Add error utils (#146)

This commit satisfies a few needs that we have in various projects:

- When catching a throwable from some kind of operation, we want to
  be able to test whether the throwable is an error.
- Furthermore, since the Error interface in TypeScript is pretty simple,
  we want to be able to test for different properties on an error
  (`code`, `stack`, etc.).
- We want to wrap an error produced by a lower level part of the system
  with a different message, but preserve the original error using the
  `cause` property (note: this property was added in Node 18, so for
  older Nodes, we use the `pony-cause` library to set this).
- We want to be able to take a throwable and produce an error that has a
  stacktrace. This is particularly useful for working with the
  `fs.promises` module, which (as of Node 22) [does not produce proper
  stacktraces][1].
- We want to be able to get a message from a throwable.

[1]: nodejs/node#30944

---------

Co-authored-by: Jongsun Suh <[email protected]>
Co-authored-by: legobeat <[email protected]>

package.json

@@ -52,6 +52,7 @@
     "@scure/base": "^1.1.3",
     "@types/debug": "^4.1.7",
     "debug": "^4.3.4",
+    "pony-cause": "^2.1.10",
     "semver": "^7.5.4",
     "superstruct": "^1.0.3"
   },

src/assert.ts

@@ -1,21 +1,12 @@
 import type { Struct } from 'superstruct';
 import { assert as assertSuperstruct } from 'superstruct';
 
+import { getErrorMessage } from './errors';
+
 export type AssertionErrorConstructor =
   | (new (args: { message: string }) => Error)
   | ((args: { message: string }) => Error);
 
-/**
- * Type guard for determining whether the given value is an error object with a
- * `message` property, such as an instance of Error.
- *
- * @param error - The object to check.
- * @returns True or false, depending on the result.
- */
-function isErrorWithMessage(error: unknown): error is { message: string } {
-  return typeof error === 'object' && error !== null && 'message' in error;
-}
-
 /**
  * Check if a value is a constructor, i.e., a function that can be called with
  * the `new` keyword.
@@ -31,22 +22,18 @@ function isConstructable(
 }
 
 /**
- * Get the error message from an unknown error object. If the error object has
- * a `message` property, that property is returned. Otherwise, the stringified
- * error object is returned.
+ * Attempts to obtain the message from a possible error object. If it is
+ * possible to do so, any trailing period will be removed from the message;
+ * otherwise an empty string is returned.
  *
  * @param error - The error object to get the message from.
- * @returns The error message.
+ * @returns The message without any trailing period if `error` is an object
+ * with a `message` property; the string version of `error` without any trailing
+ * period if it is not `undefined` or `null`; otherwise an empty string.
  */
-function getErrorMessage(error: unknown): string {
-  const message = isErrorWithMessage(error) ? error.message : String(error);
-
-  // If the error ends with a period, remove it, as we'll add our own period.
-  if (message.endsWith('.')) {
-    return message.slice(0, -1);
-  }
-
-  return message;
+function getErrorMessageWithoutTrailingPeriod(error: unknown): string {
+  // We'll add our own period.
+  return getErrorMessage(error).replace(/\.$/u, '');
 }
 
 /**
@@ -127,7 +114,10 @@ export function assertStruct<Type, Schema>(
   try {
     assertSuperstruct(value, struct);
   } catch (error) {
-    throw getError(ErrorWrapper, `${errorPrefix}: ${getErrorMessage(error)}.`);
+    throw getError(
+      ErrorWrapper,
+      `${errorPrefix}: ${getErrorMessageWithoutTrailingPeriod(error)}.`,
+    );
   }
 }
 

src/errors.test.ts

@@ -0,0 +1,284 @@
+import fs from 'fs';
+
+import {
+  getErrorMessage,
+  isErrorWithCode,
+  isErrorWithMessage,
+  isErrorWithStack,
+  wrapError,
+} from './errors';
+
+describe('isErrorWithCode', () => {
+  it('returns true if given an object that includes a "code" property', () => {
+    expect(
+      isErrorWithCode({ code: 'some code', message: 'some message' }),
+    ).toBe(true);
+  });
+
+  it('returns false if given null', () => {
+    expect(isErrorWithCode(null)).toBe(false);
+  });
+
+  it('returns false if given undefined', () => {
+    expect(isErrorWithCode(undefined)).toBe(false);
+  });
+
+  it('returns false if given something that is not typeof object', () => {
+    expect(isErrorWithCode(12345)).toBe(false);
+  });
+
+  it('returns false if given an empty object', () => {
+    expect(isErrorWithCode({})).toBe(false);
+  });
+
+  it('returns false if given a non-empty object that does not have a "code" property', () => {
+    expect(isErrorWithCode({ message: 'some message' })).toBe(false);
+  });
+});
+
+describe('isErrorWithMessage', () => {
+  it('returns true if given an object that includes a "message" property', () => {
+    expect(
+      isErrorWithMessage({ code: 'some code', message: 'some message' }),
+    ).toBe(true);
+  });
+
+  it('returns false if given null', () => {
+    expect(isErrorWithMessage(null)).toBe(false);
+  });
+
+  it('returns false if given undefined', () => {
+    expect(isErrorWithMessage(undefined)).toBe(false);
+  });
+
+  it('returns false if given something that is not typeof object', () => {
+    expect(isErrorWithMessage(12345)).toBe(false);
+  });
+
+  it('returns false if given an empty object', () => {
+    expect(isErrorWithMessage({})).toBe(false);
+  });
+
+  it('returns false if given a non-empty object that does not have a "message" property', () => {
+    expect(isErrorWithMessage({ code: 'some code' })).toBe(false);
+  });
+});
+
+describe('isErrorWithStack', () => {
+  it('returns true if given an object that includes a "stack" property', () => {
+    expect(isErrorWithStack({ code: 'some code', stack: 'some stack' })).toBe(
+      true,
+    );
+  });
+
+  it('returns false if given null', () => {
+    expect(isErrorWithStack(null)).toBe(false);
+  });
+
+  it('returns false if given undefined', () => {
+    expect(isErrorWithStack(undefined)).toBe(false);
+  });
+
+  it('returns false if given something that is not typeof object', () => {
+    expect(isErrorWithStack(12345)).toBe(false);
+  });
+
+  it('returns false if given an empty object', () => {
+    expect(isErrorWithStack({})).toBe(false);
+  });
+
+  it('returns false if given a non-empty object that does not have a "stack" property', () => {
+    expect(
+      isErrorWithStack({ code: 'some code', message: 'some message' }),
+    ).toBe(false);
+  });
+});
+
+describe('wrapError', () => {
+  describe('if the original error is an Error instance not generated by fs.promises', () => {
+    it('returns a new Error with the given message', () => {
+      const originalError = new Error('oops');
+
+      const newError = wrapError(originalError, 'Some message');
+
+      expect(newError.message).toBe('Some message');
+    });
+
+    it('links to the original error via "cause"', () => {
+      const originalError = new Error('oops');
+
+      const newError = wrapError(originalError, 'Some message');
+
+      expect(newError.cause).toBe(originalError);
+    });
+
+    it('copies over any "code" property that exists on the given Error', () => {
+      const originalError = new Error('oops');
+      // @ts-expect-error The Error interface doesn't have a "code" property
+      originalError.code = 'CODE';
+
+      const newError = wrapError(originalError, 'Some message');
+
+      expect(newError.code).toBe('CODE');
+    });
+  });
+
+  describe('if the original error was generated by fs.promises', () => {
+    it('returns a new Error with the given message', async () => {
+      let originalError;
+      try {
+        await fs.promises.readFile('/tmp/nonexistent', 'utf8');
+      } catch (error: any) {
+        originalError = error;
+      }
+
+      const newError = wrapError(originalError, 'Some message');
+
+      expect(newError.message).toBe('Some message');
+    });
+
+    it("links to the original error via 'cause'", async () => {
+      let originalError;
+      try {
+        await fs.promises.readFile('/tmp/nonexistent', 'utf8');
+      } catch (error: any) {
+        originalError = error;
+      }
+
+      const newError = wrapError(originalError, 'Some message');
+
+      expect(newError.cause).toBe(originalError);
+    });
+
+    it('copies over any "code" property that exists on the given Error', async () => {
+      let originalError;
+      try {
+        await fs.promises.readFile('/tmp/nonexistent', 'utf8');
+      } catch (error: any) {
+        originalError = error;
+      }
+
+      const newError = wrapError(originalError, 'Some message');
+
+      expect(newError.code).toBe('ENOENT');
+    });
+  });
+
+  describe('if the original error is an object but not an Error instance', () => {
+    describe('if the message is a non-empty string', () => {
+      it('combines a string version of the original error and message together in a new Error', () => {
+        const originalError = { some: 'error' };
+
+        const newError = wrapError(originalError, 'Some message');
+
+        expect(newError.message).toBe('[object Object]: Some message');
+      });
+
+      it('does not set a cause on the new Error', async () => {
+        const originalError = { some: 'error' };
+
+        const newError = wrapError(originalError, 'Some message');
+
+        expect(newError.cause).toBeUndefined();
+      });
+
+      it('does not set a code on the new Error', async () => {
+        const originalError = { some: 'error' };
+
+        const newError = wrapError(originalError, 'Some message');
+
+        expect(newError.code).toBeUndefined();
+      });
+    });
+
+    describe('if the message is an empty string', () => {
+      it('places a string version of the original error in a new Error object without an additional message', () => {
+        const originalError = { some: 'error' };
+
+        const newError = wrapError(originalError, '');
+
+        expect(newError.message).toBe('[object Object]');
+      });
+
+      it('does not set a cause on the new Error', async () => {
+        const originalError = { some: 'error' };
+
+        const newError = wrapError(originalError, '');
+
+        expect(newError.cause).toBeUndefined();
+      });
+
+      it('does not set a code on the new Error', async () => {
+        const originalError = { some: 'error' };
+
+        const newError = wrapError(originalError, '');
+
+        expect(newError.code).toBeUndefined();
+      });
+    });
+  });
+
+  describe('if the original error is a string', () => {
+    describe('if the message is a non-empty string', () => {
+      it('combines the original error and message together in a new Error', () => {
+        const newError = wrapError('Some original message', 'Some message');
+
+        expect(newError.message).toBe('Some original message: Some message');
+      });
+
+      it('does not set a cause on the new Error', () => {
+        const newError = wrapError('Some original message', 'Some message');
+
+        expect(newError.cause).toBeUndefined();
+      });
+
+      it('does not set a code on the new Error', () => {
+        const newError = wrapError('Some original message', 'Some message');
+
+        expect(newError.code).toBeUndefined();
+      });
+    });
+
+    describe('if the message is an empty string', () => {
+      it('places the original error in a new Error object without an additional message', () => {
+        const newError = wrapError('Some original message', '');
+
+        expect(newError.message).toBe('Some original message');
+      });
+
+      it('does not set a cause on the new Error', () => {
+        const newError = wrapError('Some original message', '');
+
+        expect(newError.cause).toBeUndefined();
+      });
+
+      it('does not set a code on the new Error', () => {
+        const newError = wrapError('Some original message', '');
+
+        expect(newError.code).toBeUndefined();
+      });
+    });
+  });
+});
+
+describe('getErrorMessage', () => {
+  it("returns the value of the 'message' property from the given object if it is present", () => {
+    expect(getErrorMessage({ message: 'hello' })).toBe('hello');
+  });
+
+  it("returns the result of calling .toString() on the given object if it has no 'message' property", () => {
+    expect(getErrorMessage({ foo: 'bar' })).toBe('[object Object]');
+  });
+
+  it('returns the result of calling .toString() on the given non-object', () => {
+    expect(getErrorMessage(42)).toBe('42');
+  });
+
+  it('returns an empty string if given null', () => {
+    expect(getErrorMessage(null)).toBe('');
+  });
+
+  it('returns an empty string if given undefined', () => {
+    expect(getErrorMessage(undefined)).toBe('');
+  });
+});