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Validating State Snapshot Functionality in sveltejs/svelte

This test suite validates the snapshot functionality in Svelte’s internal clone module, focusing on state management and object cloning capabilities. The tests ensure proper handling of various data types and edge cases when creating immutable snapshots of application state.

Test Coverage Overview

The test suite provides comprehensive coverage of the snapshot functionality across different data types and scenarios.

Tests primitive values, arrays, and objects
Validates proxied state behavior
Handles circular references and class instances
Tests custom class implementations with toJSON
Verifies behavior with uncloneable values and properties

Implementation Analysis

The testing approach employs Vitest for unit testing, utilizing assertion methods to verify clone behavior and state immutability. The implementation leverages TypeScript’s type system and includes custom warning capture mechanisms for error scenarios.

Key patterns include proxy state management, deep equality checking, and class inheritance testing.

Technical Details

Testing Framework: Vitest
Language: TypeScript
Key Dependencies: Svelte internal proxy system
Test Utilities: Custom warning capture function
Assertion Methods: equal, deepEqual, notEqual

Best Practices Demonstrated

The test suite exemplifies high-quality testing practices through comprehensive edge case coverage and robust error handling.

Isolated test cases with clear purpose
Thorough validation of error scenarios
Type-safe testing approaches
Mock console warning capture
Complex object structure testing

sveltejs/svelte

packages/svelte/src/internal/shared/clone.test.ts

            
import { snapshot } from './clone';
import { assert, test } from 'vitest';
import { proxy } from '../client/proxy';

function capture_warnings() {
	const warnings: string[] = [];

	// eslint-disable-next-line no-console
	const warn = console.warn;

	// eslint-disable-next-line no-console
	console.warn = (message) => warnings.push(message);

	return () => {
		// eslint-disable-next-line no-console
		console.warn = warn;
		return warnings;
	};
}

test('primitive', () => {
	assert.equal(42, snapshot(42));
});

test('array', () => {
	const array = [1, 2, 3];
	const copy = snapshot(array);

	assert.deepEqual(copy, array);
	assert.notEqual(copy, array);
});

test('object', () => {
	const object = { a: 1, b: 2, c: 3 };
	const copy = snapshot(object);

	assert.deepEqual(copy, object);
	assert.notEqual(copy, object);
});

test('proxied state', () => {
	const object = proxy({
		a: {
			b: {
				c: 1
			}
		}
	});

	const copy = snapshot(object);

	assert.deepEqual(copy, object);
	assert.notEqual(copy, object);

	object.a.b.c = 2;
	assert.equal(copy.a.b.c, 1);
});

test('cycles', () => {
	const object: { self?: any } = {};
	object.self = object;
	const copy = snapshot(object);

	assert.equal(copy.self, copy);
});

test('class with state field', () => {
	class Foo {
		x = 1;
		#y = 2;

		get y() {
			return this.#y;
		}
	}

	const copy = snapshot(new Foo());

	// @ts-expect-error I can't figure out a way to exclude prototype properties
	assert.deepEqual(copy, { x: 1 });
});

test('class with toJSON', () => {
	class Foo {
		x = 1;
		#y = 2;

		get y() {
			return this.#y;
		}

		toJSON() {
			return {
				x: this.x,
				y: this.y
			};
		}
	}

	const copy = snapshot(new Foo());

	assert.deepEqual(copy, { x: 1, y: 2 });
});

test('reactive class', () => {
	class SvelteMap<T, U> extends Map<T, U> {
		constructor(init?: Iterable<[T, U]>) {
			super(init);
		}
	}

	const map = new SvelteMap([[1, 2]]);
	const copy = snapshot(map);

	assert.ok(copy instanceof Map);
	assert.notOk(copy instanceof SvelteMap);

	assert.equal(copy.get(1), 2);
});

test('uncloneable value', () => {
	const fn = () => {};

	const warnings = capture_warnings();
	const copy = snapshot(fn);

	assert.equal(fn, copy);
	assert.deepEqual(warnings(), [
		'%c[svelte] state_snapshot_uncloneable\n%cValue cannot be cloned with `$state.snapshot` — the original value was returned\nhttps://svelte.dev/e/state_snapshot_uncloneable'
	]);
});

test('uncloneable properties', () => {
	const object = {
		a: () => {},
		b: () => {},
		c: [() => {}, () => {}, () => {}, () => {}, () => {}, () => {}, () => {}, () => {}]
	};

	const warnings = capture_warnings();
	const copy = snapshot(object);

	assert.notEqual(object, copy);
	assert.equal(object.a, copy.a);
	assert.equal(object.b, copy.b);

	assert.notEqual(object.c, copy.c);
	assert.equal(object.c[0], copy.c[0]);

	assert.deepEqual(warnings(), [
		`%c[svelte] state_snapshot_uncloneable
%cThe following properties cannot be cloned with \`$state.snapshot\` — the return value contains the originals:

- <value>.a
- <value>.b
- <value>.c[0]
- <value>.c[1]
- <value>.c[2]
- <value>.c[3]
- <value>.c[4]
- <value>.c[5]
- <value>.c[6]
- <value>.c[7]
https://svelte.dev/e/state_snapshot_uncloneable`
	]);
});

test('many uncloneable properties', () => {
	const array = Array.from({ length: 100 }, () => () => {});

	const warnings = capture_warnings();
	snapshot(array);

	assert.deepEqual(warnings(), [
		`%c[svelte] state_snapshot_uncloneable
%cThe following properties cannot be cloned with \`$state.snapshot\` — the return value contains the originals:

- <value>[0]
- <value>[1]
- <value>[2]
- <value>[3]
- <value>[4]
- <value>[5]
- <value>[6]
- ...and 93 more
https://svelte.dev/e/state_snapshot_uncloneable`
	]);
});