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What is notStrictEqual in the Assert module of Node.js?

Talha Ashar

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The notStrictEqual method of the assert module in Node.js uses the !== operator (SameValue Comparison) to check for strict inequality between two elements.

Strict inequality compares both the types and the values of provided elements.

The process is illustrated below:

To use the notStrictEqual method, you will need to install the assert module using the command prompt, as shown below:

npm install assert


After the installation is complete, you will need to import the assert module into the program, as shown below:

const assert = require('assert');


The prototype of the notStrictEqual method is shown below:

notStrictEqual(actual, expected[, message])


Parameters

The notStrictEqual method takes the following parameters:

• actual: The first of the two values to compare.

• expected: The second of the two values to compare.

• message: An optional parameter that holds the error message in case of an AssertionError. If this parameter is left empty, a default message is assigned.

Return value

If the values are strictly equal, then the notStrictEqual method throws an AssertionError and the program terminates; otherwise, execution continues as normal.

In case of an error, the message property of the AssertionError is set equal to the message parameter. If the message parameter is not provided, a default value is assigned to the message property of the AssertionError.

Example

The code below shows how the notStrictEqual method works in Node.js.

const assert = require('assert');// evaluating first expressiontry{  assert.notStrictEqual(10, 10, "Assertion Error: The values are strictly equal.")  console.log("No error.")}catch(error){  console.log(error.message)}// evaluating second expressiontry{  assert.notStrictEqual(10, "10", "Assertion Error: The values are strictly equal.")  console.log("No error.")}catch(error){  console.log(error.message)}// evaluating third expressiontry{  assert.notStrictEqual(10, 5, "Assertion Error: The values are strictly equal.")  console.log("No error.")}catch(error){  console.log(error.message)}

Explanation

The code above uses $3$ different expressions to show the behavior of the notStrictEqual method.

In the first expression in line $5$, the actual and expected parameters are both integers with the value $10$, so the notStrictEqual method throws an error, which triggers the catch branch of the try-catch block.

The code outputs the message associated with the error, i.e., the string provided as the message parameter to the notStrictEqual method in line $5$.

In the second expression in line $14$, the actual and expected parameters are again $10$, but they have different types. The actual parameter is an integer, whereas the expected parameter is a string. Since the notStrictEqual method checks the types, it considers the two values unequal and does not throw any errors. Therefore, only the try branch of the try-catch block executes.

In the third expression in line $23$, the actual and expected parameters are unequal, so no error is thrown. Therefore, only the try branch of the try-catch block executes.

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Talha Ashar