toBeInstanceOf()

The toBeInstanceOf() method asserts that a value is an instance of a specific class or constructor function.

Syntax

JavaScript
expect(actual).toBeInstanceOf(expected);
expect(actual).not.toBeInstanceOf(expected);

Parameters

Parameter	Type	Description
expected	Function	The constructor function or class to check against

Returns

Type	Description
void	No return value

Description

The toBeInstanceOf() method uses the instanceof operator to check if the actual value is an instance of the expected constructor or class. It checks the prototype chain to determine if the constructor function appears anywhere in the object’s prototype chain.

Usage

JavaScript
import http from 'k6/http';
import { expect } from 'https://jslib.k6.io/k6-testing/0.5.0/index.js';

class User {
  constructor(name) {
    this.name = name;
  }
}

export default function () {
  const response = http.get('https://quickpizza.grafana.com/');

  // Check response object types
  expect(response).toBeInstanceOf(Object);
  expect(response.headers).toBeInstanceOf(Object);

  // Check built-in types
  expect([1, 2, 3]).toBeInstanceOf(Array);
  expect(new Date()).toBeInstanceOf(Date);

  // Check custom classes
  const user = new User('John');
  expect(user).toBeInstanceOf(User);
  expect(user).not.toBeInstanceOf(Array);
}