Node.js assert() Function

The assert() function in Node.js is used to test conditions and validate assumptions in code, throwing an error if a condition evaluates to false.

• Part of the built-in assert module used for testing and debugging.
• Throws an Assertion Error if the provided condition is not truthy.
• Helps detect bugs by validating expected outcomes during development.

Syntax:

assert(value[, message])

• value: This parameter holds the expression that needs to be evaluated. It is of any type.
• 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 (optional)

The assert module is a built-in core module in Node.js, so no separate installation is required. It can be imported directly using:

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: Uses Node.js assert module to assert a falsy value (0), catches the resulting error, and logs it.

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

// Function call
try {
	assert(0)
} catch(error) {
	console.log("Error:", error)
}

The project structure will look like this:

Run index.js file using below command:

node index.js

Output:

Error: AssertionError [ERR_ASSERTION]: The expression evaluated to a falsy value:

    assert(0)

    at Object.<anonymous> (C:\Users\Lenovo\Downloads\index.js:6:5)
    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: 0,
  expected: true,
  operator: '=='
}

• The assert module is used to check conditions in Node.js.
• assert(0) fails because 0 is a falsy value.
• An error is thrown when the assertion fails.
• The try...catch block catches the error.
• The error message is printed to the console.

Example 2: Uses the Node.js assert module to validate a truthy value and logs “No Error Occurred” if the assertion passes, otherwise catches and displays the error.

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

// Function call
try {
	assert(1)
	console.log("No Error Occurred")
} catch(error) {
	console.log("Error:", error)
}

Run index.js file using below command:

node index.js

Output:

No Error Occurred

• The assert module is used to check conditions in Node.js.
• assert(1) passes because 1 is a truthy value.
• No error is thrown when the assertion is true.
• The try...catch block is not triggered.
• "No Error Occurred" is printed to the console.