What is assert.deepStrictEqual() in Node.js?

To use the deepStrictEqual 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 deepStrictEqual method is shown below:

deepStrictEqual(actual, expected[, message])

Parameters

The deepStrictEqual 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 values are not strictly deep-equal, then the deepStrictEqual 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 deepStrictEqual 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.deepStrictEqual(first, second, "Assertion Error: The objects are not deep strictly equal.")
  console.log("No error.")
}
catch(error){
  console.log(error.message)
}
// evaluating second expression
try{
  assert.deepStrictEqual(first, third, "Assertion Error: The objects are not deep strictly equal.")
  console.log("No error.")
}
catch(error){
  console.log(error.message)
}
// evaluating third expression
try{
  assert.deepStrictEqual(first, fourth, "Assertion Error: The objects are not deep strictly equal.")
  console.log("No error.")
}
catch(error){
  console.log(error.message)
}
// evaluating fourth expression
try{
  assert.deepStrictEqual(first, fifth, "Assertion Error: The objects are not deep strictly 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 deepStrictEqual 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 the deepStrictEqual method does not throw any errors. Therefore, only the try branch of the try-catch block executes.

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 deepStrictEqual method requires the same types, it considers the two objects unequal and 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 deepStrictEqual method in line $21$ .

In the third expression in line $30$ , the actual and expected parameters are unequal, 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 deepStrictEqual method in line $30$ .

Finally, the expression in line $39$ involves a comparison with a Prototype. Since the Prototype is not strictly deep-equal with first, 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 deepStrictEqual method in line $39$ .

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

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