Function yield_

src/async/makeAsyncFunc.ts:55–69

        // Create a yield() function tailored for this iterator.
        var yield_ = expr => {

            // Ensure this function is executing inside a fiber.
            if (!Fiber.current) {
                throw new Error(
                    'await functions, yield functions, and value-returning suspendable ' +
                    'functions may only be called from inside a suspendable function. '
                );
            }

            // Notify waiters of the next result, then suspend the iterator.
            if (runContext.callback) runContext.callback(null, { value: expr, done: false });
            if (runContext.resolver) runContext.resolver.resolve({ value: expr, done: false });
            Fiber.yield();
        }

        // Insert the yield function as the first argument when starting the iterator.
        startupArgs[0] = yield_;