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Node.js assert.doesNotThrow() Function

Last Updated : 07 Aug, 2020
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The assert module provides a set of assertion functions for verifying invariants. The assert.doesNotThrow() function asserts that the function fn does not throw an error. Syntax: 
assert.doesNotThrow(fn[, error][, message])
Parameters: This function accepts the following parameters as mentioned above and described below:
  • fn: This parameter is a function which does not throw an error.
  • error: This parameter is a regular expression or function. It is the specified error. It is an optional parameter.
  • 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 javascript 
// Requiring the module
const assert = require('assert').strict;

// Function call
try {
    assert.doesNotThrow(
        () => {
          throw new TypeError('Wrong value');
        },
    );
} catch(error) {
    console.log("Error:", error)
}
Steps to run the program:
  1. The project structure will look like this: Image
  2. Run index.js file using below command: 
    node index.js
    Output:
    Error: AssertionError [ERR_ASSERTION]: Got unwanted exception. Actual message: "Wrong value" 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: false, code: 'ERR_ASSERTION', actual: TypeError: Wrong value at C:\Users\Lenovo\index.js:8:17 at getActual (assert.js:657:5) at Function.doesNotThrow (assert.js:805:32) at Object. (C:\Users\NEW\Assert Function\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, expected: undefined, operator: 'doesNotThrow' }
Example 2: Filename: index.js javascript 
// Requiring the module
const assert = require('assert').strict;

// Function call
try {
    assert.doesNotThrow(
        () => {
          throw new TypeError('The Wrong value error');
        },
        /abcd/,
        'Whoops'
    );
} catch(error) {
    console.log("Error:", error)
}
Steps to run the program:
  1. The project structure will look like this: Image
  2. Run index.js file using below command: 
    node index.js
    Output:
    Error: TypeError: The Wrong value error at C:\Users\Lenovo\Downloads\Geeksforgeeks Internship\NEW\Assert\index.js:8:17 at getActual (assert.js:657:5) at Function.doesNotThrow (assert.js:805:32) at Object. (C:\Internship\NEW\Assert Function\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
Reference: https://nodejs.org/dist/latest-v12.x/docs/api/assert.html#assert_assert_doesnotthrow_fn_error_message

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gouravhammad
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