@studnicky/v8/dynamic-property-access

Reports a computed member expression with a variable key when TypeScript type services show that its receiver is not an indexed collection. The rule covers writes only — assignment targets, updates (o[k]++), delete o[k], and destructuring targets. A read is silent, because a read cannot move an object into dictionary mode; only a variable-key assignment can. It is aimed at variable string keys on ordinary object shapes, where enough distinct writes force the object from fast properties into dictionary mode and later named-property lookups become hash lookups.

Literal string and numeric keys are exempt: object['name'] compiles to the same GetNamedProperty bytecode and measured identically to object.name (17.7 ms each at 50,000,000 iterations). Symbol.* keys are also exempt because they are compile-time constants with no dot spelling. Arrays, tuples, typed arrays, DataView, and strings are exempt because their indexed elements live in a separate elements store; array[index] is the fast path that the related for-of-arrays rule expects. Without type services, variable keys are not reported rather than guessed.

Fixable: No · Options: No · Suggested severity: error

Why reads are not reported

The measured hazard is a variable-key assignment driving a plain object out of fast properties: %HasFastProperties(o) returns false after enough distinct string-key writes. Reading o[k] in an expression position causes no such transition, so reporting reads flagged sites that could not exhibit the hazard.

That over-reach had a cost. A read in StructuralHash was "fixed" by rewriting value[key] as Reflect.get(value, key) — which does not remove the dynamic key, only spells it in a form the rule did not match. Measured over a realistic object walk at 200,000 iterations the two are indistinguishable (75.3ms vs 73.0ms, 0.97x), and Object.entries — the other obvious rewrite — is 5.80x slower (437.0ms) because it allocates a pair array per walk. The remedy this rule names is a Map, and a read of a JSON object arriving from JSON.parse cannot become one.

✗ Incorrect

const values: { [key: string]: string } = {};
const key = 'name';
values[key] = 'Ada';

const settings = { 'theme': 'dark' };
const key = 'theme';
const value = settings[key];

✓ Correct

const settings = { 'theme': 'dark' };
const value = settings.theme;

const values = new Map<string, string>();
values.set('name', 'Ada');
const value = values.get('name');

const numbers = [3, 5, 8];
const index = 1;
const value = numbers[index];