Node.js assert.fail() Function

The assert module provides a set of assertion functions for verifying invariants. The assert.fail() function throws an AssertionError with the provided the error message or with a default error message.

Syntax:

assert.fail([message])

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

• message This parameter holds the error message of string or error type. It is an optional parameter.

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

Installation of assert module:

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.

Example 1: Filename: index.js

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

// Function call
try {
    assert.fail();
} catch(error) {
    console.log("Error:", error)
}

Steps to run the program:

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

   node index.js

   Output:

   Error: AssertionError [ERR_ASSERTION]: Failed
   at Object. (C:\Users\Lenovo\Downloads\index.js:6:12)
   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 {
   generatedMessage: true,
   code: ‘ERR_ASSERTION’,
   actual: undefined,
   expected: undefined,
   operator: ‘fail’
   }

Example 2: Filename: index.js

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

// Function call
try {
    assert.fail(new TypeError('My custom defined error'));
} catch(error) {
    console.log("Error:", error)
}

Steps to run the program:

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

   node index.js

   Output:

   Error: TypeError: My custom defined error
   at Object. (C:\Users\Lenovo\Downloads\index.js:6:17)
   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

Reference: https://nodejs.org/dist/latest-v12.x/docs/api/assert.html#assert_assert_fail_message