Running Interactive Scripts | Web Browser Engineering

The first web applications were like the previous chapter’s guest book, with the server generating new web pages for every user action. But in the early 2000s, JavaScript-enhanced web applications, which can update pages dynamically and respond immediately to user actions, took their place. Let’s add support for this key web technology to our browser.

Installing DukPy

Actually writing a JavaScript interpreter is beyond the scope of this book,But check out a book on programming language implementation if it sounds interesting! so this chapter uses the dukpy library for executing JavaScript.

DukPy wraps a JavaScript interpreter called Duktape. The most famous JavaScript interpreters are those used in browsers: SpiderMonkey (Firefox), JavaScriptCore (Safari), and V8 (Chrome). Unlike those implementations, which are extremely fast but also extremely complex, Duktape aims to be simple and extensible, and is usually embedded inside a larger C or C++ project.For example, in a video game the high-speed graphics code is usually written in C or C++ , but the actual plot of the game is usually written in a higher-level language like JavaScript.

Like other JavaScript engines, DukPy not only executes JavaScript code, but also allows it to call exported Python functions. We’ll be using this feature to allow JavaScript code to modify the web page it’s running on.

The first step to using DukPy is installing it. On most machines, including on Windows, macOS, and Linux systems, you should be able to do this with:

If you get an error on the first line, you probably failed to install DukPy.Or, on my Linux machine, I sometimes get errors due to file ownership. You may have to do some sleuthing. If you get an error, or a segfault, on the second line, there’s a chance that Duktape failed to compile, or maybe doesn’t support your system, and you might need to debug further.

Running JavaScript Code

The test above shows how you run JavaScript code in DukPy: you just call evaljs! Let’s put this newfound knowledge to work in our browser.

On the web, JavaScript is found in <script> tags. Normally, a <script> tag has a src attribute with a relative URL that points to a JavaScript file, much like with CSS files. A <script> tag could also contain JavaScript source code between the start and end tag, but we won’t implement that.It’s a challenge for parsing, since it’s hard to avoid less-than and greater-than signs in JavaScript code. See Exercise 4-3.

Finding and downloading those scripts is similar to what we did for CSS. First, we need to find all of the scripts:

This should run before styling and layout. To try it out, create a simple web page with a script tag:

Exporting Functions

Right now, our browser just prints the last expression in a script; but in a real browser scripts must call the console.log function to print. To support that, we will need to export a function from Python into JavaScript. We’ll be exporting a lot of functions, so to avoid polluting the Tab object with many new methods, let’s put this code in a new JSContext class:

DukPy’s JSInterpreter object stores the values of all the JavaScript variables, and lets us run multiple JavaScript snippets and share variable values and other state between them.

As a side benefit of using one JSContext for all scripts, it is now possible to run two scripts and have one of them define a variable that the other uses, say on a page like this:

Suppose a.js is “var x = 2;” and b.js is “console.log(x + x)”; the variable x is set in a.js but used in b.js. In real web browsers, that’s common, since one script might define library functions that another script wants to call.

Now, to allow JavaScript to interact with the outside world, DukPy allows us to “export” functions to it. For example, we can export Python’s print function like so:

We can call an exported function from JavaScript using DukPy’s call_python function. For example:

When this JavaScript code runs, DukPy converts the JavaScript string "Hi from JS" into a Python string,This conversion works for numbers, strings, and booleans, plus arrays and dictionaries thereof, but not with fancy objects. and then passes that Python string to the print function we exported. Then print prints that string.

Since we ultimately want a console.log function, not a call_python function, we need to define a console object and then give it a log property. We can do that in JavaScript:

In case you’re not too familiar with JavaScript,Now’s a good time to brush up! this defines a variable called console, whose value is an object literal with the property log, whose value is a function that calls call_python. The interaction between the browser and JavaScript is shown in Figure 1.

We can call that JavaScript code our “JavaScript runtime”; we run it before we run any user code, so let’s stick it in a runtime.js file and execute it when the JSContext is created, before we run any user code:

Now you should be able to put console.log("Hi from JS!") into a JavaScript file, run it from your browser, and see output in your terminal. You should also be able to call console.log multiple times.

Taking a step back, when we run JavaScript in our browser, we’re mixing C code, which implements the JavaScript interpreter; Python code, which implements certain JavaScript functions; a JavaScript runtime, which wraps the Python API to look more like the JavaScript one; and of course some user code in JavaScript. There’s a lot of complexity here!

Handling Crashes

Crashes in JavaScript code are frustrating to debug. You can cause a crash by writing bad code, or by explicitly raising an exception, like so:

When a web page runs some JavaScript that crashes, the browser should ignore the crash. Web pages shouldn’t be able to crash our browser! You can implement that like this (plus changing the call site of run to pass the script):

But as you go through this chapter, you’ll also run into another type of crash: crashes in our own JavaScript runtime. We can’t ignore those, because that’s our code. Debugging these crashes is a bear: by default DukPy won’t show a backtrace, and if the runtime code calls into an exported function that crashes it gets even more confusing.

Here are a few tips to help with these crashes. First, if you get a crash inside some JavaScript function, wrap the body of the function like this:

This code catches all exceptions and prints a stack trace before re-raising them. If you instead are getting crashes inside an exported function you will need to wrap that function, on the Python side:

Debugging these issues is not easy, because all these calls between Python and JavaScript get pretty complicated. Because these bugs are hard, it’s worth approaching debugging systematically and gathering a lot of information before attempting a fix.

Returning Handles

So far, JavaScript evaluation is fun but useless, because JavaScript can’t make any kinds of modifications to the page itself. (Why even run JavaScript if it can’t do anything besides print? Who looks at a browser’s console output?) We need to allow JavaScript to modify the page.

JavaScript manipulates a web page by calling any of a large set of methods collectively called the DOM API. The DOM API is big, and it keeps getting bigger, so we won’t be implementing all, or even most, of it. But a few core functions show key elements of the full API:

We’ll implement simplified versions of these APIs.The simplifications will be minor. querySelectorAll will return an array, not this thing called a NodeList; innerHTML will only write the HTML contents of an element, and won’t allow reading those contents. This suffices to demonstrate JavaScript–browser interaction.

In JavaScript, querySelectorAll is a method on the document object, which we need to define in the JavaScript runtime:

On the Python side, querySelectorAll first has to parse the selector and then find and return the matching elements. To parse the selector, I’ll call into the CSSParser’s selector method:If you pass querySelectorAll an invalid selector, the selector call will throw an error, and DukPy will convert that Python-side exception into a JavaScript-side exception in the web script we are running, which can catch it.

Next we need to find and return all matching elements. To do that, we need the JSContext to have access to the Tab, specifically to its nodes field. So let’s pass in the Tab when creating a JSContext:

Finally, we need to return those nodes back to JavaScript. You might try something like this:

However, this throws an error:Yes, that’s a confusing error message. Is it a JSRuntimeError, an EvalError, or a TypeError? The confusion is a consequence of the complex interaction of Python, JS, and C code. (JSON, or JavaScript Object Notation, is a language-independent data format.)

What DukPy is trying to tell you is that it has no idea what to do with the Element objects that querySelectorAll returns. After all, the Element class only exists in Python, not JavaScript!

Python objects need to stay on the Python side of the browser, so JavaScript code will need to refer to them via some kind of indirection. I’ll use a simple numeric identifier, which I’ll call a handle (see Figure 2).Note the similarity to file descriptors, which give user-level applications access to kernel data structures.

We’ll need to keep track of the handle to node mapping. Let’s create a node_to_handle data structure to map nodes to handles, and a handle_to_node map that goes the other way:

Now the querySelectorAll handler can allocate handles for each node and return those handles instead:

The get_handle function should create a new handle if one doesn’t exist yet:

So now the querySelectorAll handler returns something like [1, 3, 4, 7], with each number being a handle for an element, which DukPy can easily convert into JavaScript objects without issue. Now of course, on the JavaScript side, querySelectorAll shouldn’t return a bunch of numbers: it should return a list of Node objects.In a real browser, querySelectorAll actually returns a NodeList object, for kind of abstruse reasons that aren’t relevant here. So let’s define a Node object in our runtime that wraps a handle:If your JavaScript is rusty, you might want to read up on the crazy way you define classes in JavaScript. Modern JavaScript also provides the class syntax, which is more sensible, but it’s not supported in DukPy.

We create these Node objects in querySelectorAll’s wrapper:This code creates new Node objects every time you call querySelectorAll, even if there’s already a Node for that handle. That means you can’t use equality to compare Node objects. I’ll ignore that but a real browser wouldn’t.

Wrapping Handles

One simple DOM method is getAttribute, a method on Node objects that lets you get the value of HTML attributes. Implementing getAttribute means solving the opposite problem to querySelectorAll: taking Node objects on the JavaScript side, and shipping them over to Python.

The solution is similar to querySelectorAll: instead of shipping the Node object itself, we send over its handle:

On the Python side, the getAttribute function takes two arguments, a handle and an attribute:

Note that if the attribute is not assigned, the get method will return None, which DukPy will translate to JavaScript’s null. Don’t forget to export this function as getAttribute.

We finally have enough of the DOM API to implement a little character count function for text areas:

Ideally, though we’d update the character count every time the user types into an input box. That requires running JavaScript on every key press. Let’s implement that next.

Event Handling

The browser executes JavaScript code as soon as it loads the web page, but that code often wants to change the page in response to user actions.

Here’s how that works. Any time the user interacts with the page, the browser generates events. Each event has a type, like change, click, or submit, and happens at a target element. The addEventListener method allows JavaScript to react to those events: node.addEventListener('click', func) sets func to run every time the element corresponding to node generates a click event. It’s basically Tk’s bind, but in the browser—see Figure 3. Let’s implement it.

Let’s start with generating events. I’ll create a dispatch_event method and call it whenever an event is generated. That includes, first of all, any time we click in the page:

And finally, when submitting forms but before actually sending the request to the server:

So far so good—but what should the dispatch_event method do? Well, it needs to run listeners passed to addEventListener, so those need to be stored somewhere. Since those listeners are JavaScript functions, we need to keep that data on the JavaScript side, in a variable in the runtime. I’ll call that variable LISTENERS; we’ll use it to look up handles and event types, so let’s make it map handles to a dictionary that maps event types to a list of listeners:

To dispatch an event, we need to look up the type and handle in the LISTENERS array, like this:

Note that dispatchEvent uses the call method on functions, which sets the value of this inside that function. As is standard in JavaScript, I’m setting it to the node that the event was generated on.

Here, the EVENT_DISPATCH_JS constant is a string of JavaScript code that dispatches a new event:

So when dispatch_event is called on the Python side, that runs dispatchEvent on the JavaScript side, and that in turn runs all of the event listeners. The dukpy JavaScript object in this code snippet stores the named type and handle arguments to evaljs.

With all this event-handling machinery in place, we can update the character count every time an input area changes:

Note that lengthCheck uses this to reference the input element that actually changed, as set up by dispatchEvent.

So far so good—but ideally the length check wouldn’t print to the console; it would add a warning to the web page itself. To do that, we’ll need to not only read from the page but also modify it.

Modifying the DOM

So far we’ve implemented read-only DOM methods; now we need methods that change the page. The full DOM API provides a lot of such methods, but for simplicity I’m going to implement only innerHTML, which is used like this:

In other words, innerHTML is a property of node objects, with a setter that is run when the field is modified. That setter takes the new value, which must be a string, parses it as HTML, and makes the new, parsed HTML nodes children of the original node.

Let’s implement this, starting on the JavaScript side. JavaScript has the obscure Object.defineProperty function to define setters, which DukPy supports:

In innerHTML_set, we’ll need to parse the HTML string. That turns out to be trickier than you’d think, because our browser’s HTML parser is intended to parse whole HTML documents, not these document fragments. As an expedient, close-enough hack,Real browsers follow the standardized parsing algorithm for HTML fragments. I’ll just wrap the HTML in an html and body element:

Don’t forget to export the innerHTML_set function. Note that we extract all children of the body element, because an innerHTML_set call can create multiple nodes at a time. These new nodes must now be made children of the element innerHTML_set was called on:

We update the parent pointers of those parsed child nodes because otherwise they would point to the dummy body element that we added to aid parsing.

It might look like we’re done—but try this out and you’ll realize that nothing happens when a script calls innerHTML_set. That’s because, while we have changed the HTML tree, we haven’t regenerated the layout tree or the display list, so the browser is still showing the old page.

Whenever the page changes, we need to update its rendering by calling render:Redoing layout for the whole page is often wasteful; Chapter 16 explores a more complicated algorithm that speeds this up.

JavaScript can now modify the web page!Note that while rendering will update to account for the new HTML, any added scripts or style sheets will not properly load, and removed style sheets will (incorrectly) still apply. I’ve left fixing that as Exercise 9-7.

Let’s try this out in our guest book. Say we want a 100-character limit on guest book entries to prevent long, incoherent rants from making it in.

First, switch to the server codebase and add a <strong> after the guest book form. Initially this element will be empty, but we’ll write an error message into it if the paragraph gets too long.

Now the browser will request comment.js, so our server needs to serve that JavaScript file:

We can then put our little input length checker into comment.js, with the lengthCheck function modified to use innerHTML:

Try it out: write a long comment and you should see the page warning you when it grows too long. By the way, we might want to make it stand out more, so let’s go ahead and add another URL to our web server, /comment.css, with the contents:

But even though we tell the user that their comment is too long the user can submit the guest book entry anyway. Oops! Let’s fix that.

Event Defaults

So far, when an event is generated, the browser will run the listeners, and then also do whatever it normally does for that event—the default action. I’d now like JavaScript code to be able to cancel that default action.

There are a few steps involved. First of all, event listeners should receive an event object as an argument. That object should have a preventDefault method. When that method is called, the default action shouldn’t occur.

Note the do_default field, to record whether preventDefault has been called. We’ll now be passing an Event object to dispatchEvent, instead of just the event type:

Also note that dispatchEvent returns evt.do_default, which is not only standard in JavaScript but also helpful when dispatching events from Python, because Python’s dispatch_event can return that boolean to its handler:

This way, every time an event happens, the browser can check the return value of dispatch_event and stop if it is True. We have three such places in the click method:

Now our character count code can prevent the user from submitting a form: it can use a global variable to track whether or not submission is allowed, and then when submission is attempted it can check that variable and cancel that submission if necessary:

Well … impossible in this browser. But since there are browsers that don’t run JavaScript (like ours, one chapter back), we should check the length on the server side too:

Note that we shouldn’t—can’t—rely on JavaScript being executed by the browser, because the browser is the user’s agent, not ours. Ideally, web pages should be written so that they work correctly without JavaScript, but work better with it. This is called progressive enhancement, and it means we’re not replicating in JavaScript what the browser can already do.

A closing thought: while our guest book now has a little bit of JavaScript code, it’s still mostly HTML, CSS, form elements, other standard web features. In this way JavaScript extends the web instead of replacing it. This is in contrast to the recently departed Adobe Flash, and before that Java Applets, which were self-contained plug-ins that handled input and rendering on their own.

Summary

Our browser now runs JavaScript applications on behalf of websites. Granted, it supports just four methods from the vast DOM API, but even those demonstrate:

A web page can now add functionality via a clever script, instead of waiting for a browser developer to add it into the browser itself. And as a side benefit, a web page can now earn the lofty title of “web application”.

Outline

The complete set of functions, classes, and methods in our browser should now look something like this:

class URL:
    def __init__(url)

    def request(payload)

    def resolve(url)

    def __str__()

class Text:
    def __init__(text, parent)

    def __repr__()

class Element:
    def __init__(tag, attributes, parent)

    def __repr__()

def print_tree(node, indent)

def tree_to_list(tree, list)

class HTMLParser:
    SELF_CLOSING_TAGS

    HEAD_TAGS

    def __init__(body)

    def parse()

    def get_attributes(text)

    def add_text(text)

    def add_tag(tag)

    def implicit_tags(tag)

    def finish()

class CSSParser:
    def __init__(s)

    def whitespace()

    def literal(literal)

    def word()

    def ignore_until(chars)

    def pair()

    def selector()

    def body()

    def parse()

class TagSelector:
    def __init__(tag)

    def matches(node)

class DescendantSelector:
    def __init__(ancestor, descendant)

    def matches(node)

FONTS

def get_font(size, weight, style)

DEFAULT_STYLE_SHEET

INHERITED_PROPERTIES

def style(node, rules)

def cascade_priority(rule)

WIDTH, HEIGHT

HSTEP, VSTEP

class Rect:
    def __init__(left, top, right, bottom)

    def contains_point(x, y)

INPUT_WIDTH_PX

BLOCK_ELEMENTS

class DocumentLayout:
    def __init__(node)

    def layout()

    def should_paint()

    def paint()

class BlockLayout:
    def __init__(node, parent, previous)

    def layout_mode()

    def layout()

    def recurse(node)

    def new_line()

    def word(node, word)

    def input(node)

    def self_rect()

    def should_paint()

    def paint()

class LineLayout:
    def __init__(node, parent, previous)

    def layout()

    def should_paint()

    def paint()

class TextLayout:
    def __init__(node, word, parent, previous)

    def layout()

    def should_paint()

    def paint()

class InputLayout:
    def __init__(node, parent, previous)

    def layout()

    def should_paint()

    def paint()

    def self_rect()

class DrawText:
    def __init__(x1, y1, text, font, color)

    def execute(scroll, canvas)

class DrawRect:
    def __init__(rect, color)

    def execute(scroll, canvas)

class DrawLine:
    def __init__(x1, y1, x2, y2, color, thickness)

    def execute(scroll, canvas)

class DrawOutline:
    def __init__(rect, color, thickness)

    def execute(scroll, canvas)

def paint_tree(layout_object, display_list)

EVENT_DISPATCH_JS

RUNTIME_JS

class JSContext:
    def __init__(tab)

    def run(script, code)

    def dispatch_event(type, elt)

    def get_handle(elt)

    def querySelectorAll(selector_text)

    def getAttribute(handle, attr)

    def innerHTML_set(handle, s)

SCROLL_STEP

class Tab:
    def __init__(tab_height)

    def load(url, payload)

    def render()

    def draw(canvas, offset)

    def scrolldown()

    def click(x, y)

    def go_back()

    def submit_form(elt)

    def keypress(char)

class Chrome:
    def __init__(browser)

    def tab_rect(i)

    def paint()

    def click(x, y)

    def keypress(char)

    def enter()

    def blur()

class Browser:
    def __init__()

    def draw()

    def new_tab(url)

    def handle_down(e)

    def handle_click(e)

    def handle_key(e)

    def handle_enter(e)

Exercises

9-1 Node.children. Add support for the children property on JavaScript Nodes. Node.children returns the immediate Element children of a node, as an array. Text children are not included.The DOM method childNodes gives access to both elements and text nodes.

9-2 createElement. The document.createElement method creates a new element, which can be attached to the document with the appendChild and insertBefore methods on Nodes; unlike innerHTML, there’s no parsing involved. Implement all three methods.

9-3 removeChild. The removeChild method on Nodes detaches the provided child and returns it, bringing that child—and its subtree—back into a detached state. (It can then be re-attached elsewhere, with appendChild and insertBefore, or deleted.) Implement this method. It’s more challenging to implement this one, because you’ll need to also remove the subtree from the Python side.

9-4 IDs. When an HTML element has an id attribute, a JavaScript variable pointing to that element is predefined. So, if a page has a <div id="foo"></div>, then there’s a variable foo referring to that node.This is standard behavior. Implement this in your browser. Make sure to handle the case of nodes being added and removed (such as with innerHTML).

9-5 Event bubbling. Right now, you can attach a click handler to a (anchor) elements, but not to anything else. Fix this. One challenge you’ll face is that when you click on an element, you also click on all its ancestors. On the web, this sort of quirk is handled by event bubbling: when an event is generated on an element, listeners are run not just on that element but also on its ancestors. Implement event bubbling, and make sure listeners can call stopPropagation on the event object to stop bubbling the event up the tree. Double-check that clicking on links still works, and make sure preventDefault still successfully prevents clicks on a link from actually following the link.

9-6 Serializing HTML. Reading from innerHTML should return a string containing HTML source code. That source code should reflect the current attributes of the element; for example:

Implement this behavior for innerHTML as a getter. Also implement outerHTML, which differs from innerHTML in that it contains the element itself, not just its children.

9-7 Script-added scripts and style sheets. The innerHTML API could cause <script> or <link> elements to be added to the document, but currently our browser does not load them when this happens. Fix this. Likewise, when a <link> element is removed from the document, its style sheet should be removed from the global list; implement that as well.Note that, unlike a style sheet, a removed <script>’s evaluated code still exists for the lifetime of the web page. Can you see why it has to be that way?