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Node.js assert() Function

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  • Difficulty Level : Easy
  • Last Updated : 16 Jun, 2022
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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:

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


Steps to run the program:

  • 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. (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 

javascript




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


Steps to run the program:

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

Output:

No Error Occurred

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


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