Node + TypeScript + Rollup + Jest

Monday, 8 April 2019

Node + TypeScript + Rollup + Jest

To install Jest:

$ npm install jest --save-dev

Jest CLI

$ npx jest --help
Usage: jest [--config=<pathToConfigFile>] [TestPathPattern]

Options:
--help, -h Show help [boolean]
--version, -v Print the version and exit [boolean]
--all The opposite of `onlyChanged`. If `onlyChanged`
is set by default, running jest with `--all`
will force Jest to run all tests instead of
running only tests related to changed files.
--automock Automock all files by default. [boolean]
--bail, -b Exit the test suite immediately after `n` number
of failing tests.
--browser Respect the "browser" field in package.json when
resolving modules. Some packages export
different versions based on whether they are
operating in node.js or a browser. [boolean]
--cache Whether to use the transform cache. Disable the
cache using --no-cache. [boolean]
--cacheDirectory The directory where Jest should store its cached
dependency information. [string]
--changedFilesWithAncestor Runs tests related to the current changes and
the changes made in the last commit. Behaves
similarly to `--onlyChanged`. [boolean]
--changedSince Runs tests related to the changes since the
provided branch. If the current branch has
diverged from the given branch, then only
changes made locally will be tested. Behaves
similarly to `--onlyChanged`. [string]
--ci Whether to run Jest in continuous integration
(CI) mode. This option is on by default in most
popular CI environments. It will prevent
snapshots from being written unless explicitly
requested. [boolean] [default: false]
--clearCache Clears the configured Jest cache directory and
then exits. Default directory can be found by
calling jest --showConfig [boolean]
--clearMocks Automatically clear mock calls and instances
between every test. Equivalent to calling
jest.clearAllMocks() between each test.[boolean]
--collectCoverage Alias for --coverage. [boolean]
--collectCoverageFrom A glob pattern relative to <rootDir> matching
the files that coverage info needs to be
collected from. [string]
--collectCoverageOnlyFrom Explicit list of paths coverage will be
restricted to. [array]
--color Forces test results output color highlighting
(even if stdout is not a TTY). Set to false if
you would like to have no colors. [boolean]
--colors Alias for `--color`. [boolean]
--config, -c The path to a jest config file specifying how to
find and execute tests. If no rootDir is set in
the config, the directory containing the config
file is assumed to be the rootDir for the
project.This can also be a JSON encoded value
which Jest will use as configuration. [string]
--coverage Indicates that test coverage information should
be collected and reported in the output.
[boolean]
--coverageDirectory The directory where Jest should output its
coverage files. [string]
--coveragePathIgnorePatterns An array of regexp pattern strings that are
matched against all file paths before executing
the test. If the file pathmatches any of the
patterns, coverage information will be skipped.
[array]
--coverageReporters A list of reporter names that Jest uses when
writing coverage reports. Any istanbul reporter
can be used. [array]
--coverageThreshold A JSON string with which will be used to
configure minimum threshold enforcement for
coverage results [string]
--debug Print debugging info about your jest config.
[boolean]
--detectLeaks **EXPERIMENTAL**: Detect memory leaks in tests.
After executing a test, it will try to garbage
collect the global object used, and fail if it
was leaked [boolean] [default: false]
--detectOpenHandles Print out remaining open handles preventing Jest
from exiting at the end of a test run. Implies
`runInBand`. [boolean] [default: false]
--env The test environment used for all tests. This
can point to any file or node module. Examples:
`jsdom`, `node` or `path/to/my-environment.js`
[string]
--errorOnDeprecated Make calling deprecated APIs throw helpful error
messages. [boolean] [default: false]
--expand, -e Use this flag to show full diffs instead of a
patch. [boolean]
--filter Path to a module exporting a filtering function.
This method receives a list of tests which can
be manipulated to exclude tests from running.
Especially useful when used in conjunction with
a testing infrastructure to filter known broken
tests. [string]
--findRelatedTests Find related tests for a list of source files
that were passed in as arguments. Useful for
pre-commit hook integration to run the minimal
amount of tests necessary. [boolean]
--forceExit Force Jest to exit after all tests have
completed running. This is useful when resources
set up by test code cannot be adequately cleaned
up. [boolean]
--globalSetup The path to a module that runs before All Tests.
[string]
--globalTeardown The path to a module that runs after All Tests.
[string]
--globals A JSON string with map of global variables that
need to be available in all test environments.
[string]
--haste A JSON string with map of variables for the
haste module system [string]
--init Generate a basic configuration file [boolean]
--json Prints the test results in JSON. This mode will
send all other test output and user messages to
stderr. [boolean]
--lastCommit Run all tests affected by file changes in the
last commit made. Behaves similarly to
`--onlyChanged`. [boolean]
--listTests Lists all tests Jest will run given the
arguments and exits. Most useful in a CI system
together with `--findRelatedTests` to determine
the tests Jest will run based on specific files
[boolean] [default: false]
--logHeapUsage Logs the heap usage after every test. Useful to
debug memory leaks. Use together with
`--runInBand` and `--expose-gc` in node.
[boolean]
--mapCoverage Maps code coverage reports against original
source code when transformers supply source
maps.

DEPRECATED [boolean]
--maxConcurrency Specifies the maximum number of tests that are
allowed to runconcurrently. This only affects
tests using `test.concurrent`.
[number] [default: 5]
--maxWorkers, -w Specifies the maximum number of workers the
worker-pool will spawn for running tests. This
defaults to the number of the cores available on
your machine. (its usually best not to override
this default) [number]
--moduleDirectories An array of directory names to be searched
recursively up from the requiring module's
location. [array]
--moduleFileExtensions An array of file extensions your modules use.
If you require modules without specifying a
file extension, these are the extensions Jest
will look for. [array]
--moduleNameMapper A JSON string with a map from regular
expressions to module names that allow to stub
out resources, like images or styles with a
single module [string]
--modulePathIgnorePatterns An array of regexp pattern strings that are
matched against all module paths before those
paths are to be considered "visible" to the
module loader. [array]
--modulePaths An alternative API to setting the NODE_PATH env
variable, modulePaths is an array of absolute
paths to additional locations to search when
resolving modules. [array]
--noStackTrace Disables stack trace in test results output
[boolean]
--notify Activates notifications for test results.
[boolean]
--notifyMode Specifies when notifications will appear for
test results.
[string] [default: "failure-change"]
--onlyChanged, -o Attempts to identify which tests to run based on
which files have changed in the current
repository. Only works if you're running tests
in a git or hg repository at the moment.
[boolean]
--onlyFailures, -f Run tests that failed in the previous execution.
[boolean]
--outputFile Write test results to a file when the --json
option is also specified. [string]
--passWithNoTests Will not fail if no tests are found (for example
while using `--testPathPattern`.)
[boolean] [default: false]
--preset A preset that is used as a base for Jest's
configuration. [string]
--prettierPath The path to the "prettier" module used for
inline snapshots. [string]
--projects A list of projects that use Jest to run all
tests of all projects in a single instance of
Jest. [array]
--reporters A list of custom reporters for the test suite.
[array]
--resetMocks Automatically reset mock state between every
test. Equivalent to calling jest.resetAllMocks()
between each test. [boolean]
--resetModules If enabled, the module registry for every test
file will be reset before running each
individual test. [boolean]
--resolver A JSON string which allows the use of a custom
resolver. [string]
--restoreMocks Automatically restore mock state and
implementation between every test. Equivalent to
calling jest.restoreAllMocks() between each
test. [boolean]
--rootDir The root directory that Jest should scan for
tests and modules within. [string]
--roots A list of paths to directories that Jest should
use to search for files in. [array]
--runInBand, -i Run all tests serially in the current process
(rather than creating a worker pool of child
processes that run tests). This is sometimes
useful for debugging, but such use cases are
pretty rare. [boolean]
--runTestsByPath Used when provided patterns are exact file
paths. This avoids converting them into a
regular expression and matching it against every
single file. [boolean] [default: false]
--runner Allows to use a custom runner instead of Jest's
default test runner. [string]
--setupFiles A list of paths to modules that run some code
to configure or set up the testing environment
before each test. [array]
--setupFilesAfterEnv A list of paths to modules that run some code
to configure or set up the testing framework
before each test [array]
--showConfig Print your jest config and then exits. [boolean]
--silent Prevent tests from printing messages through the
console. [boolean]
--skipFilter Disables the filter provided by --filter. Useful
for CI jobs, or local enforcement when fixing
tests. [boolean]
--snapshotSerializers A list of paths to snapshot serializer modules
Jest should use for snapshot testing. [array]
--testEnvironment Alias for --env [string]
--testEnvironmentOptions Test environment options that will be passed to
the testEnvironment. The relevant options depend
on the environment. [string]
--testFailureExitCode Exit code of `jest` command if the test run
failed [string]
--testLocationInResults Add `location` information to the test results
[boolean] [default: false]
--testMatch The glob patterns Jest uses to detect test
files. [array]
--testNamePattern, -t Run only tests with a name that matches the
regex pattern. [string]
--testPathIgnorePatterns An array of regexp pattern strings that are
matched against all test paths before executing
the test. If the test path matches any of the
patterns, it will be skipped. [array]
--testPathPattern A regexp pattern string that is matched against
all tests paths before executing the test.
[array]
--testRegex A string or array of string regexp patterns that
Jest uses to detect test files. [array]
--testResultsProcessor Allows the use of a custom results processor.
This processor must be a node module that
exports a function expecting as the first
argument the result object. [string]
--testRunner Allows to specify a custom test runner. The
default is `jasmine2`. A path to a custom test
runner can be provided:
`<rootDir>/path/to/testRunner.js`. [string]
--testSequencer Allows to specify a custom test sequencer. The
default is `@jest/test-sequencer`. A path to a
custom test sequencer can be provided:
`<rootDir>/path/to/testSequencer.js` [string]
--testURL This option sets the URL for the jsdom
environment. [string]
--timers Setting this value to fake allows the use of
fake timers for functions such as setTimeout.
[string]
--transform A JSON string which maps from regular
expressions to paths to transformers. [string]
--transformIgnorePatterns An array of regexp pattern strings that are
matched against all source file paths before
transformation. [array]
--unmockedModulePathPatterns An array of regexp pattern strings that are
matched against all modules before the module
loader will automatically return a mock for
them. [array]
--updateSnapshot, -u Use this flag to re-record snapshots. Can be
used together with a test suite pattern or with
`--testNamePattern` to re-record snapshot for
test matching the pattern [boolean]
--useStderr Divert all output to stderr. [boolean]
--verbose Display individual test results with the test
suite hierarchy. [boolean]
--watch Watch files for changes and rerun tests related
to changed files. If you want to re-run all
tests when a file has changed, use the
`--watchAll` option. [boolean]
--watchAll Watch files for changes and rerun all tests. If
you want to re-run only the tests related to the
changed files, use the `--watch` option.
[boolean]
--watchPathIgnorePatterns An array of regexp pattern strings that are
matched against all paths before trigger test
re-run in watch mode. If the test path matches
any of the patterns, it will be skipped. [array]
--watchman Whether to use watchman for file crawling.
Disable using --no-watchman. [boolean]

Documentation: https://jestjs.io/

---

To create a basic config file:

$ npx jest --init

This will crate config file after prompting you couple of questions:

$ npx jest --init

The following questions will help Jest to create a suitable configuration for your project

✔ Would you like to use Jest when running "test" script in "package.json"? … yes
✔ Choose the test environment that will be used for testing › node
✔ Do you want Jest to add coverage reports? … yes
✔ Automatically clear mock calls and instances between every test? … yes

✏️ Modified /../demo-typescript/package.json

📝 Configuration file created at /../demo-typescript/jest.config.js

jest.config.js can look like this:

// For a detailed explanation regarding each configuration property, visit:
// https://jestjs.io/docs/en/configuration.html

module.exports = {
// All imported modules in your tests should be mocked automatically
// automock: false,

// Stop running tests after `n` failures
// bail: 0,

// Respect "browser" field in package.json when resolving modules
// browser: false,

// The directory where Jest should store its cached dependency information
// cacheDirectory: "/tmp/jest_rs",

// Automatically clear mock calls and instances between every test
clearMocks: true,

// Indicates whether the coverage information should be collected while executing the test
// collectCoverage: false,

// An array of glob patterns indicating a set of files for which coverage information should be collected
// collectCoverageFrom: null,

// The directory where Jest should output its coverage files
coverageDirectory: "coverage",

// An array of regexp pattern strings used to skip coverage collection
// coveragePathIgnorePatterns: [
// "/node_modules/"
// ],

// A list of reporter names that Jest uses when writing coverage reports
// coverageReporters: [
// "json",
// "text",
// "lcov",
// "clover"
// ],

// An object that configures minimum threshold enforcement for coverage results
// coverageThreshold: null,

// A path to a custom dependency extractor
// dependencyExtractor: null,

// Make calling deprecated APIs throw helpful error messages
// errorOnDeprecated: false,

// Force coverage collection from ignored files using an array of glob patterns
// forceCoverageMatch: [],

// A path to a module which exports an async function that is triggered once before all test suites
// globalSetup: null,

// A path to a module which exports an async function that is triggered once after all test suites
// globalTeardown: null,

// A set of global variables that need to be available in all test environments
// globals: {},

// An array of directory names to be searched recursively up from the requiring module's location
// moduleDirectories: [
// "node_modules"
// ],

// An array of file extensions your modules use
// moduleFileExtensions: [
// "js",
// "json",
// "jsx",
// "ts",
// "tsx",
// "node"
// ],

// A map from regular expressions to module names that allow to stub out resources with a single module
// moduleNameMapper: {},

// An array of regexp pattern strings, matched against all module paths before considered 'visible' to the module loader
// modulePathIgnorePatterns: [],

// Activates notifications for test results
// notify: false,

// An enum that specifies notification mode. Requires { notify: true }
// notifyMode: "failure-change",

// A preset that is used as a base for Jest's configuration
preset: "ts-jest",

// Run tests from one or more projects
// projects: null,

// Use this configuration option to add custom reporters to Jest
// reporters: undefined,

// Automatically reset mock state between every test
// resetMocks: false,

// Reset the module registry before running each individual test
// resetModules: false,

// A path to a custom resolver
// resolver: null,

// Automatically restore mock state between every test
// restoreMocks: false,

// The root directory that Jest should scan for tests and modules within
// rootDir: null,

// A list of paths to directories that Jest should use to search for files in
// roots: [
// "<rootDir>"
// ],

// Allows you to use a custom runner instead of Jest's default test runner
// runner: "jest-runner",

// The paths to modules that run some code to configure or set up the testing environment before each test
// setupFiles: [],

// A list of paths to modules that run some code to configure or set up the testing framework before each test
// setupFilesAfterEnv: [],

// A list of paths to snapshot serializer modules Jest should use for snapshot testing
// snapshotSerializers: [],

// The test environment that will be used for testing
testEnvironment: "node",

// Options that will be passed to the testEnvironment
// testEnvironmentOptions: {},

// Adds a location field to test results
// testLocationInResults: false,

// The glob patterns Jest uses to detect test files
// testMatch: [
// "**/__tests__/**/*.[jt]s?(x)",
// "**/?(*.)+(spec|test).[tj]s?(x)"
// ],

// An array of regexp pattern strings that are matched against all test paths, matched tests are skipped
// testPathIgnorePatterns: [
// "/node_modules/"
// ],

// The regexp pattern or array of patterns that Jest uses to detect test files
// testRegex: [],

// This option allows the use of a custom results processor
// testResultsProcessor: null,

// This option allows use of a custom test runner
// testRunner: "jasmine2",

// This option sets the URL for the jsdom environment. It is reflected in properties such as location.href
// testURL: "http://localhost",

// Setting this value to "fake" allows the use of fake timers for functions such as "setTimeout"
// timers: "real",

// A map from regular expressions to paths to transformers
// transform: {
// "^.+\\.tsx?$": "ts-jest"
// },

// An array of regexp pattern strings that are matched against all source file paths, matched files will skip transformation
// transformIgnorePatterns: [
// "/node_modules/"
// ],

// An array of regexp pattern strings that are matched against all modules before the module loader will automatically return a mock for them
// unmockedModulePathPatterns: undefined,

// Indicates whether each individual test should be reported during the run
verbose: true,

// An array of regexp patterns that are matched against all source file paths before re-running tests in watch mode
// watchPathIgnorePatterns: [],

// Whether to use watchman for file crawling
// watchman: true,
};

---

Config file can also be created via ts-jest:

$ npx ts-jest config:init

...but first you need to install ts-jest:

$ npm install jest typescript ts-jest --save-dev

ts-jest creates jest.config.js file with the following content:

module.exports = {
preset: 'ts-jest',
testEnvironment: 'node',
};

If jest.config.js already exists, the previous command will fail with this error message:

ts-jest[cli] (FATAL) Configuration file jest.config.js already exists.

To see the list of all Jest options:

$ npx jest --help
Usage: jest [--config=<pathToConfigFile>] [TestPathPattern]

Options:
--help, -h Show help [boolean]
--version, -v Print the version and exit [boolean]
--all The opposite of `onlyChanged`. If `onlyChanged`
is set by default, running jest with `--all`
will force Jest to run all tests instead of
running only tests related to changed files.
--automock Automock all files by default. [boolean]
--bail, -b Exit the test suite immediately after `n` number
of failing tests.
--browser Respect the "browser" field in package.json when
resolving modules. Some packages export
different versions based on whether they are
operating in node.js or a browser. [boolean]
--cache Whether to use the transform cache. Disable the
cache using --no-cache. [boolean]
--cacheDirectory The directory where Jest should store its cached
dependency information. [string]
--changedFilesWithAncestor Runs tests related to the current changes and
the changes made in the last commit. Behaves
similarly to `--onlyChanged`. [boolean]
--changedSince Runs tests related to the changes since the
provided branch. If the current branch has
diverged from the given branch, then only
changes made locally will be tested. Behaves
similarly to `--onlyChanged`. [string]
--ci Whether to run Jest in continuous integration
(CI) mode. This option is on by default in most
popular CI environments. It will prevent
snapshots from being written unless explicitly
requested. [boolean] [default: false]
--clearCache Clears the configured Jest cache directory and
then exits. Default directory can be found by
calling jest --showConfig [boolean]
--clearMocks Automatically clear mock calls and instances
between every test. Equivalent to calling
jest.clearAllMocks() between each test.[boolean]
--collectCoverage Alias for --coverage. [boolean]
--collectCoverageFrom A glob pattern relative to <rootDir> matching
the files that coverage info needs to be
collected from. [string]
--collectCoverageOnlyFrom Explicit list of paths coverage will be
restricted to. [array]
--color Forces test results output color highlighting
(even if stdout is not a TTY). Set to false if
you would like to have no colors. [boolean]
--colors Alias for `--color`. [boolean]
--config, -c The path to a jest config file specifying how to
find and execute tests. If no rootDir is set in
the config, the directory containing the config
file is assumed to be the rootDir for the
project.This can also be a JSON encoded value
which Jest will use as configuration. [string]
--coverage Indicates that test coverage information should
be collected and reported in the output.
[boolean]
--coverageDirectory The directory where Jest should output its
coverage files. [string]
--coveragePathIgnorePatterns An array of regexp pattern strings that are
matched against all file paths before executing
the test. If the file pathmatches any of the
patterns, coverage information will be skipped.
[array]
--coverageReporters A list of reporter names that Jest uses when
writing coverage reports. Any istanbul reporter
can be used. [array]
--coverageThreshold A JSON string with which will be used to
configure minimum threshold enforcement for
coverage results [string]
--debug Print debugging info about your jest config.
[boolean]
--detectLeaks **EXPERIMENTAL**: Detect memory leaks in tests.
After executing a test, it will try to garbage
collect the global object used, and fail if it
was leaked [boolean] [default: false]
--detectOpenHandles Print out remaining open handles preventing Jest
from exiting at the end of a test run.
[boolean] [default: false]
--env The test environment used for all tests. This
can point to any file or node module. Examples:
`jsdom`, `node` or `path/to/my-environment.js`
[string]
--errorOnDeprecated Make calling deprecated APIs throw helpful error
messages. [boolean] [default: false]
--expand, -e Use this flag to show full diffs instead of a
patch. [boolean]
--filter Path to a module exporting a filtering function.
This method receives a list of tests which can
be manipulated to exclude tests from running.
Especially useful when used in conjunction with
a testing infrastructure to filter known broken
tests. [string]
--findRelatedTests Find related tests for a list of source files
that were passed in as arguments. Useful for
pre-commit hook integration to run the minimal
amount of tests necessary. [boolean]
--forceExit Force Jest to exit after all tests have
completed running. This is useful when resources
set up by test code cannot be adequately cleaned
up. [boolean]
--globalSetup The path to a module that runs before All Tests.
[string]
--globalTeardown The path to a module that runs after All Tests.
[string]
--globals A JSON string with map of global variables that
need to be available in all test environments.
[string]
--haste A JSON string with map of variables for the
haste module system [string]
--init Generate a basic configuration file [boolean]
--json Prints the test results in JSON. This mode will
send all other test output and user messages to
stderr. [boolean]
--lastCommit Run all tests affected by file changes in the
last commit made. Behaves similarly to
`--onlyChanged`. [boolean]
--listTests Lists all tests Jest will run given the
arguments and exits. Most useful in a CI system
together with `--findRelatedTests` to determine
the tests Jest will run based on specific files
[boolean] [default: false]
--logHeapUsage Logs the heap usage after every test. Useful to
debug memory leaks. Use together with
`--runInBand` and `--expose-gc` in node.
[boolean]
--mapCoverage Maps code coverage reports against original
source code when transformers supply source
maps.

DEPRECATED [boolean]
--maxConcurrency Specifies the maximum number of tests that are
allowed to runconcurrently. This only affects
tests using `test.concurrent`.
[number] [default: 5]
--maxWorkers, -w Specifies the maximum number of workers the
worker-pool will spawn for running tests. This
defaults to the number of the cores available on
your machine. (its usually best not to override
this default) [number]
--moduleDirectories An array of directory names to be searched
recursively up from the requiring module's
location. [array]
--moduleFileExtensions An array of file extensions your modules use.
If you require modules without specifying a
file extension, these are the extensions Jest
will look for. [array]
--moduleNameMapper A JSON string with a map from regular
expressions to module names that allow to stub
out resources, like images or styles with a
single module [string]
--modulePathIgnorePatterns An array of regexp pattern strings that are
matched against all module paths before those
paths are to be considered "visible" to the
module loader. [array]
--modulePaths An alternative API to setting the NODE_PATH env
variable, modulePaths is an array of absolute
paths to additional locations to search when
resolving modules. [array]
--noStackTrace Disables stack trace in test results output
[boolean]
--notify Activates notifications for test results.
[boolean]
--notifyMode Specifies when notifications will appear for
test results.
[string] [default: "failure-change"]
--onlyChanged, -o Attempts to identify which tests to run based on
which files have changed in the current
repository. Only works if you're running tests
in a git or hg repository at the moment.
[boolean]
--onlyFailures, -f Run tests that failed in the previous execution.
[boolean]
--outputFile Write test results to a file when the --json
option is also specified. [string]
--passWithNoTests Will not fail if no tests are found (for example
while using `--testPathPattern`.)
[boolean] [default: false]
--preset A preset that is used as a base for Jest's
configuration. [string]
--prettierPath The path to the "prettier" module used for
inline snapshots. [string]
--projects A list of projects that use Jest to run all
tests of all projects in a single instance of
Jest. [array]
--reporters A list of custom reporters for the test suite.
[array]
--resetMocks Automatically reset mock state between every
test. Equivalent to calling jest.resetAllMocks()
between each test. [boolean]
--resetModules If enabled, the module registry for every test
file will be reset before running each
individual test. [boolean]
--resolver A JSON string which allows the use of a custom
resolver. [string]
--restoreMocks Automatically restore mock state and
implementation between every test. Equivalent to
calling jest.restoreAllMocks() between each
test. [boolean]
--rootDir The root directory that Jest should scan for
tests and modules within. [string]
--roots A list of paths to directories that Jest should
use to search for files in. [array]
--runInBand, -i Run all tests serially in the current process
(rather than creating a worker pool of child
processes that run tests). This is sometimes
useful for debugging, but such use cases are
pretty rare. [boolean]
--runTestsByPath Used when provided patterns are exact file
paths. This avoids converting them into a
regular expression and matching it against every
single file. [boolean] [default: false]
--runner Allows to use a custom runner instead of Jest's
default test runner. [string]
--setupFiles A list of paths to modules that run some code
to configure or set up the testing environment
before each test. [array]
--setupFilesAfterEnv A list of paths to modules that run some code
to configure or set up the testing framework
before each test [array]
--showConfig Print your jest config and then exits. [boolean]
--silent Prevent tests from printing messages through the
console. [boolean]
--skipFilter Disables the filter provided by --filter. Useful
for CI jobs, or local enforcement when fixing
tests. [boolean]
--snapshotSerializers A list of paths to snapshot serializer modules
Jest should use for snapshot testing. [array]
--testEnvironment Alias for --env [string]
--testEnvironmentOptions Test environment options that will be passed to
the testEnvironment. The relevant options depend
on the environment. [string]
--testFailureExitCode Exit code of `jest` command if the test run
failed [string]
--testLocationInResults Add `location` information to the test results
[boolean] [default: false]
--testMatch The glob patterns Jest uses to detect test
files. [array]
--testNamePattern, -t Run only tests with a name that matches the
regex pattern. [string]
--testPathIgnorePatterns An array of regexp pattern strings that are
matched against all test paths before executing
the test. If the test path matches any of the
patterns, it will be skipped. [array]
--testPathPattern A regexp pattern string that is matched against
all tests paths before executing the test.
[array]
--testRegex A string or array of string regexp patterns that
Jest uses to detect test files. [array]
--testResultsProcessor Allows the use of a custom results processor.
This processor must be a node module that
exports a function expecting as the first
argument the result object. [string]
--testRunner Allows to specify a custom test runner. The
default is `jasmine2`. A path to a custom test
runner can be provided:
`<rootDir>/path/to/testRunner.js`. [string]
--testSequencer Allows to specify a custom test sequencer. The
default is `@jest/test-sequencer`. A path to a
custom test sequencer can be provided:
`<rootDir>/path/to/testSequencer.js` [string]
--testURL This option sets the URL for the jsdom
environment. [string]
--timers Setting this value to fake allows the use of
fake timers for functions such as setTimeout.
[string]
--transform A JSON string which maps from regular
expressions to paths to transformers. [string]
--transformIgnorePatterns An array of regexp pattern strings that are
matched against all source file paths before
transformation. [array]
--unmockedModulePathPatterns An array of regexp pattern strings that are
matched against all modules before the module
loader will automatically return a mock for
them. [array]
--updateSnapshot, -u Use this flag to re-record snapshots. Can be
used together with a test suite pattern or with
`--testNamePattern` to re-record snapshot for
test matching the pattern [boolean]
--useStderr Divert all output to stderr. [boolean]
--verbose Display individual test results with the test
suite hierarchy. [boolean]
--watch Watch files for changes and rerun tests related
to changed files. If you want to re-run all
tests when a file has changed, use the
`--watchAll` option. [boolean]
--watchAll Watch files for changes and rerun all tests. If
you want to re-run only the tests related to the
changed files, use the `--watch` option.
[boolean]
--watchPathIgnorePatterns An array of regexp pattern strings that are
matched against all paths before trigger test
re-run in watch mode. If the test path matches
any of the patterns, it will be skipped. [array]
--watchman Whether to use watchman for file crawling.
Disable using --no-watchman. [boolean]

Documentation: https://jestjs.io/

---

const greeter = require('../src/modules/greeter');

test('greeter ', () => {
expect(greeter("ABC")).toBe('Hello, ABC');
});

Intellisense will mark an error on test with following message:

Cannot find name 'test'. Do you need to install type definitions for a test runner? Try `npm i @types/jest` or `npm i @types/mocha` and then add `jest` or `mocha` to the types field in your tsconfig.ts(2582)

So all we have to do is:

$ npm install @types/jest --save-dev

Using Jest with TypeScript

Where to place Jest test file?

Right next to the source file that contains SUT (Subject Under Test).

Organizing Tests in Jest

Unit tests run against specific lines of code. So it makes sense to place them right next to that code.

|- /main| |- index.js| |- index.test.js

Why I Recommend Unit Tests in the Src Folder

The approach I like to take is that you throw the unit tests right in the app and src directories with all the other application code. Why I do this? There are basically three reasons:

You can easily ensure all javascript files have a corresponding test file.

You can easily switch between the test file and app code file.

It's easier for command line generators to create code and spec files for you this way.

Jest documentation is allowing it [source]:

(...) if you have tests in source files named with .t.js extension (...)

How to set up Jest & Enzyme like a boss

testMatch tells Jest which files it will test. I pass in 2 configurations here, one being test all files in any folder named __tests__ or test all files that end with spec.js|.jsx or test.js|.jsx

Before we run a test, we need to create one. So let’s start. In your app/src/ folder create a file called WelcomeMessage.js. In the same folder create a file called WelcomeMessage.test.js

We can tell TS compiler not to compile test files/folders by specifying exclude property in tsconfig.json:

"exclude": [
"./__tests__/",
"**/*.test.ts"
]