Node.js - assert.fail() Function

---

---

The Node.js assert.fail() function will throw an AssertionError along with the error message we passed as a parameter or with the default error message. If the message, we passed is an instance of an Error then it will be thrown instead of the AssertionError.

The assert module provides a set of assertion functions for verifying invariants. The Node.js assert.fail() function is an inbuilt function of the assert module of Node.js.

Syntax

Following is the syntax of Node.js assert.fail() function −

assert.fail([message]);

Parameters

• message − (optional) String or Error type can be passed as an input into this parameter. By default, it will print 'Failed'.

Return Value

This method will return an AssertionError along with the message passed in the parameter to the output.

Example

In the example below, we are passing a text to the message parameter of the Node.js assert.fail() function.

const assert = require('assert');
const err = 'Warning';
assert.fail(err);

Output

When we compile and run the code, the function will throw AssertionError along with the message to the output.

assert.js:79
   throw new AssertionError(obj);
   ^
   
AssertionError [ERR_ASSERTION]: Warning
   at Object.<anonymous> (/home/cg/root/639c30a0a5fb2/main.js:4:8)
   at Module._compile (internal/modules/cjs/loader.js:702:30)
   at Object.Module._extensions..js (internal/modules/cjs/loader.js:713:10)
   at Module.load (internal/modules/cjs/loader.js:612:32)
   at tryModuleLoad (internal/modules/cjs/loader.js:551:12)
   at Function.Module._load (internal/modules/cjs/loader.js:543:3)
   at Function.Module.runMain (internal/modules/cjs/loader.js:744:10)
   at startup (internal/bootstrap/node.js:238:19)
   at bootstrapNodeJSCore (internal/bootstrap/node.js:572:3)

Example

In the example below, we are passing an instance of an Error to the message parameter of the Node.js assert.fail() function.

const assert = require('assert');

const err = new TypeError('Alert, this is a warning...')
assert.fail(err);

Output

When we compile and run the code, the function will not throw AssertionError instead; the Error will be thrown.

assert.js:77
   if (obj.message instanceof Error) throw obj.message;
   ^
   
TypeError: Alert, this is a warning...
   at Object.<anonymous> (/home/cg/root/639c30a0a5fb2/main.js:3:13)
   at Module._compile (internal/modules/cjs/loader.js:702:30)
   at Object.Module._extensions..js (internal/modules/cjs/loader.js:713:10)
   at Module.load (internal/modules/cjs/loader.js:612:32)
   at tryModuleLoad (internal/modules/cjs/loader.js:551:12)
   at Function.Module._load (internal/modules/cjs/loader.js:543:3)
   at Function.Module.runMain (internal/modules/cjs/loader.js:744:10)
   at startup (internal/bootstrap/node.js:238:19)
   at bootstrapNodeJSCore (internal/bootstrap/node.js:572:3)

Example

In the example below, we are not passing any text to the message parameter to the function. So it will assign the default message as 'Failed'.

const assert = require('assert');
assert.fail();

Output

On executing the above program, it will generate the following output −

assert.js:79
   throw new AssertionError(obj);
   ^
   
AssertionError [ERR_ASSERTION]: Failed
   at Object.<anonymous> (/home/cg/root/639c30a0a5fb2/main.js:3:8)
   at Module._compile (internal/modules/cjs/loader.js:702:30)
   at Object.Module._extensions..js (internal/modules/cjs/loader.js:713:10)
   at Module.load (internal/modules/cjs/loader.js:612:32)
   at tryModuleLoad (internal/modules/cjs/loader.js:551:12)
   at Function.Module._load (internal/modules/cjs/loader.js:543:3)
   at Function.Module.runMain (internal/modules/cjs/loader.js:744:10)
   at startup (internal/bootstrap/node.js:238:19)
   at bootstrapNodeJSCore (internal/bootstrap/node.js:572:3)