Blog


Tiny browser features: JSBrowser crash resistance

7 days ago

JSBrowser is a basic browser built as a Win10 JavaScript UWP app around the WebView HTML element. Its fun and relatively simple to implement tiny browser features in JavaScript and in this post I'm implementing crash resistance.

The normal DOM mechanisms for creating an HTML WebView create an in-process WebView, in which the WebView runs on a unique UI thread. But we can use the MSWebView constructor instead to create an out-of-process WebView in which the WebView runs in its own distinct WebView process. Unlike an in-process WebView, Web content running in an out-of-process WebView can only crash the WebView process and not the app process.

        this.replaceWebView = () => {
let webview = document.querySelector("#WebView");
// Cannot access webview.src - anything that would need to communicate with the webview process may fail
let oldSrc = browser.currentUrl;
const webviewParent = webview.parentElement;
webview = new MSWebView();
Object.assign(this, {
"webview": webview
webview.setAttribute("id", "WebView");

// During startup our currentUrl field is blank. If the WebView has crashed
// and we were on a URI then we may obtain it from this property.
if (browser.currentUrl && browser.currentUrl != "") {

I run replaceWebView during startup to replace the in-process WebView created via HTML markup with an out-of-process WebView. I could be doing more to dynamically copy styles, attributes, etc but I know what I need to set on the WebView and just do that.

When a WebView process crashes the corresponding WebView object is no longer useful and a new WebView element must be created. In fact if the old WebView object is used it may throw and will no longer have valid state. Accordingly when the WebView crashes I run replaceWebView again. Additionally, I need to store the last URI we've navigated to (browser.currentUrl in the above) since the crashed WebView object won't know what URI it is on after it crashes.

            webview.addEventListener("MSWebViewProcessExited", () => { 
if (browser.currentUrl === browser.lastCrashUrl) { ++browser.lastCrashUrlCrashCount;
else {
browser.lastCrashUrl = browser.currentUrl;
browser.lastCrashUrlCrashCount = 1;
// If we crash again and again on the same URI, maybe stop trying to load that URI.
if (browser.lastCrashUrlCrashCount >= 3) {
browser.lastCrashUrl = "";
browser.lastCrashUrlCrashCount = 0;
browser.currentUrl = browser.startPage;

I also keep track of the last URI that we recovered and how many times we've recovered that same URI. If the same URI crashes more than 3 times in a row then I assume that it will keep happening and I navigate to the start URI instead.

PermalinkCommentsbrowser javascript jsbrowser uwp webview win10

Tiny browser features: JSBrowser zoom

10 days ago

JSBrowser is a basic browser built as a Win10 JavaScript UWP app around the WebView HTML element. Its fun and relatively simple to implement tiny browser features in JavaScript and in this post I'm implementing zoom.

My plan to implement zoom is to add a zoom slider to the settings div that controls the scale of the WebView element via CSS transform. My resulting zoom change is in git and you can try the whole thing out in my JSBrowser fork.


I can implement the zoom settings slider as a range type input HTML element. This conveniently provides me a min, max, and step property and suits exactly my purposes. I chose some values that I thought would be reasonable so the browser can scale between half to 3x by increments of one quarter. This is a tiny browser feature after all so there's no custom zoom entry.

<a><label for="webviewZoom">Zoom</label><input type="range" min="50" max="300" step="25" value="100" id="webviewZoom" /></a>

To let the user know this slider is for controlling zoom, I make a label HTML element that says Zoom. The label HTML element has a for attribute which takes the id of another HTML element. This lets the browser know what the label is labelling and lets the browser do things like when the label is clicked to put focus on the slider.


There are no explicit scale APIs for WebView so to change the size of the content in the WebView we use CSS.

        this.applyWebviewZoom = state => {
const minValue = this.webviewZoom.getAttribute("min");
const maxValue = this.webviewZoom.getAttribute("max");
const scaleValue = Math.max(Math.min(parseInt(this.webviewZoom.value, 10), maxValue), minValue) / 100;

// Use setAttribute so they all change together to avoid weird visual glitches
this.webview.setAttribute("style", [
["width", (100 / scaleValue) + "%"],
["height", "calc(" + (-40 / scaleValue) + "px + " + (100 / scaleValue) + "%)"],
["transform", "scale(" + scaleValue + ")"]
].map(pair => pair[0] + ": " + pair[1]).join("; "));

Because the user changes the scale at runtime I accordingly replace the static CSS for the WebView element with the script above to programmatically modify the style of the WebView. I change the style with one setAttribute call to do my best to avoid the browser performing unnecessary work or displaying the WebView in an intermediate and incomplete state. Applying the scale to the element is as simple as adding 'transform: scale(X)' but then there are two interesting problems.

The first is that the size of the WebView is also scaled not just the content within it. To keep the WebView the same effective size so that it still fits properly into our browser UI, we must compensate for the scale in the WebView width and height. Accordingly, you can see that we scale up by scaleValue and then in width and height we divide by the scaleValue.

transform-origin: 0% 0%;

The other issue is that by default the scale transform's origin is the center of the WebView element. This means when scaled up all sides of the WebView would expand out. But when modifying the width and height those apply relative to the upper left of the element so our inverse scale application to the width and height above aren't quite enough. We also have to change the origin of the scale transform to match the origin of the changes to the width and height.

PermalinkCommentsbrowser css-transform javascript JS jsbrowser uwp webview win10

Tweet from Šime Vidas

2016 Nov 17, 3:14
Chrome Canary has enabled scroll anchoring which prevents “jumps” when the layout above the viewport changes (demo: https://output.jsbin.com/parujo/quiet#heading2 )

Tweet from David Risney

2016 Aug 31, 6:09
CSP about:blank frame inherits CSP from parent. about:blank child window doesn't despite having same origin http://output.jsbin.com/zesiwituyu/ 

Data breakpoints in JavaScript

2016 Jun 17, 5:44

The other day I had to debug a JavaScript UWA that was failing when trying to use an undefined property. In a previous OS build this code would run and the property was defined. I wanted something similar to windbg/cdb's ba command that lets me set a breakpoint on read or writes to a memory location so I could see what was creating the object in the previous OS build and what that code was doing now in the current OS build. I couldn't find such a breakpoint mechanism in Visual Studio or F12 so I wrote a little script to approximate JavaScript data breakpoints.

The script creates a stub object with a getter and setter. It actually performs the get or set but also calls debugger; to break in the debugger. In order to handle my case of needing to break when window.object1.object2 was created or accessed, I further had it recursively set up such stub objects for the matching property names.

Its not perfect because it is an enumerable property and shows up in hasOwnProperty and likely other places. But for your average code that checks for the existence of a property via if (object.property) it works well.

PermalinkCommentsdebug debugging javascript

Tweet from David Risney

2016 Jun 5, 4:00
@FremyCompany It uses https://github.com/nmrugg/stockfish.js  the asm.js port of the StockFish chess AI (the asm.js part is why Edge wins)

Tweet from David Risney

2016 Jun 1, 1:51
Cool JS image diff'ing including browser webcam based demos https://twitter.com/lonekorean/status/737630487913455616 

Tweet from David_Risney

2016 Feb 7, 9:13
Pretty shredded polygons in html & js https://www.clicktorelease.com/code/polygon-shredder/ …

JavaScript Types and WinRT Types

2016 Jan 21, 5:35

MSDN covers the topic of JavaScript and WinRT type conversions provided by Chakra (JavaScript Representation of Windows Runtime Types and Considerations when Using the Windows Runtime API), but for the questions I get about it I’ll try to lay out some specifics of that discussion more plainly. I’ve made a TL;DR JavaScript types and WinRT types summary table.

WinRT Conversion JavaScript
Struct ↔️ JavaScript object with matching property names
Class or interface instance JavaScript object with matching property names
Windows.Foundation.Collections.IPropertySet JavaScript object with arbitrary property names
Any DOM object

Chakra, the JavaScript engine powering the Edge browser and JavaScript Windows Store apps, does the work to project WinRT into JavaScript. It is responsible for, among other things, converting back and forth between JavaScript types and WinRT types. Some basics are intuitive, like a JavaScript string is converted back and forth with WinRT’s string representation. For other basic types check out the MSDN links at the top of the page. For structs, interface instances, class instances, and objects things are more complicated.

A struct, class instance, or interface instance in WinRT is projected into JavaScript as a JavaScript object with corresponding property names and values. This JavaScript object representation of a WinRT type can be passed into other WinRT APIs that take the same underlying type as a parameter. This JavaScript object is special in that Chakra keeps a reference to the underlying WinRT object and so it can be reused with other WinRT APIs.

However, if you start with plain JavaScript objects and want to interact with WinRT APIs that take non-basic WinRT types, your options are less plentiful. You can use a plain JavaScript object as a WinRT struct, so long as the property names on the JavaScript object match the WinRT struct’s. Chakra will implicitly create an instance of the WinRT struct for you when you call a WinRT method that takes that WinRT struct as a parameter and fill in the WinRT struct’s values with the values from the corresponding properties on your JavaScript object.

// C# WinRT component
public struct ExampleStruct
public string String;
public int Int;

public sealed class ExampleStructContainer
ExampleStruct value;
public void Set(ExampleStruct value)
this.value = value;

public ExampleStruct Get()
return this.value;

// JS code
var structContainer = new ExampleWinRTComponent.ExampleNamespace.ExampleStructContainer();
structContainer.set({ string: "abc", int: 123 });
console.log("structContainer.get(): " + JSON.stringify(structContainer.get()));
// structContainer.get(): {"string":"abc","int":123}

You cannot have a plain JavaScript object and use it as a WinRT class instance or WinRT interface instance. Chakra does not provide such a conversion even with ES6 classes.

You cannot take a JavaScript object with arbitrary property names that are unknown at compile time and don’t correspond to a specific WinRT struct and pass that into a WinRT method. If you need to do this, you have to write additional JavaScript code to explicitly convert your arbitrary JavaScript object into an array of property name and value pairs or something else that could be represented in WinRT.

However, the other direction you can do. An instance of a Windows.Foundation.Collections.IPropertySet implementation in WinRT is projected into JavaScript as a JavaScript object with property names and values corresponding to the key and value pairs in the IPropertySet. In this way you can project a WinRT object as a JavaScript object with arbitrary property names and types. But again, the reverse is not possible. Chakra will not convert an arbitrary JavaScript object into an IPropertySet.

// C# WinRT component
public sealed class PropertySetContainer
private Windows.Foundation.Collections.IPropertySet otherValue = null;

public Windows.Foundation.Collections.IPropertySet other
return otherValue;
otherValue = value;

public sealed class PropertySet : Windows.Foundation.Collections.IPropertySet
private IDictionary map = new Dictionary();

public PropertySet()
map.Add("abc", "def");
map.Add("ghi", "jkl");
map.Add("mno", "pqr");
// ... rest of PropertySet implementation is simple wrapper around the map member.

// JS code
var propertySet = new ExampleWinRTComponent.ExampleNamespace.PropertySet();
console.log("propertySet: " + JSON.stringify(propertySet));
// propertySet: {"abc":"def","ghi":"jkl","mno":"pqr"}

var propertySetContainer = new ExampleWinRTComponent.ExampleNamespace.PropertySetContainer();
propertySetContainer.other = propertySet;
console.log("propertySetContainer.other: " + JSON.stringify(propertySetContainer.other));
// propertySetContainer.other: {"abc":"def","ghi":"jkl","mno":"pqr"}

try {
propertySetContainer.other = { "123": "456", "789": "012" };
catch (e) {
console.error("Error setting propertySetContainer.other: " + e);
// Error setting propertySetContainer.other: TypeError: Type mismatch

There’s also no way to implicitly convert a DOM object into a WinRT type. If you want to write third party WinRT code that interacts with the DOM, you must do so indirectly and explicitly in JavaScript code that is interacting with your third party WinRT. You’ll have to extract the information you want from your DOM objects to pass into WinRT methods and similarly have to pass messages out from WinRT that say what actions the JavaScript should perform on the DOM.

PermalinkCommentschakra development javascript winrt

Retweet of anildash

2016 Jan 3, 9:59
Oh! @jsnell is way ahead of me: https://sixcolors.com/post/2015/12/the-best-christmas-gift-in/ … Programming BB-8 with a kid-friendly app! Thanks to @anoopr for the link.

Retweet of JustRogDigiTec

2015 Nov 29, 2:58
@JustRogDigiTec A fiddle as well http://jsfiddle.net/xb3gheqb/  in case it clarifies. Note: An older browser may not demo properly

Retweet of Esquiring

2015 Nov 23, 10:32
#TIL someone virtualized a bunch of networked Tamagotchis and has an AI caring for them. http://tamahive.spritesserver.nl/  pic.twitter.com/3GUJ30RNGj

Retweet of ohunt

2015 Sep 16, 1:57
Is your site/app not loading resources in ios9? WebKit now blocks mixed content: You can't load CSS or JS over http from https _ever_.

Retweet of SwiftOnSecurity

2015 Aug 16, 1:09
.@realnzall Don't worry, Tay has a physical hard drive wiper with verification pass and optional label printer 😁 pic.twitter.com/PgAqBKXgCH

Tweet from David_Risney

2015 Aug 4, 3:08
Very helpful site to determine which channels you're likely able to receive OTA based on your address or zip code: http://gomohu.com/xbox/ 

Retweet of BrandonLive

2015 Jul 22, 2:37
Someone please make a way for @electronjs apps to run in WWAHost on Win10. Chromium is such a miserable pig. Plus, Store support!

laughingsquid: Live Band Performs ‘Mario Kart’ Song as Man...

2015 May 27, 3:11


Live Band Performs ‘Mario Kart’ Song as Man Races Down the Video Game Track


laughingsquid: A Real Hedgehog Scurries Along to Music From the...

2015 Apr 23, 1:17


A Real Hedgehog Scurries Along to Music From the ‘Sonic the Hedgehog’ Video Game While Collecting Golden Rings


RIP CadburdyShe died yesterday. Besides the normal grief its...

2015 Apr 8, 2:43

RIP Cadburdy

She died yesterday. Besides the normal grief its strange being the adult and dealing with a deceased pet.


laughingsquid:‘Find a Fish’, The Classic Surreal Intermission...

2015 Apr 8, 5:02


‘Find a Fish’, The Classic Surreal Intermission Sketch From Monty Python’s 1983 Film ‘The Meaning of Life’

Surprisingly often quoted ‘a fish fish fishy, oh!’ with my roommates in college.

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