How Does Svelte Actually Work? part 1
Tags: svelte, javascript
Published: 2019-12-05 16:36:05

---
title: How Does Svelte Actually Work? part 1
published: true
description: A conversational, thinking-out-loud tour of the JavaScript that Svelte outputs
tags: svelte, javascript
---
Here's part 2:

{% link https://dev.to/zev/how-does-svelte-actually-work-part-2-3gbp %}

A friend put Svelte on the map for me this summer. Rather than tout its performance relative to the frameworks of the day, he touted the bite-sizedness and readability of the JavaScript it generates when compiled. 

I'm writing a course that uses Svelte (and FastAPI and some other snazzy things) and am realizing that I could use some deeper knowledge of how Svelte operates: Specifically, how the code works that Svelte compiles to.

I'll post my insights as they come about, so this is part 1 of `x`.

# First Steps

I used the template provided by the Svelte project by doing
`npx degit sveltejs/template my-svelte-project; cd $_; npm install`.

Then I ran `npm run dev` to compile the included component and start the development server.

This produced [`build/bundle.js`](https://gist.github.com/zevaverbach/83901aed1230fbd3adb01b96a7be0572), the beast we'll be dissecting.

# Start at the Bottom

```js
// build/bundle.js (all code blocks are from this file unless otherwise specified)
...
const app = new App({
    target: document.body,
    props: {
    	name: 'world'
    }
});

return app;

}());
//# sourceMappingURL=bundle.js.map
```

I didn't know what a source map is, but having Googled it and inspected `bundle.js.map` a little, I've decided not to try to decipher it just yet!

Those parens at the end tell me that the `app` var on line 3 of `bundle.js` 

```js
...
var app = (function () {
...
```

stores the result of `return app`, as everything on the right-hand side of that  👆👆 `=` is an anonymous function which immediately calls itself.

Then, the above block, starting with `const app`, is identical to the logic in `main.js`. 

```js
// src/main.js

import App from './App.svelte';

const app = new App({
	target: document.body,
	props: {
	name: 'world',
	}
});

export default app;
```

Searching for `main.js` in the Rollup config file that came with this sample app, I see

```js
// rollup.config.js
...
    input: 'src/main.js',
...
```

Okay, I'm reminded that this is where the Svelte app is defined, as configured in `rollup.config.js`.

# The App: First Hypothesis

It looks like the `App` class has `get` and `set` methods on it, each called `name`. 

```js
...
class App extends SvelteComponentDev {
    constructor(options) {
        super(options);
        init(this, options, instance, create_fragment, safe_not_equal, { name: 0 });

        dispatch_dev("SvelteRegisterComponent", {
            component: this,
            tagName: "App",
            options,
            id: create_fragment.name
        });

        const { ctx } = this.$$;
        const props = options.props || ({});

        if (/*name*/ ctx[0] === undefined && !("name" in props)) {
            console.warn("<App> was created without expected prop 'name'");
        }
    }

    get name() {
        throw new Error("<App>: Props cannot be read directly from the component instance unless compiling with 'accessors: true' or '<svelte:options accessors/>'");
    }

    set name(value) {
        throw new Error("<App>: Props cannot be set directly on the component instance unless compiling with 'accessors: true' or '<svelte:options accessors/>'");
    }
}

...
```

I hypothesize that if I give `App` another prop, there will be a pair of `get` and `set` for that as well.

## Testing Hypothesis #1

```html
<!-- src/App.svelte -->

<script>
	export let name; 
	export let number; // new
</script>

```

Sure enough, these methods have appeared:

```js
...
get name() {
    throw new Error("<App>: Props cannot be read directly from the component instance unless compiling with 'accessors: true' or '<svelte:options accessors/>'");
}

set name(value) {
    throw new Error("<App>: Props cannot be set directly on the component instance unless compiling with 'accessors: true' or '<svelte:options accessors/>'");
}

get number() {
    throw new Error("<App>: Props cannot be read directly from the component instance unless compiling with 'accessors: true' or '<svelte:options accessors/>'");
}

set number(value) {
    throw new Error("<App>: Props cannot be set directly on the component instance unless compiling with 'accessors: true' or '<svelte:options accessors/>'");
}
...
```

So that's how that works. I don't know much about how getters/setters work in JS classes, but I'm guessing it's like in Python: They trigger when you try to get or set an instance attribute.

Then there's this in the constructor of `App`:

```js
if (/*name*/ ctx[0] === undefined && !("name" in props)) {
    console.warn("<App> was created without expected prop 'name'");
}

if (/*number*/ ctx[1] === undefined && !("number" in props)) {
    console.warn("<App> was created without expected prop 'number'");
}
```
This `ctx` thing is mysterious, and it's popped off of the even more mysterious `this.$$`. 

```js
class App extends SvelteComponentDev {
    constructor(options) {
        ...
        const { ctx } = this.$$;
...
```

We'll come back to these.

Before continuing, let's update `main.js` to provide a value for the `number` prop.

```js
// src/main.js
...
const app = new App({
    target: document.body,
    props: {
    	name: 'world',
        number: 42
    }
});
```

# Everything Starts in `create_fragment`

```js
function create_fragment(ctx) {
    let main;
    let h1;
    let t0;
    let t1;
    let t2;
    let t3;
    let p;
    let t4;
    let a;
    let t6;

    const block = {
        c: function create() {
            main = element("main");
            h1 = element("h1");
            t0 = text("Hello ");
            t1 = text(/*name*/ ctx[0]);
            t2 = text("!");
            t3 = space();
            p = element("p");
            t4 = text("Visit the ");
            a = element("a");
            a.textContent = "Svelte tutorial";
            t6 = text(" to learn how to build Svelte apps.");
            attr_dev(h1, "class", "svelte-1tky8bj");
            add_location(h1, file, 5, 1, 46);
            attr_dev(a, "href", "https://svelte.dev/tutorial");
            add_location(a, file, 6, 14, 83);
            add_location(p, file, 6, 1, 70);
            attr_dev(main, "class", "svelte-1tky8bj");
            add_location(main, file, 4, 0, 38);
        },
        l: function claim(nodes) {
            throw new Error("options.hydrate only works if the component was compiled with the `hydratable: true` option");
        },
        m: function mount(target, anchor) {
            insert_dev(target, main, anchor);
            append_dev(main, h1);
            append_dev(h1, t0);
            append_dev(h1, t1);
            append_dev(h1, t2);
            append_dev(main, t3);
            append_dev(main, p);
            append_dev(p, t4);
            append_dev(p, a);
            append_dev(p, t6);
        },
        p: function update(ctx, [dirty]) {
            if (dirty & /*name*/ 1) set_data_dev(t1, /*name*/ ctx[0]);
        },
        i: noop,
        o: noop,
        d: function destroy(detaching) {
            if (detaching) detach_dev(main);
        }
    };

    dispatch_dev("SvelteRegisterBlock", {
        block,
        id: create_fragment.name,
        type: "component",
        source: "",
        ctx
    });

    return block;
}



```

`create_fragment` is a function that takes a single argument `ctx`, and its job is primarily to create and render DOM elements; it returns `block`.

## `block`

`block` is an object whose most important attributes are `c` (create), `m` (mount), `p` (update), `d` (destroy). 

### `c` (create)

`block.c`'s value is a factory function called `create`, which

```js
    c: function create() {
        main = element("main");
        h1 = element("h1");
        t0 = text("Hello ");
        t1 = text(/*name*/ ctx[0]);
        t2 = text("!");
        t3 = space();
        p = element("p");
        t4 = text("Visit the ");
        a = element("a");
        a.textContent = "Svelte tutorial";
        t6 = text(" to learn how to build Svelte apps.")
        ...
```
 
1) creates a bunch of DOM elements and text nodes
2) assigns them each to a variable declared at the start of `create_fragment`
 
Then it

```js
    ...
    attr_dev(h1, "class", "svelte-1tky8bj");
    add_location(h1, file, 5, 1, 46);
    attr_dev(a, "href", "https://svelte.dev/tutorial");
    add_location(a, file, 6, 14, 83);
    add_location(p, file, 6, 1, 70);
    attr_dev(main, "class", "svelte-1tky8bj");
    add_location(main, file, 4, 0, 38);
}
```

3) sets attributes (like 'class' and 'href') on the elements
4) dispatches an event for each attribute-setting (more on that later: we can safely ignore these events forever).
5) adds metadata to each element (`__svelte_meta`) detailing exactly where it's defined in the `src` modules.
 
### `m` (mount)

`block.m`'s value is a factory function called `mount`, which, y'know, adds each element and text node to the DOM in the appropriate place.

```js
    m: function mount(target, anchor) {
        insert_dev(target, main, anchor);
        append_dev(main, h1);
        append_dev(h1, t0);
        append_dev(h1, t1);
        append_dev(h1, t2);
        append_dev(main, t3);
        append_dev(main, p);
        append_dev(p, t4);
        append_dev(p, a);
        append_dev(p, t6);
    },

```

### `p` (update)

`block.p`'s value is _not_ a factory function, but a plain old function which seems to

```js
    p: function update(ctx, [dirty]) {
        if (dirty & /*name*/ 1) set_data_dev(t1, /*name*/ ctx[0]);
    },
```

1) do something with bits that I don't understand, but probably just checks whether there's anything to update (`dirty`) 
2) if the new value (`ctx[0]`) differs from `t1`'s value (`undefined` by default),
3) update `t1`'s value -- it's a text node, as a reminder

### Hypothesis #2

I notice here that the prop we added in the first hypothesis, `number`, doesn't appear in the `update` function. I'm thinking this is because it's not used anywhere in the component: It's an unused prop. 

#### Testing Hypothesis #2

```html
<!-- src/App.svelte -->
...
<main>
    <h1>Hello {name}!</h1>
    <p>Your lucky number is {number}.</p> <!-- 👈👈👈 new -->
    <p>Visit the <a href="https://svelte.dev/tutorial">Svelte tutorial</a> to learn how to build Svelte apps.</p>
</main>
...
```

```js
// build/bundle.js
...
    p: function update(ctx, [dirty]) {
        if (dirty & /*name*/ 1) set_data_dev(t1, /*name*/ ctx[0]);
        if (dirty & /*number*/ 2) set_data_dev(t5, /*number*/ ctx[1]);
    },
...
```

Ding ding ding! I'm still not sure about this `if (dirty & 2)` business; we'll kick that can for now.

### `d` (destroy)

`block.d`'s value is a function which -- shock and awe -- removes an element from the DOM.

```js
    d: function destroy(detaching) {
        if (detaching) detach_dev(main);
```

# Where is `block` consumed? 

`create_fragment` is only called once in `bundle.js`, which makes sleuthing pretty easy:

```js
    ...
    $$.fragment = create_fragment ? create_fragment($$.ctx) : false;
    ...
```

This is inside of the monster `init` function, which is itself called only in the constructor of the `class App` definition. What is this `create_fragment ? ...` ternary about? It seems like `create_fragment` will always be truthy, given that it... exists? The more fruitful question is probably where and how is `$$.fragment` used? Where? In three places, it turns out. How?

## `init`

```js
...
   function init(component, options, instance, create_fragment, not_equal, props, dirty = [-1]) {
    const parent_component = current_component;
    set_current_component(component);
    const prop_values = options.props || {};
    const $$ = component.$$ = {
        fragment: null,
        ctx: null,
        // state
        props,
        update: noop,
        not_equal,
        bound: blank_object(),
        // lifecycle
        on_mount: [],
        on_destroy: [],
        before_update: [],
        after_update: [],
        context: new Map(parent_component ? parent_component.$$.context : []),
        // everything else
        callbacks: blank_object(),
        dirty
    };
    let ready = false;
    $$.ctx = instance
        ? instance(component, prop_values, (i, ret, value = ret) => {
            if ($$.ctx && not_equal($$.ctx[i], $$.ctx[i] = value)) {
                if ($$.bound[i])
                    $$.bound[i](value);
                if (ready)
                    make_dirty(component, i);
            }
            return ret;
        })
        : [];
    $$.update();
    ready = true;
    run_all($$.before_update);
    // `false` as a special case of no DOM component
    $$.fragment = create_fragment ? create_fragment($$.ctx) : false;
    if (options.target) {
        if (options.hydrate) {
            // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
            $$.fragment && $$.fragment.l(children(options.target));
        }
        else {
            // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
            $$.fragment && $$.fragment.c();
        }
        if (options.intro)
            transition_in(component.$$.fragment);
        mount_component(component, options.target, options.anchor);
        flush();
    }
    set_current_component(parent_component);
}

...
```

`$$.fragment` is referred to three times directly after its creation in `init`. Since only `target` is in the `options` of the sample app, we'll ignore all but the second, `$$.fragment && $$.fragment.c();`. Similar to the previous step, I don't understand the boolean check here of `$$.fragment && ...`, but what's notable is that `fragment`'s `c` method is called, which will create—but not mount—all the elements and text nodes, giving the elements metadata about their pre-compiled location in `App.svelte`.

Since `init` is called inside the constructor of `App`, we know the above will be executed at runtime.

### Backtracking: What About `$$`?

Real quick: `$$` is defined early in `init`.

```js
...
        const $$ = component.$$ = {
            fragment: null,
            ctx: null,
            // state
            props,
            update: noop,
            not_equal,
            bound: blank_object(),
            // lifecycle
            on_mount: [],
            on_destroy: [],
            before_update: [],
            after_update: [],
            context: new Map(parent_component ? parent_component.$$.context : []),
            // everything else
            callbacks: blank_object(),
            dirty
        };
...

```
Mystery solved!

## `update`

```js
    function update($$) {
        if ($$.fragment !== null) {
            $$.update();
            run_all($$.before_update);
            $$.fragment && $$.fragment.p($$.ctx, $$.dirty);
            $$.dirty = [-1];
            $$.after_update.forEach(add_render_callback);
        }
    }
```

We can ignore almost all of this. `$$.update` is assigned to `noop` which does nothing at all. We'll also assume `$$.fragment` isn't null (how could it be??). Then, `$$.before_update` is currently an empty array, so we'll wait for more app complexity before studying `run_all($$.before_update)`. Similarly, `$$.after_update.forEach(add_render_callback)` we can ignore because `$$.after_update` is also an empty array.

That leaves only

```js
    $$.fragment && $$.fragment.p($$.ctx, $$.dirty);
    $$.dirty = [-1];
```

Looking around `bundle.js` I'm pretty confident that `$$.dirty = [-1]` means there are no pending changes to the app's state. This means that after updating the DOM in the line above it, `$$.fragment.p($$.ctx, $$.dirty)`, we're indicating that all necessary changes have been made.

That makes the only action-packed line `$$.fragment.p($$.ctx, $$.dirty)`, to update the DOM with any changes to 
`$$.ctx`.

## `$$.ctx`

`$$.ctx` seems to be where the app's state lives. Its calculation is a little complex:

```js
    $$.ctx = instance
        ? instance(component, prop_values, (i, ret, value = ret) => {
            if ($$.ctx && not_equal($$.ctx[i], $$.ctx[i] = value)) {
                if ($$.bound[i])
                    $$.bound[i](value);
                if (ready)
                    make_dirty(component, i);
            }
            return ret;
        })
```

The `instance` function is what generates it:

```js
    function instance($$self, $$props, $$invalidate) {
    	let { name } = $$props;
    	let { number } = $$props;
    	const writable_props = ["name", "number"];

    	Object.keys($$props).forEach(key => {
    		if (!~writable_props.indexOf(key) && key.slice(0, 2) !== "$$") console.warn(`<App> was created with unknown prop '${key}'`);
    	});

    	$$self.$set = $$props => {
    		if ("name" in $$props) $$invalidate(0, name = $$props.name);
    		if ("number" in $$props) $$invalidate(1, number = $$props.number);
    	};

    	$$self.$capture_state = () => {
    		return { name, number };
    	};

    	$$self.$inject_state = $$props => {
    		if ("name" in $$props) $$invalidate(0, name = $$props.name);
    		if ("number" in $$props) $$invalidate(1, number = $$props.number);
    	};

    	return [name, number];
    }
```

`instance` destructures our props, `name` and `number`, and passes them right through, unchanged, to `$$.ctx`. 

Therefore, `$$.ctx` is equal to `["world", 42]`: Not as complex as I expected; we'll come back to all these side effects happening here between the seeming pass-through of props.

As seen earlier, `$$.fragment.p($$.ctx, $$.dirty)` is calling this function:

```js
    function update(ctx, [dirty]) {
      if (dirty & /*name*/ 1) set_data_dev(t1, /*name*/ ctx[0]);
      if (dirty & /*number*/ 2) set_data_dev(t5, /*number*/ ctx[1]);
    }
```

Okay, time to figure out what this `dirty & x` business is about. It seems like `dirty` contains indices of what elements need updating, but why not find out the specifics?:

```js
    p: function update(ctx, [dirty]) {
        if (dirty & /*name*/ 1) {
            console.log(`dirty 1 was dirty: ${dirty}`)
            set_data_dev(t1, /*name*/ ctx[0]);
        } else {
            console.log(`dirty 1 wasn't dirty: ${dirty}`)
        }
        if (dirty & /*name*/ 2) {
            console.log(`dirty 2 was dirty: ${dirty}`)
            set_data_dev(t5, /*name*/ ctx[0]);
        } else {
            console.log(`dirty 2 wasn't dirty: ${dirty}`)
        }
        console.log(typeof dirty)
    },
```

In order to trigger `update` without building some UI, to trigger these informative `console.log`s, we need to manipulate the app's state manually:

# `app` in Action

Circling back to the `instance` function, the more meaningful work it performs (the "side effects") is in binding three methods—`$set`, `$capture_state`, and `$inject_state`—to `$$self`, which is `App`.

Did I mention we can inspect our `App` instance, `app`, in the console? It's another lovely feature of Svelte: Since it compiles down to vanilla Javascript, `app` is in the global scope of a browser rendering it, without any special plugins or other somersaults! Armed with that knowledge, let's play with these new methods in the Javascript console:

```js
>> app.$capture_state()
   ► Object { name: "world", number: 42 }
>> app.$set({name: "Whirl"})
   undefined
   dirty 1 was dirty: 1
   dirty 2 wasn't dirty: 1
   number
>> app.$capture_state()
   ► Object { name: "Whirl", number: 42 }
>> app.$inject_state({number: 24})
   undefined
   undefined
   dirty 1 wasn't dirty: 2 
   dirty 2 was dirty: 2
   number
>> app.$capture_state()
   ► Object { name: "Whirl", number: 24 }
```

The page looks like this now:

![A screenshot showing that the updated props have also changed in the rendered page.](https://thepracticaldev.s3.amazonaws.com/i/1cia44g9gwvtqb5l01qi.png)

Several discoveries here: 

1) `$capture_state` gives the current state of the app as an object.
2) `$set` and `$inject_state` seem to both update the app's state via an object.
3) `dirty`, when it's not equal to `[-1]`, is a positive integer seemingly referring to the props by a 1-based index.
4) These props are updated in the rendered page.

One more mystery to unravel:

```js
>> app.name
   Error: <App>: Props cannot be read directly from the component instance unless compiling with 'accessors: true' or
   '<svelte:options accessors/>'
>> app.name = 'hi'
   Error: <App>: Props cannot be set directly on the component instance unless compiling with 'accessors: true' or
   '<  svelte:options accessors/>'
```

That's the purpose of the `set` and `get` methods from earlier: Enforce that the compiled code doesn't set and get props directly on the `App` instance, but that it uses... the included machinery?  

# Next Time

Join us next time to unwrap the mysteries of

1) What is the difference between `app.$set` and `app.$inject_state`, if any?
2) How does `bundle.js` change with increasing app complexity? Multiple components, for example, or dynamically re-rendering props/state.
3) What is `__svelte_meta` for?
4) Where and when does `mount` actually get called?
5) Can `dirty` ever contain anything besides a single integer? In other words, are elements updated one after the next, or can `update` sometimes operate on more than one element at a run?
6) When are components and elements destroyed? Are Svelte and Rollup as efficient about unnecessary re-renders as billed?
7) How does all this fit together? Asked another way, is it possible to have a basic understanding of how a web framework we use actually works?

# Random Notes
[According to Svelte's tweet response to me](https://twitter.com/zevav/status/1202593636631941123), the events emitted at various points in `bundle.js` are strictly for dev tooling. This is why we can ignore them.