diff --git a/is.ts b/is.ts
index 574afcd..d32b173 100644
--- a/is.ts
+++ b/is.ts
@@ -1,3 +1,4 @@
+import type { ErrorLike } from "./deps/scrapbox.ts";
 // These code are based on https://deno.land/x/unknownutil@v1.1.0/is.ts
 
 export const isNone = (value: unknown): value is undefined | null =>
@@ -8,3 +9,28 @@ export const isNumber = (value: unknown): value is number =>
   typeof value === "number";
 export const isArray = <T>(value: unknown): value is T[] =>
   Array.isArray(value);
+export const isObject = (value: unknown): value is Record<string, unknown> =>
+  typeof value === "object" && value !== null;
+
+export const isErrorLike = (e: unknown): e is ErrorLike => {
+  if (!isObject(e)) return false;
+  return (e.name === undefined || typeof e.name === "string") &&
+    typeof e.message === "string";
+};
+
+/** 与えられたobjectもしくはJSONテキストをErrorLikeに変換できるかどうか試す
+ *
+ * @param e 試したいobjectもしくはテキスト
+ * @return 変換できなかったら`false`を返す。変換できたらそのobjectを返す
+ */
+export const tryToErrorLike = (e: unknown): false | ErrorLike => {
+  try {
+    const json = typeof e === "string" ? JSON.parse(e) : e;
+    if (!isErrorLike(json)) return false;
+    return json;
+  } catch (e2: unknown) {
+    if (e2 instanceof SyntaxError) return false;
+    // JSONのparse error以外はそのまま投げる
+    throw e2;
+  }
+};
diff --git a/rest/auth.ts b/rest/auth.ts
new file mode 100644
index 0000000..3fa37d1
--- /dev/null
+++ b/rest/auth.ts
@@ -0,0 +1,28 @@
+import { getProfile } from "./profile.ts";
+import { BaseOptions } from "./util.ts";
+
+// scrapbox.io内なら`window._csrf`にCSRF tokenが入っている
+declare global {
+  interface Window {
+    _csrf?: string;
+  }
+}
+
+/** HTTP headerのCookieに入れる文字列を作る
+ *
+ * @param sid connect.sidに入っている文字列
+ */
+export const cookie = (sid: string): string => `connect.sid=${sid}`;
+
+/** CSRF tokenを取得する
+ *
+ * @param init 認証情報など
+ */
+export const getCSRFToken = async (
+  init?: BaseOptions,
+): Promise<string> => {
+  if (window._csrf) return window._csrf;
+
+  const user = await getProfile(init);
+  return user.csrfToken;
+};
diff --git a/rest/error.ts b/rest/error.ts
new file mode 100644
index 0000000..3b90966
--- /dev/null
+++ b/rest/error.ts
@@ -0,0 +1,26 @@
+export class UnexpectedResponseError extends Error {
+  name = "UnexpectedResponseError";
+  status: number;
+  statusText: string;
+  body: string;
+  path: URL;
+
+  constructor(
+    init: { status: number; statusText: string; body: string; path: URL },
+  ) {
+    super(
+      `${init.status} ${init.statusText} when fetching ${init.path.toString()}`,
+    );
+
+    this.status = init.status;
+    this.statusText = init.statusText;
+    this.body = init.body;
+    this.path = init.path;
+
+    // @ts-ignore only available on V8
+    if (Error.captureStackTrace) {
+      // @ts-ignore only available on V8
+      Error.captureStackTrace(this, UnexpectedResponseError);
+    }
+  }
+}
diff --git a/rest/mod.ts b/rest/mod.ts
index 708592a..9a45976 100644
--- a/rest/mod.ts
+++ b/rest/mod.ts
@@ -3,3 +3,6 @@ export * from "./project.ts";
 export * from "./profile.ts";
 export * from "./replaceLinks.ts";
 export * from "./page-data.ts";
+export * from "./auth.ts";
+export * from "./util.ts";
+export * from "./error.ts";
diff --git a/rest/page-data.ts b/rest/page-data.ts
index e0bee80..57a9f27 100644
--- a/rest/page-data.ts
+++ b/rest/page-data.ts
@@ -6,39 +6,27 @@ import type {
   NotLoggedInError,
   NotPrivilegeError,
 } from "../deps/scrapbox.ts";
-import {
-  cookie,
-  getCSRFToken,
-  makeCustomError,
-  tryToErrorLike,
-} from "./utils.ts";
-import type { Result } from "./utils.ts";
-
-/** `importPages`の認証情報 */
-export interface ImportInit {
-  /** connect.sid */ sid: string;
-  /** CSRF token
-   *
-   * If it isn't set, automatically get CSRF token from scrapbox.io server.
-   */
-  csrf?: string;
-}
+import { cookie, getCSRFToken } from "./auth.ts";
+import { UnexpectedResponseError } from "./error.ts";
+import { tryToErrorLike } from "../is.ts";
+import { BaseOptions, ExtendedOptions, Result, setDefaults } from "./util.ts";
 /** projectにページをインポートする
  *
  * @param project - インポート先のprojectの名前
  * @param data - インポートするページデータ
  */
-export async function importPages(
+export const importPages = async (
   project: string,
   data: ImportedData<boolean>,
-  { sid, csrf }: ImportInit,
+  init: ExtendedOptions,
 ): Promise<
   Result<string, ErrorLike>
-> {
+> => {
   if (data.pages.length === 0) {
     return { ok: true, value: "No pages to import." };
   }
 
+  const { sid, hostName, fetch, csrf } = setDefaults(init ?? {});
   const formData = new FormData();
   formData.append(
     "import-file",
@@ -47,90 +35,80 @@ export async function importPages(
     }),
   );
   formData.append("name", "undefined");
+  const path = `https://${hostName}/api/page-data/import/${project}.json`;
 
-  csrf ??= await getCSRFToken(sid);
-
-  const path = `https://scrapbox.io/api/page-data/import/${project}.json`;
   const res = await fetch(
     path,
     {
       method: "POST",
       headers: {
-        Cookie: cookie(sid),
+        ...(sid ? { Cookie: cookie(sid) } : {}),
         Accept: "application/json, text/plain, */*",
-        "X-CSRF-TOKEN": csrf,
+        "X-CSRF-TOKEN": csrf ?? await getCSRFToken(init),
       },
       body: formData,
     },
   );
 
   if (!res.ok) {
-    if (res.status === 503) {
-      throw makeCustomError("ServerError", "503 Service Unavailable");
-    }
-    const value = tryToErrorLike(await res.text());
+    const text = await res.json();
+    const value = tryToErrorLike(text);
     if (!value) {
-      throw makeCustomError(
-        "UnexpectedError",
-        `Unexpected error has occuerd when fetching "${path}"`,
-      );
+      throw new UnexpectedResponseError({
+        path: new URL(path),
+        ...res,
+        body: await res.text(),
+      });
     }
     return { ok: false, value };
   }
+
   const { message } = (await res.json()) as { message: string };
   return { ok: true, value: message };
-}
+};
 
 /** `exportPages`の認証情報 */
-export interface ExportInit<withMetadata extends true | false> {
-  /** connect.sid */ sid: string;
+export interface ExportInit<withMetadata extends true | false>
+  extends BaseOptions {
   /** whether to includes metadata */ metadata: withMetadata;
 }
 /** projectの全ページをエクスポートする
  *
  * @param project exportしたいproject
  */
-export async function exportPages<withMetadata extends true | false>(
+export const exportPages = async <withMetadata extends true | false>(
   project: string,
-  { sid, metadata }: ExportInit<withMetadata>,
+  init: ExportInit<withMetadata>,
 ): Promise<
   Result<
     ExportedData<withMetadata>,
     NotFoundError | NotPrivilegeError | NotLoggedInError
   >
-> {
+> => {
+  const { sid, hostName, fetch, metadata } = setDefaults(init ?? {});
   const path =
-    `https://scrapbox.io/api/page-data/export/${project}.json?metadata=${metadata}`;
+    `https://${hostName}/api/page-data/export/${project}.json?metadata=${metadata}`;
   const res = await fetch(
     path,
-    {
-      headers: {
-        Cookie: cookie(sid),
-      },
-    },
+    sid ? { headers: { Cookie: cookie(sid) } } : undefined,
   );
 
   if (!res.ok) {
-    const error = (await res.json());
-    return { ok: false, ...error };
-  }
-  if (!res.ok) {
-    const value = tryToErrorLike(await res.text()) as
-      | false
-      | NotFoundError
-      | NotPrivilegeError
-      | NotLoggedInError;
+    const text = await res.json();
+    const value = tryToErrorLike(text);
     if (!value) {
-      throw makeCustomError(
-        "UnexpectedError",
-        `Unexpected error has occuerd when fetching "${path}"`,
-      );
+      throw new UnexpectedResponseError({
+        path: new URL(path),
+        ...res,
+        body: await res.text(),
+      });
     }
     return {
       ok: false,
-      value,
+      value: value as NotFoundError | NotPrivilegeError | NotLoggedInError,
     };
   }
+
   const value = (await res.json()) as ExportedData<withMetadata>;
   return { ok: true, value };
-}
+};
diff --git a/rest/pages.ts b/rest/pages.ts
index 84a0c55..361a17f 100644
--- a/rest/pages.ts
+++ b/rest/pages.ts
@@ -5,14 +5,15 @@ import type {
   Page,
   PageList,
 } from "../deps/scrapbox.ts";
-import { cookie, makeCustomError, tryToErrorLike } from "./utils.ts";
+import { cookie } from "./auth.ts";
+import { UnexpectedResponseError } from "./error.ts";
+import { tryToErrorLike } from "../is.ts";
 import { encodeTitleURI } from "../title.ts";
-import type { Result } from "./utils.ts";
+import { BaseOptions, Result, setDefaults } from "./util.ts";
 
 /** Options for `getPage()` */
-export interface GetPageOption {
+export interface GetPageOption extends BaseOptions {
   /** use `followRename` */ followRename?: boolean;
-  /** connect.sid */ sid?: string;
 }
 /** 指定したページのJSONデータを取得する
  *
@@ -20,7 +21,7 @@ export interface GetPageOption {
  * @param title 取得したいページのtitle 大文字小文字は問わない
  * @param options オプション
  */
-export async function getPage(
+export const getPage = async (
   project: string,
   title: string,
   options?: GetPageOption,
@@ -29,45 +30,39 @@ export async function getPage(
     Page,
     NotFoundError | NotLoggedInError | NotMemberError
   >
-> {
-  const path = `https://scrapbox.io/api/pages/${project}/${
+> => {
+  const { sid, hostName, fetch, followRename } = setDefaults(options ?? {});
+  const path = `https://${hostName}/api/pages/${project}/${
     encodeTitleURI(title)
-  }?followRename=${options?.followRename ?? true}`;
-
+  }?followRename=${followRename ?? true}`;
   const res = await fetch(
     path,
-    options?.sid
-      ? {
-        headers: {
-          Cookie: cookie(options.sid),
-        },
-      }
-      : undefined,
+    sid ? { headers: { Cookie: cookie(sid) } } : undefined,
   );
-
   if (!res.ok) {
-    const value = tryToErrorLike(await res.text()) as
-      | false
-      | NotFoundError
-      | NotLoggedInError
-      | NotMemberError;
+    const text = await res.text();
+    const value = tryToErrorLike(text);
     if (!value) {
-      throw makeCustomError(
-        "UnexpectedError",
-        `Unexpected error has occuerd when fetching "${path}"`,
-      );
+      throw new UnexpectedResponseError({
+        path: new URL(path),
+        ...res,
+        body: text,
+      });
     }
     return {
       ok: false,
-      value,
+      value: value as
+        | NotFoundError
+        | NotLoggedInError
+        | NotMemberError,
     };
   }
   const value = (await res.json()) as Page;
   return { ok: true, value };
-}
+};
 
 /** Options for `listPages()` */
-export interface ListPagesOption {
+export interface ListPagesOption extends BaseOptions {
   /** the sort of page list to return
    *
    * @default "updated"
@@ -91,15 +86,13 @@ export interface ListPagesOption {
    * @default 100
    */
   limit?: number;
-  /** connect.sid */
-  sid?: string;
 }
 /** 指定したprojectのページを一覧する
  *
  * @param project 一覧したいproject
  * @param options オプション 取得範囲や並び順を決める
  */
-export async function listPages(
+export const listPages = async (
   project: string,
   options?: ListPagesOption,
 ): Promise<
@@ -107,42 +100,38 @@ export async function listPages(
     PageList,
     NotFoundError | NotLoggedInError | NotMemberError
   >
-> {
-  const { sort, limit, skip } = options ?? {};
+> => {
+  const { sid, hostName, fetch, sort, limit, skip } = setDefaults(
+    options ?? {},
+  );
   const params = new URLSearchParams();
   if (sort !== undefined) params.append("sort", sort);
   if (limit !== undefined) params.append("limit", `${limit}`);
   if (skip !== undefined) params.append("skip", `${skip}`);
-  const path = `https://scrapbox.io/api/pages/${project}?${params.toString()}`;
+  const path = `https://${hostName}/api/pages/${project}?${params.toString()}`;
 
   const res = await fetch(
     path,
-    options?.sid
-      ? {
-        headers: {
-          Cookie: cookie(options.sid),
-        },
-      }
-      : undefined,
+    sid ? { headers: { Cookie: cookie(sid) } } : undefined,
   );
-
   if (!res.ok) {
-    const value = tryToErrorLike(await res.text()) as
-      | false
-      | NotFoundError
-      | NotLoggedInError
-      | NotMemberError;
+    const text = await res.text();
+    const value = tryToErrorLike(text);
     if (!value) {
-      throw makeCustomError(
-        "UnexpectedError",
-        `Unexpected error has occuerd when fetching "${path}"`,
-      );
+      throw new UnexpectedResponseError({
+        path: new URL(path),
+        ...res,
+        body: text,
+      });
     }
     return {
       ok: false,
-      value,
+      value: value as
+        | NotFoundError
+        | NotLoggedInError
+        | NotMemberError,
     };
   }
   const value = (await res.json()) as PageList;
   return { ok: true, value };
-}
+};
diff --git a/rest/profile.ts b/rest/profile.ts
index 60b95fe..8bddc5d 100644
--- a/rest/profile.ts
+++ b/rest/profile.ts
@@ -1,33 +1,27 @@
 import type { GuestUser, MemberUser } from "../deps/scrapbox.ts";
-import { cookie, makeCustomError } from "./utils.ts";
+import { cookie } from "./auth.ts";
+import { BaseOptions, setDefaults } from "./util.ts";
+import { UnexpectedResponseError } from "./error.ts";
 
-export interface ProfileInit {
-  /** connect.sid */ sid: string;
-}
 /** get user profile
  *
  * @param init connect.sid etc.
  */
-export async function getProfile(
-  init?: ProfileInit,
-): Promise<MemberUser | GuestUser> {
-  const path = "https://scrapbox.io/api/users/me";
+export const getProfile = async (
+  init?: BaseOptions,
+): Promise<MemberUser | GuestUser> => {
+  const { sid, hostName, fetch } = setDefaults(init ?? {});
+  const path = `https://${hostName}/api/users/me`;
   const res = await fetch(
     path,
-    init?.sid
-      ? {
-        headers: {
-          Cookie: cookie(init.sid),
-        },
-      }
-      : undefined,
+    sid ? { headers: { Cookie: cookie(sid) } } : undefined,
   );
-
   if (!res.ok) {
-    throw makeCustomError(
-      "UnexpectedError",
-      `Unexpected error has occuerd when fetching "${path}"`,
-    );
+    throw new UnexpectedResponseError({
+      path: new URL(path),
+      ...res,
+      body: await res.text(),
+    });
   }
   return (await res.json()) as MemberUser | GuestUser;
-}
+};
diff --git a/rest/project.ts b/rest/project.ts
index 8886e0a..18e832e 100644
--- a/rest/project.ts
+++ b/rest/project.ts
@@ -1,9 +1,3 @@
-export async function getProjectId(project: string) {
-  const res = await fetch(`https://scrapbox.io/api/projects/${project}`);
-  const json = (await res.json()) as MemberProject;
-  return json.id;
-}
-
 import type {
   MemberProject,
   NotFoundError,
@@ -11,51 +5,48 @@ import type {
   NotMemberError,
   NotMemberProject,
 } from "../deps/scrapbox.ts";
-import { cookie, makeCustomError, Result, tryToErrorLike } from "./utils.ts";
+import { cookie } from "./auth.ts";
+import { UnexpectedResponseError } from "./error.ts";
+import { tryToErrorLike } from "../is.ts";
+import { BaseOptions, Result, setDefaults } from "./util.ts";
 
-export interface ProjectInit {
-  /** connect.sid */ sid: string;
-}
 /** get the project information
  *
  * @param project project name to get
  * @param init connect.sid etc.
  */
-export async function getProject(
+export const getProject = async (
   project: string,
-  init?: ProjectInit,
+  init?: BaseOptions,
 ): Promise<
   Result<
     MemberProject | NotMemberProject,
     NotFoundError | NotMemberError | NotLoggedInError
   >
-> {
-  const path = `https://scrapbox.io/api/projects/${project}`;
+> => {
+  const { sid, hostName, fetch } = setDefaults(init ?? {});
+  const path = `https://${hostName}/api/projects/${project}`;
   const res = await fetch(
     path,
-    init?.sid
-      ? {
-        headers: {
-          Cookie: cookie(init.sid),
-        },
-      }
-      : undefined,
+    sid ? { headers: { Cookie: cookie(sid) } } : undefined,
   );
 
   if (!res.ok) {
-    const value = tryToErrorLike(await res.json()) as
-      | false
-      | NotFoundError
-      | NotMemberError
-      | NotLoggedInError;
+    const text = await res.json();
+    const value = tryToErrorLike(text);
     if (!value) {
-      throw makeCustomError(
-        "UnexpectedError",
-        `Unexpected error has occuerd when fetching "${path}"`,
-      );
+      throw new UnexpectedResponseError({
+        path: new URL(path),
+        ...res,
+        body: await res.text(),
+      });
     }
-    return { ok: false, value };
+    return {
+      ok: false,
+      value: value as NotFoundError | NotMemberError | NotLoggedInError,
+    };
   }
+
   const value = (await res.json()) as MemberProject | NotMemberProject;
   return { ok: true, value };
-}
+};
diff --git a/rest/replaceLinks.ts b/rest/replaceLinks.ts
index e8ead4a..83634eb 100644
--- a/rest/replaceLinks.ts
+++ b/rest/replaceLinks.ts
@@ -3,23 +3,10 @@ import type {
   NotLoggedInError,
   NotMemberError,
 } from "../deps/scrapbox.ts";
-import {
-  cookie,
-  getCSRFToken,
-  makeCustomError,
-  tryToErrorLike,
-} from "./utils.ts";
-import type { Result } from "./utils.ts";
-
-/** `replaceLinks`の認証情報 */
-export interface ReplaceLinksInit {
-  /** connect.sid */ sid: string;
-  /** CSRF token
-   *
-   * If it isn't set, automatically get CSRF token from scrapbox.io server.
-   */
-  csrf?: string;
-}
+import { cookie, getCSRFToken } from "./auth.ts";
+import { UnexpectedResponseError } from "./error.ts";
+import { tryToErrorLike } from "../is.ts";
+import { ExtendedOptions, Result, setDefaults } from "./util.ts";
 
 /** 指定したproject内の全てのリンクを書き換える
  *
@@ -29,23 +16,22 @@ export interface ReplaceLinksInit {
  * @param project これで指定したproject内の全てのリンクが置換対象となる
  * @param from 置換前のリンク
  * @param to 置換後のリンク
- * @param options connect.sidなど
+ * @param init connect.sidなど
  * @return 置換されたリンクがあったページの数
  */
-export async function replaceLinks(
+export const replaceLinks = async (
   project: string,
   from: string,
   to: string,
-  init?: ReplaceLinksInit,
+  init?: ExtendedOptions,
 ): Promise<
   Result<
     number,
     NotFoundError | NotLoggedInError | NotMemberError
   >
-> {
-  const path = `https://scrapbox.io/api/pages/${project}/replace/links`;
-  const sid = init?.sid;
-  const csrf = init?.csrf ?? await getCSRFToken(sid);
+> => {
+  const { sid, hostName, fetch, csrf } = setDefaults(init ?? {});
+  const path = `https://${hostName}/api/pages/${project}/replace/links`;
 
   const res = await fetch(
     path,
@@ -53,36 +39,30 @@ export async function replaceLinks(
       method: "POST",
       headers: {
         "Content-Type": "application/json;charset=utf-8",
-        "X-CSRF-TOKEN": csrf,
-        ...(sid
-          ? {
-            Cookie: cookie(sid),
-          }
-          : {}),
+        "X-CSRF-TOKEN": csrf ?? await getCSRFToken(init),
+        ...(sid ? { Cookie: cookie(sid) } : {}),
       },
       body: JSON.stringify({ from, to }),
     },
   );
 
   if (!res.ok) {
-    const value = tryToErrorLike(await res.text()) as
-      | false
-      | NotFoundError
-      | NotLoggedInError
-      | NotMemberError;
+    const text = await res.json();
+    const value = tryToErrorLike(text);
     if (!value) {
-      throw makeCustomError(
-        "UnexpectedError",
-        `Unexpected error has occuerd when fetching "${path}"`,
-      );
+      throw new UnexpectedResponseError({
+        path: new URL(path),
+        ...res,
+        body: await res.text(),
+      });
     }
     return {
       ok: false,
-      value,
+      value: value as NotFoundError | NotMemberError | NotLoggedInError,
     };
   }
 
   // messageには"2 pages have been successfully updated!"というような文字列が入っているはず
   const { message } = (await res.json()) as { message: string };
   return { ok: true, value: parseInt(message.match(/\d+/)?.[0] ?? "0") };
-}
+};
diff --git a/rest/util.ts b/rest/util.ts
new file mode 100644
index 0000000..2cd2a2a
--- /dev/null
+++ b/rest/util.ts
@@ -0,0 +1,51 @@
+/** 正常値と異常値を格納する型 */
+export type Result<T, E> = { ok: true; value: T } | { ok: false; value: E };
+
+/** networkからdataをとってくる処理
+ *
+ * interfaceは`fetch()`と同じ
+ */
+export type Fetch = (
+  input: string | Request,
+  init?: RequestInit,
+) => Promise<Response>;
+
+/** 全てのREST APIに共通するopitons */
+export interface BaseOptions {
+  /** connect.sid
+   *
+   * private projectのデータやscrapbox accountに紐付いたデータを取得する際に必要な認証情報
+   */
+  sid?: string;
+
+  /** データの取得に使う処理
+   *
+   * @default fetch
+   */
+  fetch?: Fetch;
+
+  /** REST APIのdomain
+   *
+   * オンプレ版scrapboxなどだと、scrapbox.io以外のhost nameになるので、予め変えられるようにしておく
+   *
+   * @default "scrapbox.io"
+   */
+  hostName?: string;
+}
+/** BaseeOptionsにCSRF情報を入れたもの */
+export interface ExtendedOptions extends BaseOptions {
+  /** CSRF token
+   *
+   * If it isn't set, automatically get CSRF token from scrapbox.io server.
+   */
+  csrf?: string;
+}
+
+/** BaseOptionsの既定値を埋める */
+export const setDefaults = <T extends BaseOptions = BaseOptions>(
+  options: T,
+): Omit<T, "fetch" | "hostName"> & Required<Omit<BaseOptions, "sid">> => {
+  const { fetch = globalThis.fetch, hostName = "scrapbox.io", ...rest } =
+    options;
+  return { fetch, hostName, ...rest };
+};
diff --git a/rest/utils.ts b/rest/utils.ts
deleted file mode 100644
index c0c5ce6..0000000
--- a/rest/utils.ts
+++ /dev/null
@@ -1,61 +0,0 @@
-import type { ErrorLike } from "../deps/scrapbox.ts";
-import { getProfile } from "./profile.ts";
-
-// scrapbox.io内なら`window._csrf`にCSRF tokenが入っている
-declare global {
-  interface Window {
-    _csrf?: string;
-  }
-}
-
-/** HTTP headerのCookieに入れる文字列を作る
- *
- * @param sid connect.sidに入っている文字列
- */
-export const cookie = (sid: string) => `connect.sid=${sid}`;
-
-export type Result<T, E> = { ok: true; value: T } | { ok: false; value: E };
-/** CSRF tokenを取得する
- *
- * @param sid - connect.sidに入っている文字列。不正な文字列を入れてもCSRF tokenを取得できるみたい
- */
-export async function getCSRFToken(
-  sid?: string,
-): Promise<string> {
-  if (window._csrf) return window._csrf;
-
-  const user = await getProfile(sid ? { sid } : undefined);
-  return user.csrfToken;
-}
-
-// cf. https://blog.uhy.ooo/entry/2021-04-09/typescript-is-any-as/#%E3%83%A6%E3%83%BC%E3%82%B6%E3%83%BC%E5%AE%9A%E7%BE%A9%E5%9E%8B%E3%82%AC%E3%83%BC%E3%83%89%E3%81%AE%E5%BC%95%E6%95%B0%E3%81%AE%E5%9E%8B%E3%82%92%E3%81%A9%E3%81%86%E3%81%99%E3%82%8B%E3%81%8B
-function isNotNullish(data: unknown): data is Record<string, unknown> {
-  return data != null;
-}
-function isErrorLike(e: unknown): e is ErrorLike {
-  if (!isNotNullish(e)) return false;
-  return (e.name === undefined || typeof e.name === "string") &&
-    typeof e.message === "string";
-}
-/** 与えられたobjectもしくはJSONテキストをErrorLikeに変換できるかどうか試す
- *
- * @param e 試したいobjectもしくはテキスト
- */
-export function tryToErrorLike(e: unknown): false | ErrorLike {
-  try {
-    const json = typeof e === "string" ? JSON.parse(e) : e;
-    if (!isErrorLike(json)) return false;
-    return json;
-  } catch (e2: unknown) {
-    if (e2 instanceof SyntaxError) return false;
-    throw e2;
-  }
-}
-
-/** classを使わずにカスタムエラーを作る */
-export function makeCustomError(name: string, message: string) {
-  const error = new Error();
-  error.name = name;
-  error.message = message;
-  return error;
-}