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Node.js assert() Function

  • Last Updated : 07 Aug, 2020

The assert module provides a set of assertion functions for verifying invariants. In assert() function, if the value is not truth, then a AssertionError is thrown with a message property set equal to the value of the message parameter.

Syntax:

assert(value[, message])

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

  • value: This parameter holds the expression that need 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:



  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(0)
} 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]: The expression evaluated to a falsy value:

    assert(0)

    at Object. (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: ‘==’
    }

Example 2: Filename: index.js




// Requiring the module
const assert = require('assert').strict;
  
// Function call
try {
    assert(1)
    console.log("No Error Occured")
} 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:

    No Error Occured
    

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




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