Node.js assert.rejects() Function

Last Updated : 07 Aug, 2020

The assert module provides a set of assertion functions for verifying invariants. The assert.rejects() function awaits the asyncFn promise or if the asyncFn is a function then it immediately calls the function and awaits the returned promise to complete and after that it will then check that the promise is rejected. Syntax:

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

Parameters: This function accepts following parameters as mentioned above and described below:

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.

Return Value: This function returns assertion error of object type. Installation of assert module:

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.
After installing the assert module, you can check your assert version in command prompt using the command.
```
npm version assert
```
After that, you can just create a folder and add a file for example, index.js as shown below.

Example 1: Filename: index.js

javascript

// 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")  }); })();

Steps to run the program:

The project structure will look like this:
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)

Example 2: Filename: index.js

javascript

// 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")  }); })();