## Code like Rich Harris (@Rich-Harris) · compiler-grade precision

Rich Harris writes library code with the discipline of a compiler author: every class owns exactly one responsibility, every piece of mutable state is named and tracked explicitly, and invalid input throws immediately with a precise message instead of silently degrading. Data structures are pointer-based and mutations are surgical — relinking a doubly-linked list in a single focused pass with no helper indirection. Boolean flags are packed into bit fields to avoid object allocation and property-access overhead. Small utility functions are given overloaded type signatures that cover every legal call shape, and error codes are string constants declared alphabetically in one place so that `grep` and `switch` always find the canonical definition.

### `MagicString.js` — constructor: explicit field manifest via `Object.defineProperties`
[source](https://github.com/Rich-Harris/magic-string/blob/410fd4d080d8bf0b5be900c16c8ba11276fd8749/src/MagicString.js)
```js
export default class MagicString {
	constructor(string, options = {}) {
		const chunk = new Chunk(0, string.length, string);

		Object.defineProperties(this, {
			original: { writable: true, value: string },
			outro: { writable: true, value: '' },
			intro: { writable: true, value: '' },
			firstChunk: { writable: true, value: chunk },
			lastChunk: { writable: true, value: chunk },
			lastSearchedChunk: { writable: true, value: chunk },
			byStart: { writable: true, value: {} },
			byEnd: { writable: true, value: {} },
			filename: { writable: true, value: options.filename },
			indentExclusionRanges: { writable: true, value: options.indentExclusionRanges },
			sourcemapLocations: { writable: true, value: new BitSet() },
			storedNames: { writable: true, value: {} },
			indentStr: { writable: true, value: undefined },
			ignoreList: { writable: true, value: options.ignoreList },
			offset: { writable: true, value: options.offset || 0 },
		});

		if (DEBUG) {
			Object.defineProperty(this, 'stats', { value: new Stats() });
		}

		this.byStart[0] = chunk;
		this.byEnd[string.length] = chunk;
	}
```
Using `Object.defineProperties` instead of `this.x =` assignments makes every field's writability explicit and prevents accidental enumeration — every property of the object is a deliberate, named decision rather than an incidental assignment.

### `MagicString.js` — `update`: validate first, then mutate
[source](https://github.com/Rich-Harris/magic-string/blob/410fd4d080d8bf0b5be900c16c8ba11276fd8749/src/MagicString.js)
```js
	update(start, end, content, options) {
		start = start + this.offset;
		end = end + this.offset;

		if (typeof content !== 'string') throw new TypeError('replacement content must be a string');

		if (this.original.length !== 0) {
			while (start < 0) start += this.original.length;
			while (end < 0) end += this.original.length;
		}

		if (end > this.original.length) throw new Error('end is out of bounds');
		if (start === end)
			throw new Error(
				'Cannot overwrite a zero-length range – use appendLeft or prependRight instead',
			);

		if (DEBUG) this.stats.time('overwrite');

		this._split(start);
		this._split(end);
```
Every guard fires before any state is touched: type check, Python-style negative-index normalization, out-of-bounds check, zero-length check — each with an actionable message naming the correct alternative. Only after all guards pass does structural mutation begin.

### `blank.ts` — frozen sentinels replace re-allocation
[source](https://github.com/rollup/rollup/blob/5e0066d92defee0097f10fb814e63f60b2a7b612/src/utils/blank.ts)
```ts
export const BLANK: Record<string, unknown> = Object.freeze(Object.create(null));
export const EMPTY_OBJECT = Object.freeze({});
export const EMPTY_ARRAY = Object.freeze([]);
export const EMPTY_SET = Object.freeze(
	new (class extends Set {
		add(): never {
			throw new Error('Cannot add to empty set');
		}
	})()
);
```
Constants that could be `{}` or `[]` are instead named, frozen, and (for `BLANK`) proto-less so they are safe as `Record<string, unknown>` without `hasOwnProperty` guards. `EMPTY_SET` goes further: it subclasses `Set` to make mutation a thrown error, catching callers that accidentally write to a sentinel they should only read from.

### `BitFlags.ts` — boolean state packed into a `const enum` bit field
[source](https://github.com/rollup/rollup/blob/5e0066d92defee0097f10fb814e63f60b2a7b612/src/ast/nodes/shared/BitFlags.ts)
```ts
export const enum Flag {
	included = 1 << 0,
	deoptimized = 1 << 1,
	tdzAccessDefined = 1 << 2,
	tdzAccess = 1 << 3,
	assignmentDeoptimized = 1 << 4,
	bound = 1 << 5,
	isUndefined = 1 << 6,
	optional = 1 << 7,
	async = 1 << 8,
	deoptimizedReturn = 1 << 9,
	computed = 1 << 10,
	hasLostTrack = 1 << 11,
	hasUnknownDeoptimizedInteger = 1 << 12,
	hasUnknownDeoptimizedProperty = 1 << 13,
	directlyIncluded = 1 << 14,
	deoptimizeBody = 1 << 15,
	isBranchResolutionAnalysed = 1 << 16,
	await = 1 << 17,
	method = 1 << 18,
	shorthand = 1 << 19,
	tail = 1 << 20,
	prefix = 1 << 21,
	generator = 1 << 22,
	expression = 1 << 23,
	destructuringDeoptimized = 1 << 24,
	hasDeoptimizedCache = 1 << 25,
	hasEffects = 1 << 26,
	checkedForWarnings = 1 << 27,
	shouldIncludeDynamicAttributes = 1 << 28
}

export function isFlagSet(flags: number, flag: Flag): boolean {
	return (flags & flag) !== 0;
}

export function setFlag(flags: number, flag: Flag, value: boolean): number {
	return (flags & ~flag) | (-value & flag);
}
```
Twenty-nine boolean properties of AST nodes are packed into a single integer rather than stored as object fields, reducing allocation cost across millions of nodes. The two helper functions are the only gateway — every get and set in the codebase goes through `isFlagSet`/`setFlag`, keeping the bit arithmetic in one place.

### `Queue.ts` — minimal class with one private loop
[source](https://github.com/rollup/rollup/blob/5e0066d92defee0097f10fb814e63f60b2a7b612/src/utils/Queue.ts)
```ts
type Task<T> = () => Promise<T>;

interface QueueItem {
	reject: (reason?: unknown) => void;
	resolve: (value: any) => void;
	task: Task<unknown>;
}

export default class Queue {
	private readonly queue: QueueItem[] = [];
	private workerCount = 0;

	constructor(private maxParallel: number) {}

	run<T>(task: Task<T>): Promise<T> {
		return new Promise((resolve, reject) => {
			this.queue.push({ reject, resolve, task });
			this.work();
		});
	}

	private async work(): Promise<void> {
		if (this.workerCount >= this.maxParallel) return;
		this.workerCount++;

		let entry: QueueItem | undefined;
		while ((entry = this.queue.shift())) {
			const { reject, resolve, task } = entry;

			try {
				const result = await task();
				resolve(result);
			} catch (error) {
				reject(error);
			}
		}

		this.workerCount--;
	}
}
```
Forty lines, two fields, two methods — nothing more. `run` is the only public surface; `work` is entirely private. The `while` loop drains however many items are queued without recursion, and `workerCount` guards against spawning more concurrent workers than `maxParallel` without any external dependency or complex locking.

### `logs.ts` — error handling: structured log objects and a single throw gateway
[source](https://github.com/rollup/rollup/blob/5e0066d92defee0097f10fb814e63f60b2a7b612/src/utils/logs.ts)
```ts
export function error(base: Error | RollupLog): never {
	throw base instanceof Error ? base : getRollupError(base);
}

export function getRollupError(base: RollupLog): Error & RollupLog {
	augmentLogMessage(base);
	const errorInstance = Object.assign(new Error(base.message), base);
	Object.defineProperty(errorInstance, 'name', {
		value: 'RollupError',
		writable: true
	});
	return errorInstance;
}

export function augmentCodeLocation(
	properties: RollupLog,
	pos: number | { column: number; line: number },
	source: string,
	id: string
): void {
	if (typeof pos === 'object') {
		const { line, column } = pos;
		properties.loc = { column, file: id, line };
	} else {
		properties.pos = pos;
		const location = locate(source, pos, { offsetLine: 1 });
		if (!location) {
			return;
		}
		const { line, column } = location;
		properties.loc = { column, file: id, line };
	}

	if (properties.frame === undefined) {
		const { line, column } = properties.loc;
		properties.frame = getCodeFrame(source, line, column);
	}
}
```
`error()` is the single throw site in the entire codebase — everything else builds a `RollupLog` plain object and passes it here. `getRollupError` uses `Object.assign` + `Object.defineProperty` to attach the log's structured fields to a real `Error` instance (so stack traces work) while overriding `name` to `'RollupError'` without making it enumerable. Error construction and throwing are two separate, testable operations.

### `MagicString.test.js` — test shape: flat `describe`/`it`, assert-only, one concept per case
[source](https://github.com/Rich-Harris/magic-string/blob/410fd4d080d8bf0b5be900c16c8ba11276fd8749/test/MagicString.test.js)
```js
	describe('append', () => {
		it('should append content', () => {
			const s = new MagicString('abcdefghijkl');

			s.append('xyz');
			assert.equal(s.toString(), 'abcdefghijklxyz');

			s.append('xyz');
			assert.equal(s.toString(), 'abcdefghijklxyzxyz');
		});

		it('should return this', () => {
			const s = new MagicString('abcdefghijkl');
			assert.strictEqual(s.append('xyz'), s);
		});

		it('should throw when given non-string content', () => {
			const s = new MagicString('');
			assert.throws(() => s.append([]), TypeError);
		});
	});
```
Tests are organized by method name, not by scenario type. Each `it` proves exactly one thing — idempotent accumulation, fluent return, type rejection — with no setup helpers, no `beforeEach`, and no mocking. Node's built-in `assert` is used directly; the test file imports nothing more than the class under test and the assert module.