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

Talha Ashar

Grokking Modern System Design Interview for Engineers & Managers

Ace your System Design Interview and take your career to the next level. Learn to handle the design of applications like Netflix, Quora, Facebook, Uber, and many more in a 45-min interview. Learn the RESHADED framework for architecting web-scale applications by determining requirements, constraints, and assumptions before diving into a step-by-step design process.

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 expression
try{
assert.notStrictEqual(10, 10, "Assertion Error: The values are strictly equal.")
console.log("No error.")
}
catch(error){
console.log(error.message)
}
// evaluating second expression
try{
assert.notStrictEqual(10, "10", "Assertion Error: The values are strictly equal.")
console.log("No error.")
}
catch(error){
console.log(error.message)
}
// evaluating third expression
try{
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 33 different expressions to show the behavior of the notStrictEqual method.

In the first expression in line 55, the actual and expected parameters are both integers with the value 1010, 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 55.

In the second expression in line 1414, the actual and expected parameters are again 1010, 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 2323, 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
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Grokking Modern System Design Interview for Engineers & Managers

Ace your System Design Interview and take your career to the next level. Learn to handle the design of applications like Netflix, Quora, Facebook, Uber, and many more in a 45-min interview. Learn the RESHADED framework for architecting web-scale applications by determining requirements, constraints, and assumptions before diving into a step-by-step design process.

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