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# What is the notDeepEqual method of the assert module in Node.js?

Talha Ashar

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The notDeepEqual method of the assert module in Node.js uses the != operator (Abstract Equality Comparison) to check for inequality between two objects.

Deep equality means that the values of child objects are also compared.

The process is illustrated below:

Note: You can view a list of rules for the != operator here.

To use the notDeepEqual 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 notDeepEqual method is shown below:

notDeepEqual(actual, expected[, message])

## Parameters

The notDeepEqual method takes the following parameters:

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

• expected: The second of the two objects 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 objects are equal, then the notDeepEqual 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 notDeepEqual method works in Node.js:

const assert = require('assert');

// initializing objects
const first = { a : { b : 10 } };
const second = { a : { b : 10 } };
const third = { a : { b : '10' } };
const fourth = { a : { b : 30 } };
const fifth = Object.create(first);

// evaluating first expression
try{
assert.notDeepEqual(first, second, "Assertion Error: The objects are deep equal.")
console.log("No error.")
}
catch(error){
console.log(error.message)
}

// evaluating second expression
try{
assert.notDeepEqual(first, third, "Assertion Error: The objects are deep equal.")
console.log("No error.")
}
catch(error){
console.log(error.message)
}

// evaluating third expression
try{
assert.notDeepEqual(first, fourth, "Assertion Error: The objects are deep equal.")
console.log("No error.")
}
catch(error){
console.log(error.message)
}

// evaluating fourth expression
try{
assert.notDeepEqual(first, fifth, "Assertion Error: The objects are deep equal.")
console.log("No error.")
}
catch(error){
console.log(error.message)
}

## Explanation

The code above uses $4$ different expressions to show the behavior of the notDeepEqual method.

First, $5$ different objects are initialized. The objects first and second are identical. The object third has the same structure as first, but third has a value of type string rather than an integer. Similarly, fourth also has the same structure as first, but the value is different, i.e., $30$. Finally, fifth is a Prototype Object created using first.

In the first expression in line $12$, the actual and expected parameters are identical objects with the value $10$, so an error is thrown, 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 notDeepEqual method in line $12$.

In the second expression in line $21$, the actual and expected parameters have identical structures but have different types. The actual parameter is an integer, whereas the expected parameter is a string. Since the deepEqual method only checks values, it considers the objects equal and throws an error. The error 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 notDeepEqual method in line $21$.

In the third expression in line $30$, the actual and expected parameters are unequal, so the notDeepEqual method does not throw any errors. Therefore, only the try branch of the try-catch block executes.

Finally, the expression in line $39$ involves a comparison with a Prototype. Since the implementation of the notDeepEqual method does not test Prototypes, no error is thrown. Therefore, only the try branch of the try-catch block executes.

Note: You can read up further on the notDeepEqual method and other similar functions in the official documentation.

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Talha Ashar