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mod.ts
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mod.ts
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export class FeatureFlags<
Environment extends string,
Schema extends FeatureFlagSchema,
> {
private subscriptions: Subscription<Schema>[] = [];
public readonly schema: Schema;
public readonly initialStore: TFeatureFlags<Schema>;
private _store: TFeatureFlags<Schema>;
constructor({
schema,
environment,
defaults: defaultsInput,
overrides,
subscription,
}: {
schema: Schema;
environment?: Environment;
defaults?: FeatureFlagDefaults<Environment | string, Schema>;
overrides?: Overrides<Schema>;
subscription?: Subscription<Schema>;
}) {
if (defaultsInput && !environment) {
throw new Error("Got defaults but no environment");
}
const defaults =
defaultsInput && environment && environment in defaultsInput
? defaultsInput[environment]
: undefined;
this.schema = schema;
this._store = new Proxy(
FeatureFlags.computeStore({ schema, defaults, overrides }),
{
set: (store, flag, value) => {
if (typeof flag !== "string") return false;
const valid = value === FeatureFlags.computeValue({
schema: this.schema,
flag,
current: flag in store
? store[flag as keyof typeof store]
: undefined,
manual: value,
});
if (!valid) return false;
const success = Reflect.set(store, flag, value);
if (success) this.subscriptions.map((f) => f(flag, value));
return success;
},
},
);
this.initialStore = { ...this._store };
if (subscription) this.subscriptions.push(subscription);
}
subscribe(subscription: Subscription<Schema>) {
this.subscriptions.push(subscription);
return true;
}
unsubscribe(subscription: Subscription<Schema>) {
const index = this.subscriptions.indexOf(subscription);
if (index) this.subscriptions.splice(index, 1);
return !!index;
}
get store() {
return this._store;
}
get<Flag extends FeatureFlag<Schema>>(
flag: Flag,
): FeatureFlagOption<Schema, Flag> {
return this._store[flag];
}
set<Flag extends FeatureFlag<Schema>>(
flag: Flag,
value: FeatureFlagOption<Schema, Flag>,
) {
this._store[flag] = value;
return true;
}
static computeStore<Schema extends FeatureFlagSchema>({
schema,
defaults,
overrides,
}: {
schema: Schema;
defaults?: Partial<TFeatureFlags<Schema>>;
overrides?: Overrides<Schema>;
}) {
const options = FeatureFlags.listOptionsFromSchema(schema);
return options.reduce((prev, flag) => ({
...prev,
[flag]: FeatureFlags.computeValue({
schema,
defaults,
overrides,
flag,
}),
}), {} as TFeatureFlags<Schema>);
}
static computeValue<
Schema extends FeatureFlagSchema,
Flag extends FeatureFlag<Schema>,
>({
schema,
flag,
overrides,
defaults,
current: vCurrent,
manual,
}: {
schema: Schema;
flag: Flag;
defaults?: Partial<TFeatureFlags<Schema>>;
overrides?: Overrides<Schema>;
current?: FeatureFlagValue;
manual?: FeatureFlagValue;
}): FeatureFlagValue {
const v = (v: unknown) =>
schema[flag].options.includes(v as FeatureFlagValue)
? v as FeatureFlagValue
: undefined;
const vSchema = schema[flag].options[0];
const vDefault = v(defaults?.[flag]);
const vOverride = v(
FeatureFlags.computeOverride({ schema, flag, overrides }),
);
const vManual = v(!schema[flag].readonly ? manual : undefined);
return vManual ?? vOverride ?? vDefault ?? vCurrent ?? vSchema;
}
static computeOverride<
Schema extends FeatureFlagSchema,
Flag extends FeatureFlag<Schema>,
>({
schema,
flag,
overrides,
}: {
schema: Schema;
flag: Flag;
overrides?: Overrides<Schema>;
}): FeatureFlagValue | undefined {
if (schema[flag].readonly) return undefined;
const value = overrides?.(flag);
if (!FeatureFlags.isValidValue(value)) return undefined;
return value;
}
static listOptionsFromSchema<Schema extends FeatureFlagSchema>(
schema: Schema,
) {
return Object.keys(schema) as FeatureFlag<Schema>[];
}
static isValidValue(v: unknown): v is FeatureFlagValue {
return ["string", "number", "boolean"].includes(typeof v);
}
static createOptions<
Environment extends string,
Schema extends FeatureFlagSchema,
>({
schema,
defaults,
}: {
schema: Schema;
defaults?: FeatureFlagDefaults<Environment | string, Schema>;
}) {
return { schema, defaults };
}
}
/////////////////////////////////////
///////////// TYPES /////////////
/////////////////////////////////////
export type FeatureFlagValue = boolean | number | string;
export type FeatureFlagSchema = Record<
string,
{
readonly?: boolean;
options: readonly [FeatureFlagValue, ...FeatureFlagValue[]];
}
>;
export type TFeatureFlags<Schema extends FeatureFlagSchema> = {
-readonly [T in FeatureFlag<Schema>]: FeatureFlagOption<Schema, T>;
};
export type FeatureFlagDefaults<
Environment extends string,
Schema extends FeatureFlagSchema,
> = Partial<
Record<
Environment,
Partial<TFeatureFlags<Schema>>
>
>;
export type FeatureFlag<T extends FeatureFlagSchema> = keyof T;
export type FeatureFlagOption<
Schema extends FeatureFlagSchema,
Flag extends FeatureFlag<Schema>,
> = Schema[Flag]["options"][number];
export type Overrides<Schema extends FeatureFlagSchema> = <
T extends FeatureFlag<Schema>,
>(
flag: T,
) => FeatureFlagValue | void;
export type Subscription<Schema extends FeatureFlagSchema> = <
T extends FeatureFlag<Schema>,
>(flag: T, value: FeatureFlagOption<Schema, T>) => unknown | Promise<unknown>;
// deno-lint-ignore no-explicit-any
export type AnyFeatureFlags = FeatureFlags<any, any>;