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

Talha Ashar

The deepStrictEqual method of the assert module in Node.js uses the === operator (SameValue Comparison) to check for strict equality between two objects.

Strict deep equality means that the values and types of child objects are also compared.

The process is illustrated below:

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 44 different expressions to show the behavior of the deepStrictEqual method.

First, 55 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., 3030. Finally, fifth is a Prototype Object created using first.

In the first expression in line 1212, the actual and expected parameters are identical objects with the value 1010, 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 2121, 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 2121.

In the third expression in line 3030, 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 3030.

Finally, the expression in line 3939 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 3939.

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

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