Node.js assert.rejects() Function

assert.rejects(asyncFn[, error][, message])

• asyncFn: This parameter is async function which throws an error synchronously.
• error: This parameter can be of type Class or a Regular expression or a validation function or an object where each property will be tested for. It is an optional parameter.
• message: This parameter will be the message provided by the AssertionError if the asyncFn fails to reject. It is an optional parameter.

1. You can visit the link to Install assert module. You can install this package by using this command.

   npm install assert

   Note: Installation is an optional step as it is inbuilt Node.js module.
2. After installing the assert module, you can check your assert version in command prompt using the command.

   npm version assert

3. After that, you can just create a folder and add a file for example, index.js as shown below.

// Requiring the module
 const assert = require('assert').strict;

// Function call
(async () => {
    assert.strictEqual(1,2)
    await assert.rejects(
      async () => {
        throw new TypeError('Wrong value');
      },
      (err) => {
        assert.strictEqual(err.name, 'TypeError');
        assert.strictEqual(err.message, 'Wrong value');
        return true;
      }
    ).then(() => {
        console.log("Reject Demo")
    });
})();

1. The project structure will look like this:
2. Run index.js file using below command:

   node index.js

   Output:

   (node:12704) UnhandledPromiseRejectionWarning: AssertionError [ERR_ASSERTION]: Expected values to be strictly equal: 1 !== 2 at C:\Users\Lenovo\Downloads\Geeksforgeeks Internship\index.js:25:12 at Object. (C:\Users\Lenovo\Downloads\Geeksforgeeks Internship\NEW\Assert Function \index.js:38:3) at Module._compile (internal/modules/cjs/loader.js:1138:30) at Object.Module._extensions..js (internal/modules/cjs/loader.js:1158:10) at Module.load (internal/modules/cjs/loader.js:986:32) at Function.Module._load (internal/modules/cjs/loader.js:879:14) at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:71:12) at internal/main/run_main_module.js:17:47 (node:12704) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 1)

// Requiring the module
 const assert = require('assert').strict;

// Function call
(async () => {
    assert.strictEqual(1,1)
    await assert.rejects(
      async () => {
        throw new TypeError('Wrong value');
      },
      (err) => {
        assert.strictEqual(err.name, 'TypeError');
        assert.strictEqual(err.message, 'Wrong value');
        return true;
      }
    ).then(() => {
        console.log("Reject Demo Works Successfully")
    });
})();

1. The project structure will look like this:
2. Run index.js file using below command:

   node index.js

   Output:

   Reject Demo Works Successfully