What is the actual use of toContain() in expect?

Question

As we all know expect replaced with jest. Some property of expect also changes. One of them is toContain which was known as toInclude. You will find it here : https://github.com/skovhus/jest-codemods/blob/master/src/transformers/expect.js

My problem is when i am trying to use toContain in order to check whether a object match with another object,it showing me error. But earlier with toInclude it was just one line code. So as a replacement of toInclude I find it different,not exact the same.

This array works fine.

expect([2,3,4]).toContain(4);

But when i go with object,this error come up

 expect({
        name : 'Adil',
        age : 23
    }).toContain({
        age : 23
    });

This is the error

Error: expect(object).toContain(value)
Expected object:

{"age": 23, "name": "Adil"}
To contain value:

{"age": 23}

Answer 1

.toContain is for checking that an item is in an array

If you want to check the value of an object's property then you can use .toHaveProperty - here are the docs

so your example would be

expect({
    name : 'Adil',
    age : 23
}).toHaveProperty('age', 23);

... or to avoid learning another matcher you could just do:

expect({
    name : 'Adil',
    age : 23
}.age).toBe(23);

Answer 2

Its used when you want to check the existence of an item within an array. Its similar to python's x in [1,2,3]. Note that it will not give you the index of the first occurrence. It will just return a boolean