Jest runs TypeScript single tests and increases coverage

This article shows you how to use Jest to run TS single tests without compiling, and increase single test coverage to avoid under-testing or code corruption. If you encounter problems, refer to Jest Test ESM

Start the configuration

You can select the following manual configuration mode if you are worried about one-click configuration on the CLI

CLI One-click configuration

TS

$ tnpx create-app-test --type ts
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JS

$ tnpx create-app-test --type js
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Note that coverage is automatically set to 100%, which can be customized by adding the parameter — Coverage =80

Manual configuration

Install dependencies

npm i -D jest typescript ts-jest @types/jest
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Generate the jest. Config. Js

npx ts-jest config:init
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Run a single test

npm test
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Single test sample

The single test writing specification comes from the single test section of applets best Practices 👮.

import { isFunction, isPromise, values } from '.. /src/lib/lite-lodash'

describe('lite-lodash'.() = > {
  describe('isPromise'.() = > {
    it('should Promise.resolve be a promise'.() = > {
      const input = Promise.resolve();
      const actual = isPromise(input);
      const expected = true;

      expect(actual).toEqual(expected);
    });

    it('should new Promise be a promise'.() = > {
      const input = new Promise(() = > {});
      const actual = isPromise(input);
      const expected = true;

      expect(actual).toEqual(expected);
    });
  });
});
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Running results:

IsPromise ✓ shouldPromiseResolve be a promise ✓ shouldnew Promise be a promise (1 ms)
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Coverage reporting and threshold Settings

Thresholds are set to ensure that code quality does not deteriorate with iteration.

Before running the command, ensure that the previous code is committed. Because the configuration file will be overwritten, or jump directly to the jest.config.js section below.

Setting coverage

npx jest --init
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➤ It seems that you already have a jest configuration, do you want to override It? … yes

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

➤ Would you like to use Typescript for the configuration file? … no

➤ Choose the test environment that will be used for testing › node

➤ ➤ Do you want Jest to add coverage reports? … yes

➤ Which provider should be used to instrument code for coverage? Holds the v8

➤ ➤ Automatically clear mock calls and instances between every test? … no

📝 Configuration file created at jest.config.js

jest.config.js

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

+ coverageDirectory: "coverage",
+ coverageProvider: "v8",
+ coverageThreshold: {
    "global": {
      "branches": 100,
      "functions": 100,
      "lines": 100,
      "statements": 100
    }
  },
}
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package.json

   "scripts": {
     "build": "tsc --declaration",
     "preversion": "npm run build",
- "test": "jest"
+ "test": "jest --coverage"
   },
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.gitignore

Coverage reports do not need to be saved to the code base. For details on how to read coverage reports, see writing an article.

+coverage/
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Generating coverage reports

npm test

----------------|---------|----------|---------|---------|-------------------
File            | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s
----------------|---------|----------|---------|---------|-------------------
All files       |     100 |      100 |     100 |     100 |
 lite-lodash.ts |     100 |      100 |     100 |     100 |
----------------|---------|----------|---------|---------|-------------------
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Deliberately under-test functions to see if they fail

----------------|---------|----------|---------|---------|-------------------
File            | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s
----------------|---------|----------|---------|---------|-------------------
All files       |   72.73 |      100 |   66.67 |   72.73 |
 lite-lodash.ts |   72.73 |      100 |   66.67 |   72.73 | 5-7
----------------|---------|----------|---------|---------|-------------------
Jest: "global" coverage threshold for statements (100%) not met: 72.73%
Jest: "global" coverage threshold for lines (100%) not met: 72.73%
Jest: "global" coverage threshold for functions (100%) not met: 66.67%

npm ERR! code 1
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Single test failure Exit code is not equal to 0, resulting in CI failure. The problem of preventing code undertesting or deterioration with coverage is achieved 👍.

reference

• ts-jest

TODO

• Cli One-click configuration