The 'expect' npm package is a library for writing test assertions. It is commonly used in conjunction with testing frameworks like Jest or Mocha to validate the behavior of JavaScript code. It provides a range of assertion types and matchers that allow developers to write expressive and readable tests.
Basic Assertions
This feature allows you to assert that a value matches exactly what you expect. The 'toBe' matcher compares with ===.
expect(2 + 2).toBe(4);Object Property Assertions
With this feature, you can assert that an object has a specific property with a certain value. The 'toHaveProperty' matcher checks for the existence and value of a property in an object.
expect({ name: 'Alice', age: 30 }).toHaveProperty('name', 'Alice');Exception Testing
This feature is used to test if a function throws an exception when it is executed. The 'toThrow' matcher is used to assert that an error is thrown with a specific message.
expect(() => { throw new Error('failure'); }).toThrow('failure');Array Containment
This feature allows you to assert that an array contains a specific item. The 'toContain' matcher checks if an array includes the expected item.
expect(['Alice', 'Bob', 'Eve']).toContain('Bob');Asynchronous Assertions
This feature enables you to write assertions for asynchronous code. The 'resolves' matcher waits for a promise to resolve and then checks the resolved value.
expect(Promise.resolve('success')).resolves.toBe('success');Chai is a BDD/TDD assertion library for node and the browser that can be paired with any javascript testing framework. It offers a similar range of matchers and assertions as expect, and it provides a fluent chainable language to construct assertions.
Should.js is an expressive, readable, framework-agnostic assertion library. The key difference is in the style of assertions it offers, using a more fluent chainable API that reads like English.
Assert is a module that provides a set of assertion functions for verifying invariants. It is built into Node.js and is less feature-rich compared to expect, but it serves as a simple way to write assertions in a Node.js environment.
Jest is a delightful JavaScript Testing Framework with a focus on simplicity. It comes with its own assertion library, which provides functionality similar to expect, as expect is actually part of Jest.
jest 22.0.0
[jest-resolve] Use module.builtinModules as BUILTIN_MODULES when it exists[jest-worker] Remove debug and inspect flags from the arguments sent to the child (#5068)[jest-config] Use all --testPathPattern and <regexForTestFiles> args in testPathPattern (#5066)[jest-cli] Do not support --watch inside non-version-controlled environments (#5060)[jest-config] Escape Windows path separator in testPathPattern CLI arguments (#5054)[jest-jasmine] Register sourcemaps as node environment to improve performance with jsdom (#5045)[pretty-format] Do not call toJSON recursively (#5044)[pretty-format] Fix errors when identity-obj-proxy mocks CSS Modules (#4935)[babel-jest] Fix support for namespaced babel version 7 (#4918)[expect] fix .toThrow for promises (#4884)[jest-docblock] pragmas should preserve urls (#4837)[jest-cli] Check if npm_lifecycle_script calls Jest directly (#4629)[jest-cli] Fix --showConfig to show all configs (#4494)[jest-cli] Throw if maxWorkers doesn't have a value (#4591)[jest-cli] Use fs.realpathSync.native if available (#5031)[jest-config] Fix --passWithNoTests (#4639)[jest-config] Support rootDir tag in testEnvironment (#4579)[jest-editor-support] Fix --showConfig to support jest 20 and jest 21 (#4575)[jest-editor-support] Fix editor support test for node 4 (#4640)[jest-mock] Support mocking constructor in mockImplementationOnce (#4599)[jest-runtime] Fix manual user mocks not working with custom resolver (#4489)[jest-util] Fix runOnlyPendingTimers for setTimeout inside setImmediate (#4608)[jest-message-util] Always remove node internals from stacktraces (#4695)[jest-resolve] changes method of determining builtin modules to include missing builtins (#4740)[pretty-format] Prevent error in pretty-format for window in jsdom test env (#4750)[jest-resolve] Preserve module identity for symlinks (#4761)[jest-config] Include error message for preset json (#4766)[pretty-format] Throw PrettyFormatPluginError if a plugin halts with an exception (#4787)[expect] Keep the stack trace unchanged when PrettyFormatPluginError is thrown by pretty-format (#4787)[jest-environment-jsdom] Fix asynchronous test will fail due to timeout issue. (#4669)[jest-cli] Fix --onlyChanged path case sensitivity on Windows platform (#4730)[jest-runtime] Use realpath to match transformers (#5000)[expect] [BREAKING] Replace identity equality with Object.is in toBe matcher (#4917)[jest-message-util] Add codeframe to test assertion failures (#5087)[jest-config] Add Global Setup/Teardown options (#4716)[jest-config] Add testEnvironmentOptions to apply to jsdom options or node context. (#5003)[jest-jasmine2] Update Timeout error message to jest.timeout and display current timeout value (#4990)[jest-runner] Enable experimental detection of leaked contexts (#4895)[jest-cli] Add combined coverage threshold for directories. (#4885)[jest-mock] Add timestamps to mock state. (#4866)[eslint-plugin-jest] Add prefer-to-have-length lint rule. (#4771)[jest-environment-jsdom] [BREAKING] Upgrade to JSDOM@11 (#4770)[jest-environment-*] [BREAKING] Add Async Test Environment APIs, dispose is now teardown (#4506)[jest-cli] Add an option to clear the cache (#4430)[babel-plugin-jest-hoist] Improve error message, that the second argument of jest.mock must be an inline function (#4593)[jest-snapshot] [BREAKING] Concatenate name of test and snapshot (#4460)[jest-cli] [BREAKING] Fail if no tests are found (#3672)[jest-diff] Highlight only last of odd length leading spaces (#4558)[jest-docblock] Add docblock.print() (#4517)[jest-docblock] Add strip (#4571)[jest-docblock] Preserve leading whitespace in docblock comments (#4576)[jest-docblock] remove leading newlines from parswWithComments().comments (#4610)[jest-editor-support] Add Snapshots metadata (#4570)[jest-editor-support] Adds an 'any' to the typedef for updateFileWithJestStatus (#4636)[jest-editor-support] Better monorepo support (#4572)[jest-environment-jsdom] Add simple rAF polyfill in jsdom environment to work with React 16 (#4568)[jest-environment-node] Implement node Timer api (#4622)[jest-jasmine2] Add testPath to reporter callbacks (#4594)[jest-mock] Added support for naming mocked functions with .mockName(value) and .mockGetName() (#4586)[jest-runtime] Add module.loaded, and make module.require not enumerable (#4623)[jest-runtime] Add module.parent (#4614)[jest-runtime] Support sourcemaps in transformers (#3458)[jest-snapshot] [BREAKING] Add a serializer for jest.fn to allow a snapshot of a jest mock (#4668)[jest-worker] Initial version of parallel worker abstraction, say hello! (#4497)[jest-jasmine2] Add testLocationInResults flag to add location information per spec to test results (#4782)[jest-environment-jsdom] Update JSOM to 11.4, which includes built-in support for requestAnimationFrame (#4919)[jest-cli] Hide watch usage output when running on non-interactive environments (#4958)[jest-snapshot] Promises support for toThrowErrorMatchingSnapshot (#4946)[jest-cli] Explain which snapshots are obsolete (#5005)[docs] Add guide of using with puppeteer (#5093)[jest-util] jest-util should not depend on jest-mock (#4992)[*] [BREAKING] Drop support for Node.js version 4 (#4769)[docs] Wrap code comments at 80 characters (#4781)[eslint-plugin-jest] Removed from the Jest core repo, and moved to https://github.com/jest-community/eslint-plugin-jest (#4867)[babel-jest] Explicitly bump istanbul to newer versions (#4616)[expect] Upgrade mocha and rollup for browser testing (#4642)[docs] Add info about coveragePathIgnorePatterns (#4602)[docs] Add Vuejs series of testing with Jest (#4648)[docs] Mention about optional done argument in test function (#4556)[jest-cli] Bump node-notifier version (#4609)[jest-diff] Simplify highlight for leading and trailing spaces (#4553)[jest-get-type] Add support for date (#4621)[jest-matcher-utils] Call chalk.inverse for trailing spaces (#4578)[jest-runtime] Add .advanceTimersByTime; keep .runTimersToTime() as an alias.[docs] Include missing dependency in TestEnvironment sample code[docs] Add clarification for hook execution order[docs] Update expect.anything() sample code (#5007)FAQs
This package exports the `expect` function used in [Jest](https://jestjs.io/). You can find its documentation [on Jest's website](https://jestjs.io/docs/expect).
The npm package expect receives a total of 22,440,652 weekly downloads. As such, expect popularity was classified as popular.
We found that expect demonstrated a not healthy version release cadence and project activity because the last version was released a year ago. It has 2 open source maintainers collaborating on the project.
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