Introduction to assert-plus

The assert-plus module is a powerful assertion library in Node.js that provides a comprehensive set of assertion functions for validating inputs, parameters, and states. It extends the basic assertions provided by Node.js’s built-in assert module, offering a more feature-rich and improved developer experience.

In this article, we’ll explore a wide range of APIs offered by assert-plus and demonstrate how they can be leveraged to ensure the robustness and reliability of your Node.js applications. Each API is accompanied by a code snippet for better understanding, followed by an example application to illustrate practical usage.

Basic Assertions

assert.ok(value, [message])

Asserts that the given value is truthy. If not, it throws an error with the optional message.

  const assert = require('assert-plus');
  assert.ok(true, 'This will not throw');
  assert.ok(false, 'This will throw');

assert.equal(actual, expected, [message])

Asserts that the actual value is equal to the expected value. Throws an error if they are not equal.

  const assert = require('assert-plus');
  assert.equal(3, 3, '3 is equal to 3');
  assert.equal(3, 4, 'This will throw');

assert.strictEqual(actual, expected, [message])

Asserts that actual value is strictly equal (===) to the expected value.

  const assert = require('assert-plus');
  assert.strictEqual('hello', 'hello', 'Strings match');
  assert.strictEqual(5, '5', 'This will throw');

assert.notEqual(actual, expected, [message])

Asserts that the actual value is not equal to the expected value. Throws an error if they are equal.

  const assert = require('assert-plus');
  assert.notEqual(3, 4, 'This will not throw');
  assert.notEqual(3, 3, 'This will throw');

assert.fail([message])

Throws an assertion error with the given message.

  const assert = require('assert-plus');
  assert.fail('This will always throw');

Type Checking

assert.array(object, [message])

Asserts that the given object is an array.

  const assert = require('assert-plus');
  assert.array([1, 2, 3], 'This is an array');
  assert.array('not an array', 'This will throw');

assert.bool(object, [message])

Asserts that the given object is a boolean.

  const assert = require('assert-plus');
  assert.bool(true, 'This is a boolean');
  assert.bool('not a boolean', 'This will throw');

assert.number(object, [message])

Asserts that the given object is a number.

  const assert = require('assert-plus');
  assert.number(123, 'This is a number');
  assert.number('not a number', 'This will throw');

assert.string(object, [message])

Asserts that the given object is a string.

  const assert = require('assert-plus');
  assert.string('hello', 'This is a string');
  assert.string(123, 'This will throw');

assert.object(object, [message])

Asserts that the given object is an object.

  const assert = require('assert-plus');
  assert.object({}, 'This is an object');
  assert.object(123, 'This will throw');

assert.function(object, [message])

Asserts that the given object is a function.

  const assert = require('assert-plus');
  assert.function(function () {}, 'This is a function');
  assert.function(123, 'This will throw');

Example Application

Let’s create a simple application that utilizes the above assert-plus APIs to validate user input in an HTTP server.

  const http = require('http');
  const assert = require('assert-plus');

  const server = http.createServer((req, res) => {
    if (req.method === 'POST' && req.url === '/validate') {
      let body = '';
      req.on('data', chunk => {
        body += chunk.toString();
      });
      
      req.on('end', () => {
        const data = JSON.parse(body);
        
        try {
          assert.string(data.name, 'name must be a string');
          assert.number(data.age, 'age must be a number');
          assert.ok(data.agreeToTerms, 'must agree to terms');

          res.writeHead(200, {'Content-Type': 'application/json'});
          res.end(JSON.stringify({ message: 'Validation passed' }));
        } catch (error) {
          res.writeHead(400, {'Content-Type': 'application/json'});
          res.end(JSON.stringify({ message: error.message }));
        }
      });
    } else {
      res.writeHead(404);
      res.end();
    }
  });

  server.listen(3000, () => {
    console.log('Server listening on port 3000');
  });

In this example, we create a simple HTTP server that validates JSON input using the assert-plus module. If the input is valid, a success message is returned; otherwise, an error message is sent back to the client.

Integrating assertion libraries like assert-plus can significantly enhance the robustness and reliability of your Node.js applications by ensuring that your code behaves as expected under various conditions.

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