test: replay fetch() utility

packages/fincaps/test/replayFetch.js

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+// @ts-check
+
+/**
+ * @param {string} str
+ * ack: https://stackoverflow.com/a/7616484
+ */
+const hashCode = str => {
+  let hash = 0;
+  let i;
+  let chr;
+  if (str.length === 0) return hash;
+  for (i = 0; i < str.length; i += 1) {
+    chr = str.charCodeAt(i);
+    // eslint-disable-next-line no-bitwise
+    hash = (hash << 5) - hash + chr;
+    // eslint-disable-next-line no-bitwise
+    hash |= 0; // Convert to 32bit integer
+  }
+  return hash;
+};
+
+/**
+ * Normalize JSON RPC request ID
+ *
+ * tendermint-rpc generates ids using ambient access to Math.random()
+ * So we normalize them to a hash of the rest of the JSON.
+ *
+ * Earlier, we tried a sequence number, but it was non-deterministic
+ * with multiple interleaved requests.
+ *
+ * @param {string} argsKey
+ */
+const normalizeID = argsKey => {
+  // arbitrary string unlikely to occur in a request. from `pwgen 16 -1`
+  const placeholder = 'Ajaz1chei7ohnguv';
+
+  const noid = argsKey.replace(/\\"id\\":\d+/, `\\"id\\":${placeholder}`);
+  const id = Math.abs(hashCode(noid));
+  return noid.replace(placeholder, `${id}`);
+};
+
+/**
+ * Wrap `fetch` to capture JSON RPC IO traffic.
+ *
+ * @param {typeof window.fetch} fetch
+ * returns wraped fetch along with a .web map for use with {@link replayIO}
+ */
+export const captureIO = fetch => {
+  const web = new Map();
+  /** @param {Parameters<fetch>} args */
+  const wrapFetch = async (...args) => {
+    const key = normalizeID(JSON.stringify(args));
+    const resp = await fetch(...args);
+    return {
+      json: async () => {
+        const data = await resp.json();
+        web.set(key, data);
+        return data;
+      },
+    };
+  };
+  /** @type {typeof window.fetch} */
+  // @ts-expect-error mock
+  const f = wrapFetch;
+  return { fetch: f, web };
+};
+
+/**
+ * Replay captured JSON RPC IO.
+ *
+ * @param {Map<string, any>} web map from
+ *   JSON-stringified fetch args to fetched JSON data.
+ */
+export const replayIO = web => {
+  /** @type {typeof window.fetch} */
+  // @ts-expect-error mock
+  const f = async (...args) => {
+    const key = normalizeID(JSON.stringify(args));
+    const data = web.get(key);
+    if (!data) {
+      throw Error(`no data for ${key}`);
+    }
+    return {
+      json: async () => data,
+    };
+  };
+  return f;
+};