Stable Values

snapshotFn captures the result of a zero-argument callback once and returns a function that always replays that captured value.

import { snapshotFn } from "as-test";

const frozenNow = snapshotFn((): i64 => Date.now());

frozenNow(); // same value every call, for the rest of the run

Function	Signature	Effect
snapshotFn	snapshotFn(callback) => () => T	Evaluate callback once; return a function that replays the captured value.

When it helps

Use it to freeze a non-deterministic value so the rest of a test — and especially a snapshot assertion — stays stable:

const timestamp = snapshotFn((): i64 => Date.now());

test("serializes an event", () => {
  const event = new Event(timestamp(), "created");
  expect(serialize(event)).toMatchSnapshot();
});

Not the same as a snapshot assertion

Despite the name, snapshotFn has nothing to do with toMatchSnapshot:

• snapshotFn pins a value in memory for the current run so repeated calls agree.
• toMatchSnapshot compares a value against a baseline stored on disk across runs.

They compose well — freeze the volatile inputs with snapshotFn, then snapshot the output. When you need to replace a function's behavior rather than pin its result, use mockFn.