JEST expect function

Basic Usage
In the Jest framework, expect is a function used to create assertions in tests. Assertions are statements that evaluate whether a given condition is true. If the condition is false, the test fails.
The expect function is central to writing tests in Jest, providing a wide array of matchers to perform assertions on various data types and structures.
Here's a breakdown of how expect works and its common usage,

 expect(value)

Common Matchers
i) toBe(value): Checks for strict equality using Object.is.

 expect(2 + 2).toBe(4);

ii) toEqual(value): Checks for deep equality. Useful for comparing objects and arrays.

 const obj = { a: 1, b: 2 };
 expect(obj).toEqual({ a: 1, b: 2 });

iii) toBeNull(): Checks if the value is null.

 expect(null).toBeNull();

iv) toBeUndefined(): Checks if the value is undefined.

 expect(undefined).toBeUndefined();

v) toBeDefined(): Checks if the value is defined.

 expect(definedValue).toBeDefined();

vi) toBeTruthy(): Checks if the value is truthy.

 expect(true).toBeTruthy();

vii) toBeFalsy(): Checks if the value is falsy.

 expect(false).toBeFalsy();

viii) toContain(item): Checks if an array or iterable contains the specified item.

 expect([1, 2, 3]).toContain(2);

ix) toHaveLength(number): Checks the length of an array or string.

 expect('hello').toHaveLength(5);

x) toMatch(regexpOrString): Checks if a string matches a regular expression or string.

 expect('hello world').toMatch(/world/);

Asynchronous Code
i) Async/Await

 test('fetches data', async () => {
   const data = await fetchData();
   expect(data).toBeDefined();
 });

ii) .resolves / .rejects

 test('fetches data resolves', () => {
   return expect(fetchData()).resolves.toBe('data');
 });

 test('fetches data rejects', () => {
   return expect(fetchData()).rejects.toMatch('error');
 });

Custom Matchers

You can also define custom matchers to extend Jest's functionality

 expect.extend({
   toBeWithinRange(received, floor, ceiling) {
     const pass = received >= floor && received <= ceiling;
     if (pass) {
       return {
         message: () => `expected ${received} not to be within range ${floor} - ${ceiling}`,
         pass: true,
       };
     } else {
       return {
         message: () => `expected ${received} to be within range ${floor} - ${ceiling}`,
         pass: false,
       };
     }
   },
 });

 test('numeric ranges', () => {
   expect(100).toBeWithinRange(90, 110);
 });