Reactive data

No compiler, no bundler. The reactive store is a plain JavaScript proxy — no code generation, no AST transforms, no virtual DOM. It works the same whether the component was bundled by Vite or fetched at runtime from a CDN.

The store used by components is available standalone:

// also injected into setup scripts
import { $reactive } from "jq79"

const data = $reactive({ user: { address: { city: "NYC" } } })

data.$on("user.address.city", (value, dotKey) => { … }, { immediate: true })
data.$onAny((dotKey, value) => { … })
const stop = data.$effect(() => {
  // re-runs whenever a value it *read* changes - at whatever depth it read it
  console.log(data.user.address.city)
})

// deep mutations notify with the full dot path
data.user.address.city = "LA"

// so does deleting a key (listeners hear undefined, same as a read afterwards)
delete data.user.address.city

// effects/listeners return an unsubscribe fn
stop()

// drops this store's subscriptions to the stores nested inside it
data.$dispose()

An effect wakes for what it read, not for the object it read it through

Reaching data.user.address.city reads three objects on the way, but the effect depends on the value it actually took:

const data = $reactive({ rows: [{ label: "a" }, { label: "b" }] })

data.$effect(() => console.log(data.rows[0].label))

data.rows[1].label = "changed"  // silent: this effect never read row 1
data.rows[0].label = "changed"  // re-runs
data.rows = []                  // re-runs: replacing the array replaced the row

A write re-runs an effect when it lands on something the effect read, or inside it — replacing rows re-runs everything that read anything under rows. It does not travel the other way: touching one row does not wake an effect that only read a different one, which is what keeps a 1,000-row list costing one re-render per changed row rather than a thousand.

Replacing a list with a copy of itself one element longer or shorter is not a wholesale replacement, and isn't treated as one — the rows that stay are the same objects, so an effect that read into one of them is left alone, wherever that row has ended up. What does re-run is whatever read the list itself (a :each, its length) and whatever read a slot the shift moved, since rows[6] means "whoever sits at slot 6":

const row = data.rows[6]

data.$effect(() => console.log(row.label))           // this row, wherever it is
data.$effect(() => console.log(data.rows[6].label))  // whoever sits at slot 6

data.rows = data.rows.filter(r => r.id !== 1)        // silent / re-runs

Any other reshuffle — a sort, a reorder, two rows removed at once — re-runs everything that read under the list, as a replacement does.

The case to know about is an effect that reads an object without reading into it:

data.$effect(() => console.log(data.user))    // logs the object itself
data.user.name = "Grace"                      // silent

Nothing the effect read changed — data.user is the same object. Read the values you actually depend on (data.user.name), which is what a template interpolation like {{ user.name }} does anyway.

Two things deliberately stay coarse, because nothing finer is observable: enumerating an object (Object.keys, {...spread}, :each over a plain object) re-runs when a key is added or removed, and an array's length re-runs everything that read an element, since a truncation removes them without notifying each one.

When the effect can't read what it depends on

Reading the values you depend on is the answer almost everywhere. It stops being possible when the effect hands a value to code the store cannot watch — a chart library, a canvas draw, a request — because the reads then happen somewhere the proxy is not:

data.$effect(() => chart.render(data.series))   // reads `series`, nothing under it
data.series.points.push(4)                      // silent: nothing it read changed

deep opts that effect into waking for writes below what it read, too:

data.$effect(() => chart.render(data.series), { deep: true })
data.series.points.push(4)                      // re-runs

Use it for exactly this — a value forwarded whole to something opaque. It is not a general "watch everything" switch: an effect marked deep wakes for every write under every path it touched, which on a large object is most writes. A component's props already use it internally, which is why a parent's deep mutation reaches a child that only reads the prop object.

The object you hand it is left alone

$reactive(data) doesn't modify data — it returns a reactive view of it. Nested objects are wrapped as they're read, not rewritten in place, so data comes back exactly as you passed it in.

Two consequences worth knowing:

Objects keep a stable identity within a store: reading the same object twice hands back the same proxy, which is what lets :each diff a reordered list by reference and keep each row's DOM.

Class instances, Dates and DOM nodes pass through untouched.

Getting the plain object back

$toRaw(value) returns the object behind a store proxy — at the root or at any depth — for the code that shouldn't see a proxy at all: a library that stores what you hand it, a structuredClone, an identity comparison against the original data.

// also injected into setup scripts
import { $reactive, $toRaw } from "jq79"

const data = { user: { name: "Jesús" }, items: [{ id: 1 }] }
const store = $reactive(data)

$toRaw(store)          // data
$toRaw(store.user)     // data.user
$toRaw(store.items[0]) // data.items[0]
$toRaw({ a: 1 })       // { a: 1 } — anything that isn't a proxy comes back as it is

It hands back the live object, not a copy, so the rule above applies to it: writes made through it notify nobody.

$toRaw(store).count = 5   // the value changes, nothing re-renders
store.count = 5           // ✅

Shared state: pass a store, not an object

A plain object handed to two components is shared data, not shared state: each store notifies its own listeners, so a child writing to it updates the child and leaves the parent's DOM showing the old value.

A store handed around is shared state. It isn't re-wrapped when it lands in another store — it stays itself, with its own $on/$effect — and the store holding it subscribes to it, re-notifying its changes under the path it sits at (items.0 inside the store becomes cart.items.0 for the holder). So every component that was handed it sees every write, whoever made it:

<script :setup>
  const cart = $reactive({ items: [] })   // one store…
</script>

<CartLines :cart />                       <!-- …handed to both children -->
<CartTotal :cart />

<!-- CartLines.html -->
<script :setup="{ cart }"></script>
<button @click="cart.items = [...cart.items, 'a pear']">add</button>

CartTotal updates when CartLines writes, the parent updates when either does, and a write from outside the tree (cart.items = [], from the module that created the store) updates all three. Which makes a $reactive created outside any component a perfectly good app-wide store — import it where you need it.

A component drops its subscription to a store it was handed when it's destroyed, so a long-lived store doesn't accumulate listeners from components that are gone. Outside a component, store.$dispose() does the same by hand.

Derived values: $computed

$computed(get) keeps the result of get on .value, and recomputes it whenever anything get read changes, in any store. It's the shared-state counterpart of $:: a derived value you can export, pass as a prop, or hand to several components.

// store.js
import { $reactive, $computed } from "jq79"

export const cart = $reactive({ items: [] })
export const total = $computed(() => cart.items.reduce((sum, item) => sum + item.price, 0))

total.value                  // 0
cart.items.push({ price: 3 })
total.value                  // 3

<!-- CartTotal.html -->
<script :setup="{ total }"></script>
<p>Total: {{ total.value }}</p>

It's also available in setup scripts and factory ctx. There, it's disposed with the component that created it, so one made over a long-lived store doesn't keep running after the component is gone. For a value only the component's own template shows, $: total = … is still simpler: {{ total }} instead of {{ total.value }}.