From 3.x to 4.x

Simple migration guide to fast-check v4 starting from fast-check v3

Changes in minimal requirements

Name	New requirement	Previous requirement
Node	≥12.17.0	≥8
ECMAScript specification	ES2020	ES2017
TypeScript (optional)	≥5.0	≥4.1

Related pull requests: #5577, #5605, #5617, #5618, #5634, #5635, #5670, #5787

Update to latest v3.x

Version 4 of fast-check introduces significant changes as part of its major release, including breaking changes. However, many of these changes can be addressed while still using the latest minor release of version 3.

To ensure a smoother migration to version 4, we recommend first upgrading to the latest minor release of version 3. Then, review and address the following deprecation notices to align your codebase with supported patterns.

Changes on `date`

In version 4, the date arbitrary will generate any Date instances by default, including Invalid Date. If your code cannot handle invalid dates, you should add the noInvalidDate: true constraint to the configuration of your date builder to exclude such values.

-fc.date();
+fc.date({ noInvalidDate: true });

Related pull requests: #5589

Changes on `dictionary`

In version 4, the default behavior of dictionary will be updated to generate objects that may have null prototypes by default. As a result, unless configured otherwise, dictionary can produce both instances inheriting from the usual Object prototype and instances with no prototype.

If your code requires all generated objects to inherit from the usual Object prototype, you can set the noNullPrototype constraint to true (used to be defaulted to true in version 3). This option was introduced in version 3.13.0 and can be applied as follows:

fc.dictionary(fc.string(), fc.string(), {
  noNullPrototype: true,
  // other contraints (if any)...
});

Related pull requests: #5609

Changes on `record`

In earlier versions, the record arbitrary included a flag named withDeletedKeys. Starting with version 2.11.0, this flag was deprecated and replaced by a new flag called requiredKeys. In version 4.0.0, the deprecated withDeletedKeys flag has been removed entirely.

To migrate, update your usage of the record arbitrary as follows:

fc.record(recordModel, {
-  withDeletedKeys: true,
+  requiredKeys: [],
});
fc.record(recordModel, {
-  withDeletedKeys: false,
});

Additionally, the default behavior of record has been updated in version 4 to generate objects that may have null prototypes by default. This means that, unless configured otherwise, record will produce instances inheriting from the usual Object prototype as well as instances with no prototype.

If your code requires all generated objects to have the usual Object prototype, you can configure the noNullPrototype constraint to true. It has been introduced in version 3.13.0 and can be set as follow:

fc.record(recordModel, {
  noNullPrototype: true,
  // other contraints (if any)...
});

Related pull requests: #5578, #5597

Changes on `uuid`

Previously, the uuid arbitrary only generated UUIDs of versions 1 through 5. In version 4, we have expanded the default behavior to include versions 6, 7, and 8, which are also valid and commonly used UUID versions.

If your code relies specifically on versions 1 to 5, you can maintain the previous behavior by applying the following change:

--fc.uuid();
++fc.uuid({ version: [1, 2, 3, 4, 5] });

However, we strongly recommend either using the new default behavior or explicitly specifying the exact versions your application supports to ensure compatibility and consistency.

Related pull requests: #5633

Changes on strings

In version 4, we have made significant changes to our string arbitraries to simplify and enhance their usage.

First, we have removed arbitraries that generated single-character strings. Since generating a single character is equivalent to creating a string with a length of one, these specialized arbitraries were unnecessary. This change helps reduce the API surface and better aligns with typical use cases, as most users require multi-character strings rather than single-character ones.

Second, we have consolidated our main string arbitraries into a single string arbitrary. Previously, separate arbitraries existed for different character sets, such as ASCII and Unicode. In version 4, these have been unified into a single arbitrary that can be configured using the unit constraint to generate specific character types.

To assist with the migration, here’s how to update your existing code to the new API:

`ascii` or `asciiString`

function ascii(): fc.Arbitrary<string> {
  return fc.string({ unit: 'binary-ascii', minLength: 1, maxLength: 1 });
}

function asciiString(constraints: Omit<fc.StringConstraints, 'unit'> = {}): fc.Arbitrary<string> {
  return fc.string({ ...constraints, unit: 'binary-ascii' });
}

Related pull requests: #5636

`base64`

function base64(): fc.Arbitrary<string> {
  return fc.constantFrom(...'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789+/');
}

// We preserved fc.base64String() as it goes further than just a simple string of base64 characters.

Advanced rewriting

function base64Mapper(v: number) {
  if (v < 26) return String.fromCharCode(v + 65); // A-Z
  if (v < 52) return String.fromCharCode(v + 97 - 26); // a-z
  if (v < 62) return String.fromCharCode(v + 48 - 52); // 0-9
  return v === 62 ? '+' : '/'; // 43, 47
}

function base64Unmapper(s: unknown) {
  if (typeof s !== 'string' || s.length !== 1) {
    throw new Error('Invalid entry');
  }
  const v = s.charCodeAt(0);
  if (v >= 65 && v <= 90) return v - 65; // A-Z
  if (v >= 97 && v <= 122) return v - 97 + 26; // a-z
  if (v >= 48 && v <= 57) return v - 48 + 52; // 0-9
  return v === 43 ? 62 : v === 47 ? 63 : -1; // +/
}

function base64(): fc.Arbitrary<string> {
  return integer({ min: 0, max: 63 }).map(base64Mapper, base64Unmapper);
}

Related pull requests: #5664

`char`

function char(): fc.Arbitrary<string> {
  return fc.string({ unit: 'grapheme-ascii', minLength: 1, maxLength: 1 });
}

// We preserved fc.string().

Related pull requests: #5671

`char16bits` or `string16bits`

function char16bits(): fc.Arbitrary<string> {
  return fc.nat({ max: 0xffff }).map((n) => String.fromCharCode(n));
}

function string16bits(constraints: Omit<fc.StringConstraints, 'unit'> = {}): fc.Arbitrary<string> {
  return fc.string({ ...constraints, unit: char16bits() });
}

Related pull requests: #5666

`fullUnicode` or `fullUnicodeString`

function fullUnicode(): fc.Arbitrary<string> {
  return fc.string({ unit: 'binary', minLength: 1, maxLength: 1 });
}

function fullUnicodeString(constraints: Omit<fc.StringConstraints, 'unit'> = {}): fc.Arbitrary<string> {
  return fc.string({ ...constraints, unit: 'binary' });
}

Related pull requests: #5667

`hexa` or `hexaString`

function hexa(): fc.Arbitrary<string> {
  const items = '0123456789abcdef';
  return fc.integer({ min: 0, max: 15 }).map((n) => items[n]);
}

function hexaString(constraints: Omit<fc.StringConstraints, 'unit'> = {}): fc.Arbitrary<string> {
  return fc.string({ ...constraints, unit: hexa() });
}

Advanced rewriting

function hexa(): fc.Arbitrary<string> {
  return fc.integer({ min: 0, max: 15 }).map(
    (n) => items[n],
    (c) => items.indexOf(c),
  );
}

// hexaString unchanged!

Related pull requests: #5644

`unicode` or `unicodeString`

const gapSize = 0xdfff + 1 - 0xd800;
function unicodeMapper(v: number) {
  if (v < 0xd800) return v;
  return v + gapSize;
}
function unicode(): Arbitrary<string> {
  return integer({ min: 0, max: 0xffff - gapSize }).map((v) => String.fromCodePoint(unicodeMapper(v)));
}

function unicodeString(constraints: Omit<fc.StringConstraints, 'unit'> = {}): fc.Arbitrary<string> {
  return fc.string({ ...constraints, unit: unicode() });
}

Advanced rewriting

const gapSize = 0xdfff + 1 - 0xd800;
function unicodeMapper(v: number) {
  if (v < 0xd800) return v;
  return v + gapSize;
}
function unicodeUnmapper(v: number) {
  if (v < 0xd800) return v;
  if (v <= 0xdfff) return -1;
  return v - gapSize;
}
function unicode(): Arbitrary<string> {
  return integer({ min: 0, max: 0xffff - gapSize }).map(
    (v) => String.fromCodePoint(unicodeMapper(v)),
    (s) => {
      if (typeof s !== 'string') throw new Error('Invalid');
      if (s.length !== 1) throw new Error('Invalid');
      return unicodeUnmapper(s.codePointAt(0));
    },
  );
}

// unicodeString unchanged!

You probably don't need unicode/unicodeString

The unicode arbitrary was introduced early in fast-check, but later, fullUnicode was added to provide full Unicode support. unicode stayed limited to characters from the BMP (Basic Multilingual Plane).

If you chose unicode to support Unicode in general, you might actually need to consider fullUnicode instead.

Related pull requests: #5669

Replace any reference to `.noBias`

The .noBias method, previously available on every Arbitrary, was marked as deprecated in version 3.20.0. It has been replaced by a standalone arbitrary with the same functionality. You can prepare for compatibility with the next major version by updating your code as follows:

--myArbitrary.noBias();
++fc.noBias(myArbitrary);

Related pull requests: #5610

Replace any reference to `.noShrink`

The .noShrink method, has been marked as deprecated in version 3.20.0 and has been replaced by a standalone arbitrary doing the same. You can update your code as follows if you want to be ready for v4:

--myArbitrary.noShrink();
++fc.noShrink(myArbitrary);

Related pull requests: #5694

Replace any reference to `big{U|}int{N|}`

The arbitraries fc.bigIntN, fc.bigUintN, and fc.bigUint have been removed. Replace any usage of these with fc.bigInt. If needed, you can reimplement them as follows:

function bigIntN(n: number): fc.Arbitrary<bigint> {
  return fc.bigInt({ min: 1n << BigInt(n - 1), max: (1n << BigInt(n - 1)) - 1n });
}

function bigUintN(n: number): fc.Arbitrary<bigint> {
  return fc.bigInt({ min: 0n, max: (1n << BigInt(n)) - 1n });
}

function bigUint(max: bigint = (1n << 256n) - 1n): fc.Arbitrary<bigint> {
  return fc.bigInt({ min: 0n, max });
}

Related pull requests: #5674

Replace any reference to `stringOf`

Starting at 3.22.0, we recommend to replace any reference to stringOf by string. The following diff gives you an example of such change:

-fc.stringOf(fc.constantFrom('Hello', 'World'));
+fc.string({ unit: fc.constantFrom('Hello', 'World') });

Related pull requests: #5665

Replace any reference to `unicodeJson*`

The arbitraries unicodeJson and unicodeJsonValue have been replaced with json and jsonValue. Instead of maintaining separate versions for different character sets, the new approach consolidates them into a single arbitrary that accepts a custom charset via constraints.

To migrate, update your code as follows:

--fc.unicodeJson();
++fc.json({ stringUnit: 'binary' }); // or 'grapheme'

--fc.unicodeJsonValue();
++fc.jsonValue({ stringUnit: 'binary' }); // or 'grapheme'

This change provides greater flexibility by allowing customization of the character set directly through the constraint options.

Related pull requests: #5613

Replace any reference to `uuidV`

Introduced in version 3.21.0 for uuid, the version constraint is intended to replace uuidV. This change can already be applied in version 3 by making the following update:

--fc.uuidV(4);
++fc.uuid({ version: 4 });

Related pull requests: #5611

Update to v4.x

After applying the recommendations for migrating to the latest v3.x, transitioning to version 4 should be straightforward. However, there are still a few changes to review, either during the upgrade or as you use the updated library. These changes enhance functionality and ensure a more powerful tool by default.

Better type inference

Some typings have been enhanced to ease the user experience:

// In version 3:
fc.constant('a'); // Produces an Arbitrary<string>
fc.constant<'a'>('a'); // Produces an Arbitrary<'a'>

// In version 4:
fc.constant('a'); // Produces an Arbitrary<'a'>
fc.constant<string>('a'); // Produces an Arbitrary<string>

// In version 3:
fc.constantFrom('a', 'b'); // Produces an Arbitrary<string>
fc.constantFrom<'a' | 'b'>('a', 'b'); // Produces an Arbitrary<'a' | 'b'>

// In version 4:
fc.constantFrom('a', 'b'); // Produces an Arbitrary<'a' | 'b'>
fc.constantFrom<string[]>('a', 'b'); // Produces an Arbitrary<string>

Related pull requests: #5577, #5605

Default error reporting

We adopted a new approach to report errors by leveraging "Error Cause", which is already supported by many test runners. Previously, when your predicate threw an Error, fast-check created a new Error instance with a message that combined fast-check’s failure details with your original error message.

Now, it attaches your original error as a cause. This approach improves integration with test runners, which often parse error messages for stack trace cleanup and reporting.

If you prefer the previous behavior, you can disable this feature in version 4 by enabling the includeErrorInReport flag. You can also test this behavior in version 3 by toggling the errorWithCause flag (renamed to includeErrorInReport in version 4).

Related pull requests: #5590

Faster `scheduler`

Prefer waitNext, waitIdle, or waitFor

Since v4.2.0, waitOne and waitAll are deprecated in favor of waitNext, waitIdle, and waitFor, which behave more predictably — especially when tasks are scheduled after a few awaits, not immediately.

Use:

waitNext(n) for a known number of tasks
waitIdle() to wait until all tasks settle
waitFor(promise) to wait for a specific task, even if not yet scheduled

Since version 1.20.0, fast-check has included a primitive designed to help detect race conditions. This feature unlocked many advanced use cases and elevated the library's capabilities.

However, the previous implementation was slower than intended and allowed intermediate tasks to be created and executed between two scheduled ones. This inconsistency could lead to scenarios where code passed tests but later failed when additional microtasks were introduced. To address this, we have reworked the scheduler in version 4 to be faster, more consistent, and safer.

Consider the following example, where a scheduler instance s is used:

// `s`: an instance of scheduler provided by fast-check
s.schedule(Promise.resolve(1)).then(async () => {
  await 'something already resolved';
  s.schedule(Promise.resolve(2));
});
await s.waitAll();

In version 3, all scheduled tasks, including Promise.resolve(2), would have been executed by the end of s.waitAll(). In version 4, however, Promise.resolve(2) remains pending. This is because during the waitAll loop, the scheduler processes Promise.resolve(1) and continues execution until await 'something already resolved'. At that point, the scheduler resumes its waiting sequence, but Promise.resolve(2) has not yet been scheduled and remains unknown. As a result, waitAll finishes before executing it.

This behavior makes the scheduler more predictable and prevents subtle issues. In contrast, version 3 behaved inconsistently when processing many immediately resolved tasks, as shown below:

// `s`: an instance of scheduler provided by fast-check
s.schedule(Promise.resolve(1)).then(async () => {
  await 'something already resolved';
  await 'something already resolved';
  await 'something already resolved';
  await 'something already resolved';
  await 'something already resolved';
  s.schedule(Promise.resolve(2));
});
await s.waitAll();

On this second example version 3 would have behaved as version 4 with Promise.resolve(2) still pending. The only difference between the two examples being the number of await before the next scheduled tasks. This improvement ensures unexpected behaviors in such edge cases and ensures consistent behavior.

Related pull requests: #5600, #5604, #5614, #5615

Advanced usages

Custom reporters

The error field has been removed from the RunDetails object returned by fc.check. If you need access to the error message, use the errorInstance field instead, which was introduced in version 3.0.0.

Related pull requests: #5584

No more `nextArrayInt` on `Random`

There are several ways to build arbitraries in fast-check:

Combine some existing ones,
Map, Filter and Chain some,
Implement the interface of Arbitrary.

The breaking change only affects the last category of users. If you implemented by hand your own Arbitrary you probably leveraged the instance of Random being provided to your generate method in order to build your values.

We dropped the method called nextArrayInt from it. We recommend users to rely on nextBigInt for big numerical values.

Related pull requests: #5679

Property execution

If you have implemented a custom class that adheres to the IRawProperty API required by property runners, or if you have created a custom property runner (e.g., a custom implementation of fc.assert or fc.check), this change may affect your code.

The update requires property executors to explicitly call the runBeforeEach and runAfterEach hooks. This adjustment can already be made in version 3 by passing true as the second argument to the run method of properties.

Related pull requests: #5581

Refined serializer

In previous major releases, the stringifier algorithm produced outputs like the following:

stringify(Object.create(null)); // 'Object.create(null)'
stringify(Object.assign(Object.create(null), { a: 1 })); // 'Object.assign(Object.create(null),{"a":1})'

Starting with the new major release, the output has been refined to:

stringify(Object.create(null)); // '{__proto__:null}'
stringify(Object.assign(Object.create(null), { a: 1 })); // '{__proto__:null,"a":1}'

This change is unlikely to impact most users. However, we are highlighting it for advanced users who might rely on custom reporting capabilities or stringifier behavior to meet specific needs.

Related pull requests: #5603

Remove certain Symbol-based typings for commands

In previous versions, the typings for CommandWrapper included methods on the symbols toStringMethod and asyncToStringMethod. While these methods will still exist in JavaScript in v4, they will no longer be exposed in the TypeScript typings. As a result, the declared type will change as follows:

export declare class CommandWrapper<Model extends object, Real, RunResult, CheckAsync extends boolean>
  implements ICommand<Model, Real, RunResult, CheckAsync>
{
  readonly cmd: ICommand<Model, Real, RunResult, CheckAsync>;
-  [toStringMethod]?: () => string;
-  [asyncToStringMethod]?: () => Promise<string>;
  hasRan: boolean;
  constructor(cmd: ICommand<Model, Real, RunResult, CheckAsync>);
  check(m: Readonly<Model>): CheckAsync extends false ? boolean : Promise<boolean>;
  run(m: Model, r: Real): RunResult;
  clone(): CommandWrapper<Model, Real, RunResult, CheckAsync>;
  toString(): string;
}

A similar change affects CommandsIterable, where the cloneMethod symbol will no longer be included in the typings:

export declare class CommandsIterable<Model extends object, Real, RunResult, CheckAsync extends boolean = false>
  implements Iterable<CommandWrapper<Model, Real, RunResult, CheckAsync>>
{
  readonly commands: CommandWrapper<Model, Real, RunResult, CheckAsync>[];
  readonly metadataForReplay: () => string;
  constructor(commands: CommandWrapper<Model, Real, RunResult, CheckAsync>[], metadataForReplay: () => string);
  [Symbol.iterator](): Iterator<CommandWrapper<Model, Real, RunResult, CheckAsync>>;
-  [cloneMethod](): CommandsIterable<Model, Real, RunResult, CheckAsync>;
  toString(): string;
}

Related pull requests: #5136

Changes in minimal requirements​

Update to latest v3.x​

Changes on date​

Changes on dictionary​

Changes on record​

Changes on uuid​

Changes on strings​

ascii or asciiString​

base64​

char​

char16bits or string16bits​

fullUnicode or fullUnicodeString​

hexa or hexaString​

unicode or unicodeString​

Replace any reference to .noBias​

Replace any reference to .noShrink​

Replace any reference to big{U|}int{N|}​

Replace any reference to stringOf​

Replace any reference to unicodeJson*​

Replace any reference to uuidV​

Update to v4.x​

Better type inference​

Default error reporting​

Faster scheduler​

Advanced usages​

Custom reporters​

No more nextArrayInt on Random​

Property execution​

Refined serializer​

Remove certain Symbol-based typings for commands​

Changes in minimal requirements

Update to latest v3.x

Changes on `date`

Changes on `dictionary`

Changes on `record`

Changes on `uuid`

Changes on strings

`ascii` or `asciiString`

`base64`

`char`

`char16bits` or `string16bits`

`fullUnicode` or `fullUnicodeString`

`hexa` or `hexaString`

`unicode` or `unicodeString`

Replace any reference to `.noBias`

Replace any reference to `.noShrink`

Replace any reference to `big{U|}int{N|}`

Replace any reference to `stringOf`

Replace any reference to `unicodeJson*`

Replace any reference to `uuidV`

Update to v4.x

Better type inference

Default error reporting

Faster `scheduler`

Advanced usages

Custom reporters

No more `nextArrayInt` on `Random`

Property execution

Refined serializer

Remove certain Symbol-based typings for commands