---
title: How Does Svelte Actually Work? part 2
published: true
description: A conversational, thinking-out-loud tour of the JavaScript that Svelte outputs... part deux
tags: svelte, javascript
---
Here's part 1:
{% link https://dev.to/zev/how-does-svelte-actually-work-part-1-j9m %}
and here's the repository for what we've done so far:
{% github zevaverbach/how-svelte-works no-readme %}
(hint: we have barely changed Svelte's sample app).
# Episode Recap
On the previous episode of HDSAW we took a bewildering ride through the 400 or so lines of vanilla JavaScript the Svelte compiler produces from [the sample app](https://github.com/sveltejs/template). Here's [that vanilla JS](https://gist.github.com/zevaverbach/83901aed1230fbd3adb01b96a7be0572) for your perusal, and see below for an updated version.
## What We've Learned So Far
When we run `npm run dev` to compile the sample app, Rollup takes the contents of `main.js` and runs the Svelte compiler on it. This produces a file called `bundle.js` in the `build` directory.
`bundle.js` defines and immediately calls an anonymous function, creating an instance of `App` called `app`. The code which runs inside the constructor of `App` first renders the Svelte app, then binds a small handful of methods to the instance. These methods can be used to get and set the state of the app, including by the user in the browser's JavaScript console!
## Outstanding Questions
1) Is it a security problem that the user can directly manipulate the app's state from a browser's JavaScript console?
2) What is the difference between `app.$set` and `app.$inject_state`, if any?
3) How does `bundle.js` change with increasing app complexity? Multiple components, for example, or dynamically re-rendering props/state.
4) What is `__svelte_meta` for?
5) Where and when does `mount` actually get called?
6) 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?
7) When are components and elements destroyed? Is Svelte as efficient about unnecessary re-renders as billed?
8) What are the setters and getters on `App` for and why are they implemented the way they are?
9) 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?
We'll try to get through a few of these today, and then I'm sure more questions will emerge as we go.
# A Skinnier Bundle
This time around we're going to generate the `build.js` using `npm run build`, so as to not distract ourselves with the events that get emitted in dev mode. First, though, let's tell Rollup not to minify that file so we can actually read it:
```js
// ./rollup.config.js
...
// In dev mode, call `npm run start` once
// the bundle has been generated
!production && serve(),
// Watch the `public` directory and refresh the
// browser on changes when not in production
!production && livereload('public'),
// If we're building for production (npm run build
// instead of npm run dev), minify
// production && terser() πππ comment this out
],
watch: {
...
```
`npm run build` produces this _really quite svelte_ code, which is now only about 300 lines!:
```js
// src/bundle.js
var app = (function () {
'use strict';
function noop() { }
function run(fn) {
return fn();
}
function blank_object() {
return Object.create(null);
}
function run_all(fns) {
fns.forEach(run);
}
function is_function(thing) {
return typeof thing === 'function';
}
function safe_not_equal(a, b) {
return a != a ? b == b : a !== b || ((a && typeof a === 'object') || typeof a === 'function');
}
function append(target, node) {
target.appendChild(node);
}
function insert(target, node, anchor) {
target.insertBefore(node, anchor || null);
}
function detach(node) {
node.parentNode.removeChild(node);
}
function element(name) {
return document.createElement(name);
}
function text(data) {
return document.createTextNode(data);
}
function space() {
return text(' ');
}
function attr(node, attribute, value) {
if (value == null)
node.removeAttribute(attribute);
else if (node.getAttribute(attribute) !== value)
node.setAttribute(attribute, value);
}
function children(element) {
return Array.from(element.childNodes);
}
function set_data(text, data) {
data = '' + data;
if (text.data !== data)
text.data = data;
}
let current_component;
function set_current_component(component) {
current_component = component;
}
const dirty_components = [];
const binding_callbacks = [];
const render_callbacks = [];
const flush_callbacks = [];
const resolved_promise = Promise.resolve();
let update_scheduled = false;
function schedule_update() {
if (!update_scheduled) {
update_scheduled = true;
resolved_promise.then(flush);
}
}
function add_render_callback(fn) {
render_callbacks.push(fn);
}
function flush() {
const seen_callbacks = new Set();
do {
// first, call beforeUpdate functions
// and update components
while (dirty_components.length) {
const component = dirty_components.shift();
set_current_component(component);
update(component.$$);
}
while (binding_callbacks.length)
binding_callbacks.pop()();
// then, once components are updated, call
// afterUpdate functions. This may cause
// subsequent updates...
for (let i = 0; i < render_callbacks.length; i += 1) {
const callback = render_callbacks[i];
if (!seen_callbacks.has(callback)) {
callback();
// ...so guard against infinite loops
seen_callbacks.add(callback);
}
}
render_callbacks.length = 0;
} while (dirty_components.length);
while (flush_callbacks.length) {
flush_callbacks.pop()();
}
update_scheduled = false;
}
function update($$) {
if ($$.fragment !== null) {
$$.update();
run_all($$.before_update);
$$.fragment && $$.fragment.p($$.ctx, $$.dirty);
$$.dirty = [-1];
$$.after_update.forEach(add_render_callback);
}
}
const outroing = new Set();
function transition_in(block, local) {
if (block && block.i) {
outroing.delete(block);
block.i(local);
}
}
function mount_component(component, target, anchor) {
const { fragment, on_mount, on_destroy, after_update } = component.$$;
fragment && fragment.m(target, anchor);
// onMount happens before the initial afterUpdate
add_render_callback(() => {
const new_on_destroy = on_mount.map(run).filter(is_function);
if (on_destroy) {
on_destroy.push(...new_on_destroy);
}
else {
// Edge case - component was destroyed immediately,
// most likely as a result of a binding initialising
run_all(new_on_destroy);
}
component.$$.on_mount = [];
});
after_update.forEach(add_render_callback);
}
function destroy_component(component, detaching) {
const $$ = component.$$;
if ($$.fragment !== null) {
run_all($$.on_destroy);
$$.fragment && $$.fragment.d(detaching);
// TODO null out other refs, including component.$$ (but need to
// preserve final state?)
$$.on_destroy = $$.fragment = null;
$$.ctx = [];
}
}
function make_dirty(component, i) {
if (component.$$.dirty[0] === -1) {
dirty_components.push(component);
schedule_update();
component.$$.dirty.fill(0);
}
component.$$.dirty[(i / 31) | 0] |= (1 << (i % 31));
}
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);
}
class SvelteComponent {
$destroy() {
destroy_component(this, 1);
this.$destroy = noop;
}
$on(type, callback) {
const callbacks = (this.$$.callbacks[type] || (this.$$.callbacks[type] = []));
callbacks.push(callback);
return () => {
const index = callbacks.indexOf(callback);
if (index !== -1)
callbacks.splice(index, 1);
};
}
$set() {
// overridden by instance, if it has props
}
}
/* src/App.svelte generated by Svelte v3.16.0 */
function create_fragment(ctx) {
let main;
let h1;
let t0;
let t1;
let t2;
let t3;
let p0;
let t4;
let t5;
let t6;
let t7;
let p1;
return {
c() {
main = element("main");
h1 = element("h1");
t0 = text("Hello ");
t1 = text(/*name*/ ctx[0]);
t2 = text("!");
t3 = space();
p0 = element("p");
t4 = text("Your lucky number is ");
t5 = text(/*number*/ ctx[1]);
t6 = text(".");
t7 = space();
p1 = element("p");
p1.innerHTML = `Visit the Svelte tutorial to learn how to build Svelte apps.`;
attr(h1, "class", "svelte-1tky8bj");
attr(main, "class", "svelte-1tky8bj");
},
m(target, anchor) {
insert(target, main, anchor);
append(main, h1);
append(h1, t0);
append(h1, t1);
append(h1, t2);
append(main, t3);
append(main, p0);
append(p0, t4);
append(p0, t5);
append(p0, t6);
append(main, t7);
append(main, p1);
},
p(ctx, [dirty]) {
if (dirty & /*name*/ 1) set_data(t1, /*name*/ ctx[0]);
if (dirty & /*number*/ 2) set_data(t5, /*number*/ ctx[1]);
},
i: noop,
o: noop,
d(detaching) {
if (detaching) detach(main);
}
};
}
function instance($$self, $$props, $$invalidate) {
let { name } = $$props;
let { number } = $$props;
$$self.$set = $$props => {
if ("name" in $$props) $$invalidate(0, name = $$props.name);
if ("number" in $$props) $$invalidate(1, number = $$props.number);
};
return [name, number];
}
class App extends SvelteComponent {
constructor(options) {
super();
init(this, options, instance, create_fragment, safe_not_equal, { name: 0, number: 1 });
}
}
const app = new App({
target: document.body,
props: {
name: 'World',
number: 42
}
});
return app;
}());
//# sourceMappingURL=bundle.js.map
```
This file, `bundle.js`, is what all of the following code blocks refer to, unless otherwise noted.
# Q: Where are the elements created in `create_fragment` mounted?
Here, in `init`:
```js
...
if (options.target) {
...
ππππ
mount_component(component, options.target, options.anchor);
ππππ
flush();
...
}
```
And `mount_component`, in the first two lines of its definition, destructures `fragment` from `component.$$` then calls `fragment.m`:
```js
function mount_component(component, target, anchor) {
const { fragment, on_mount, on_destroy, after_update } = component.$$;
fragment && fragment.m(target, anchor);
...
```
As a reminder from part 1, `m` stands for "mount" (it was actually called that when compiling in dev mode), and it takes the elements created in `fragment.c` and adds them to the DOM with eminently readable and grokkable JS:
```js
m(target, anchor) {
// these πππ three parameters are equivalent to `document.body`, , and `undefined`
insert(target, main, anchor);
append(main, h1);
append(h1, t0);
append(h1, t1);
append(h1, t2);
append(main, t3);
append(main, p0);
append(p0, t4);
append(p0, t5);
append(p0, t6);
append(main, t7);
append(main, p1);
},
...
function append(target, node) {
target.appendChild(node);
}
function insert(target, node, anchor) {
target.insertBefore(node, anchor || null);
}
```
Reminder: This is one of the things that makes Svelte great, and unique among popular frameworks: The abstractions are not so deep or complex; so far, we are never far from `document.doSomething` operations.
# Disappearing Questions
Now that we've left dev mode for our explorations, it seems that a few outstanding questions have evaporated:
## What is `__svelte_meta` for?
It doesn't appear in `bundle.js` in production mode! It's for dev tooling, must be: "Error X on line Y, character Z", and so on.
Mystery solved! π΅
## What is the difference between `app.$set` and `app.$inject_state`, if any?
Well, their implementations are identical,
```js
// version of bundle.js in dev mode
function instance($$self, $$props, $$invalidate) {
...
$$self.$set = $$props => {
if ("name" in $$props) $$invalidate(0, name = $$props.name);
if ("number" in $$props) $$invalidate(1, number = $$props.number);
};
$$self.$inject_state = $$props => {
if ("name" in $$props) $$invalidate(0, name = $$props.name);
if ("number" in $$props) $$invalidate(1, number = $$props.number);
};
...
}
```
annnnnddd... `$inject_state` doesn't even get defined in the production build,
```js
// production build of bundle.js
function instance($$self, $$props, $$invalidate) {
let { name } = $$props;
let { number } = $$props;
$$self.$set = $$props => {
if ("name" in $$props) $$invalidate(0, name = $$props.name);
if ("number" in $$props) $$invalidate(1, number = $$props.number);
};
return [name, number];
}
```
so let's assume it's a very careful piece of naming to distinguish what dev tools use to set state, as compared with how state is set internally in the app.
Turns out, yep!:
{% twitter 1203024631390715912 %}
Finally, it looks like the setter and getter methods on `App` don't appear in production mode. We can therefore scratch that question off the list, as we're not concerned with the dev tooling part of the generated code for now.
# The `$$invalidate` Callback
In the `instance` function (πππ much shorter in prod), the `$set` method is bound to `app`. Presumably this is the only way prop updates propagate through the app, so it'll behoove us to understand how it's defined in the calling code:
```js
...
function init(component, options, instance, create_fragment, not_equal, props, dirty = [-1]) {
...
instance(component, prop_values, (i, ret, value = ret) => {
if ($$.ctx && not_equal($$.ctx[i], $$.ctx[i] = value)) {
if ($$.bound[i])
$$.bound[i](value);
if (ready)
make_dirty(component, i);
}
return ret;
})
```
This is hard to read, so let's write it as a normal, somewhat simplified, non-anonymous function:
```js
function invalidateProp(i, ret, value = ret) {
const currentVal = $$.ctx[i]
const newVal = value
if (currentVal !== newVal) {
// do some things with `$$.bound` and `ready` that will never trigger in the current app
}
$$.ctx[i] = newVal
return ret
}
```
So if `invalidateProp` is called like so,
```js
invalidateProp(0, name = "Whirl");
```
the only things that will happen are
1) `$$.ctx[0]` will equal "Whirl"
2) "Whirl" will be returned.
3) Nothing anywhere will consume that return value in the current app.
# Hypothesis #3: `$$.bound` and `ready` Will Come Into Play If There's Some Interactive UI
Furthermore, I'm thinking `$$invalidate`'s return value might actually get used somewhere. There's plenty more to explore ahead of this test, but let's forge ahead and see what magic the compiler has for us.
## Testing Hypothesis #3
Let's make a button to increment `number`:
```html
Hello {name}!
Your lucky number is {number}.
Visit the Svelte tutorial to learn how to build Svelte apps.