Function createCapability

packages/ai/src/activities/chat/middleware/capabilities.ts:114–162

()

 * coverage tracking keys on the name literal (runtime keys on reference).
 */
export function createCapability<TValue = unknown>(): <
  const TName extends string,
>(
  name: TName,
) => Capability<TValue, TName> {
  return <const TName extends string>(
    name: TName,
  ): Capability<TValue, TName> => {
    // Each capability owns a typed WeakMap keyed by the context object. Because
    // the value type is TValue, reads are typed with no assertion.
    const values = new WeakMap<CapabilityContext, TValue>()

    function get(ctx: CapabilityContext): TValue
    function get(
      ctx: CapabilityContext,
      opts: { optional: true },
    ): TValue | undefined
    function get(
      ctx: CapabilityContext,
      opts?: CapabilityGetOptions,
    ): TValue | undefined {
      if (!values.has(ctx)) {
        if (opts?.optional) return undefined
        throw new Error(
          `Capability "${name}" was requested but never provided. Ensure a ` +
            `middleware provides it in setup(), ordered before this consumer.`,
        )
      }
      return values.get(ctx)
    }

    const provide: CapabilityProvider<TValue> = (ctx, value) => {
      values.set(ctx, value)
      ctx.capabilities.markProvided(handle)
    }

    const pair: readonly [
      CapabilityGetter<TValue>,
      CapabilityProvider<TValue>,
    ] = [get, provide]
    // Object.assign's return type is the intersection of the tuple and the
    // props, which IS Capability<TValue, TName> — no cast needed.
    const handle = Object.assign(pair, {
      capabilityName: name,
      has: (ctx: CapabilityContext) => values.has(ctx),
    })
    return handle
  }
}