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fix: make deriveds on the server lazy again #15964

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Merged
merged 6 commits into from
May 22, 2025
Merged

fix: make deriveds on the server lazy again #15964

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e6c4e8c
fix: make deriveds on the server lazy again
dummdidumm May 20, 2025
69bd060
fix: handle basic assignment of deriveds on the server
paoloricciuti May 20, 2025
dadff16
fix: use `build_assignment_value` for deriveds assignments
paoloricciuti May 21, 2025
895e048
use once
dummdidumm May 22, 2025
846d780
Merge branch 'main' into lazy-derived-server
dummdidumm May 22, 2025
5ba3572
allow writing to public deriveds on server
dummdidumm May 22, 2025
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5 changes: 5 additions & 0 deletions .changeset/khaki-carrots-destroy.md
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@@ -0,0 +1,5 @@
---
'svelte': patch
---

fix: make deriveds on the server lazy again
2 changes: 2 additions & 0 deletions packages/svelte/src/compiler/phases/3-transform/server/transform-server.js
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Expand Up @@ -23,6 +23,7 @@ import { Identifier } from './visitors/Identifier.js';
import { IfBlock } from './visitors/IfBlock.js';
import { KeyBlock } from './visitors/KeyBlock.js';
import { LabeledStatement } from './visitors/LabeledStatement.js';
import { MemberExpression } from './visitors/MemberExpression.js';
import { PropertyDefinition } from './visitors/PropertyDefinition.js';
import { RegularElement } from './visitors/RegularElement.js';
import { RenderTag } from './visitors/RenderTag.js';
Expand All @@ -48,6 +49,7 @@ const global_visitors = {
ExpressionStatement,
Identifier,
LabeledStatement,
MemberExpression,
PropertyDefinition,
UpdateExpression,
VariableDeclaration
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5 changes: 5 additions & 0 deletions packages/svelte/src/compiler/phases/3-transform/server/visitors/AssignmentExpression.js
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Expand Up @@ -44,6 +44,11 @@ function build_assignment(operator, left, right, context) {
/** @type {Expression} */ (context.visit(right))
);
}
} else if (field && (field.type === '$derived' || field.type === '$derived.by')) {
let value = /** @type {Expression} */ (
context.visit(build_assignment_value(operator, left, right))
);
return b.call(b.member(b.this, name), value);
}
}

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2 changes: 1 addition & 1 deletion packages/svelte/src/compiler/phases/3-transform/server/visitors/CallExpression.js
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Expand Up @@ -31,7 +31,7 @@ export function CallExpression(node, context) {

if (rune === '$derived' || rune === '$derived.by') {
const fn = /** @type {Expression} */ (context.visit(node.arguments[0]));
return b.call('$.once', rune === '$derived' ? b.thunk(fn) : fn);
return b.call('$.derived', rune === '$derived' ? b.thunk(fn) : fn);
}

if (rune === '$state.snapshot') {
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23 changes: 23 additions & 0 deletions packages/svelte/src/compiler/phases/3-transform/server/visitors/MemberExpression.js
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@@ -0,0 +1,23 @@
/** @import { ClassBody, MemberExpression } from 'estree' */
/** @import { Context } from '../types.js' */
import * as b from '#compiler/builders';

/**
* @param {MemberExpression} node
* @param {Context} context
*/
export function MemberExpression(node, context) {
if (
context.state.analysis.runes &&
node.object.type === 'ThisExpression' &&
node.property.type === 'PrivateIdentifier'
) {
const field = context.state.state_fields?.get(`#${node.property.name}`);

if (field?.type === '$derived' || field?.type === '$derived.by') {
return b.call(node);
}
}

context.next();
}
7 changes: 4 additions & 3 deletions packages/svelte/src/compiler/phases/3-transform/server/visitors/PropertyDefinition.js
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Expand Up @@ -11,7 +11,7 @@ export function PropertyDefinition(node, context) {
if (context.state.analysis.runes && node.value != null && node.value.type === 'CallExpression') {
const rune = get_rune(node.value, context.state.scope);

if (rune === '$state' || rune === '$state.raw' || rune === '$derived') {
if (rune === '$state' || rune === '$state.raw') {
return {
...node,
value:
Expand All @@ -21,13 +21,14 @@ export function PropertyDefinition(node, context) {
};
}

if (rune === '$derived.by') {
if (rune === '$derived.by' || rune === '$derived') {
const fn = /** @type {Expression} */ (context.visit(node.value.arguments[0]));
return {
...node,
value:
node.value.arguments.length === 0
? null
: b.call(/** @type {Expression} */ (context.visit(node.value.arguments[0])))
: b.call('$.derived', rune === '$derived' ? b.thunk(fn) : fn)
};
}
}
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20 changes: 20 additions & 0 deletions packages/svelte/src/internal/server/index.js
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Expand Up @@ -514,3 +514,23 @@ export {
} from '../shared/validate.js';

export { escape_html as escape };

/**
* @template T
* @param {()=>T} fn
* @returns {(new_value?: T) => (T | void)}
*/
export function derived(fn) {
/**
* @type {T | undefined}
*/
let updated_value;

return function (new_value) {
if (arguments.length === 0) {
return updated_value ?? fn();
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This calls the function on every invocation now, we should wrap it with once above

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It should be called everytime or else the value will not change on reassignment no?

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If you are talking about fn it the stateful value it's dependent on changes it should be rerun no?

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We only called fn once previously and that was fine and declared as the behavior on the server. The bug introduced was that it was now called eagerly, not lazily

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I know but I still think this would be the better behavior (which I was also trying to fix on #14977 )...I guess in this case is still better to wrap fn in once however

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I'll add a change to wrap it in once then we can think about this in #14977 separately, for now we need to fix this critical bug

}
updated_value = new_value;
return updated_value;
};
}
3 changes: 3 additions & 0 deletions packages/svelte/tests/runtime-runes/samples/class-state-derived-private/_config.js
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Expand Up @@ -5,6 +5,7 @@ export default test({
html: `
<button>0</button>
<p>doubled: 0</p>
<p>tripled: 0</p>
`,

test({ assert, target }) {
Expand All @@ -17,6 +18,7 @@ export default test({
`
<button>1</button>
<p>doubled: 2</p>
<p>tripled: 3</p>
`
);

Expand All @@ -27,6 +29,7 @@ export default test({
`
<button>2</button>
<p>doubled: 4</p>
<p>tripled: 6</p>
`
);
}
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16 changes: 13 additions & 3 deletions packages/svelte/tests/runtime-runes/samples/class-state-derived-private/main.svelte
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Expand Up @@ -2,14 +2,24 @@
class Counter {
count = $state(0);
#doubled = $derived(this.count * 2);
#tripled = $derived.by(() => this.count * this.by);

get embiggened() {
constructor(by) {
this.by = by;
}

get embiggened1() {
return this.#doubled;
}

get embiggened2() {
return this.#tripled;
}
}

const counter = new Counter();
const counter = new Counter(3);
</script>

<button on:click={() => counter.count++}>{counter.count}</button>
<p>doubled: {counter.embiggened}</p>
<p>doubled: {counter.embiggened1}</p>
<p>tripled: {counter.embiggened2}</p>