How to test “this” calling context with jest?

How does one test in jest that a function is called with the right this context?

jest

this

I only was able to find how to test for arguments passed in, with toHaveBeenCalledWith. But it doesn't test this context, and I can't find any other API for this or examples.

this

1 Answer
1

JEST

Create a mock of the function then use mock.instances to check the value of this.

mock.instances

this

test('the value of this using Jest', () =>
const foo =
name: 'foo',
func: function()
return `my name is $this.name`;


const mockFunc = jest.spyOn(foo, 'func'); // spy on foo.func()

expect(foo.func()).toBe('my name is foo');
expect(mockFunc.mock.instances[0]).toBe(foo); // called on foo

mockFunc.mockClear();

const bar =
name: 'bar',
func: foo.func // use the func from foo


expect(bar.func()).toBe('my name is bar');
expect(mockFunc.mock.instances[0]).toBe(bar); // called on bar
);


SINON

For better semantics Sinon provides direct access to this using thisValue.

Sinon

this

thisValue

import * as sinon from 'sinon';

test('the value of this using Sinon', () =>
const foo =
name: 'foo',
func: function()
return `my name is $this.name`;


const spy = sinon.spy(foo, 'func'); // spy on foo.func()

expect(foo.func()).toBe('my name is foo');
expect(spy.lastCall.thisValue).toBe(foo); // called on foo

const bar =
name: 'bar',
func: foo.func // use the func from foo


expect(bar.func()).toBe('my name is bar');
expect(spy.lastCall.thisValue).toBe(bar); // called on bar
);


jest

this

instances

this

Sinon

this

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