> For the complete documentation index, see [llms.txt](https://huang-jason.gitbook.io/js-syntax/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://huang-jason.gitbook.io/js-syntax/reference-equality-shallow-equality-and-deep-equality.md).

# Reference equality, shallow equality and deep equality

`===`same reference

`shallowEqual` compare each nested object by `===`

`deepEqual` same content, and don't care about reference

`shallowEqual`When it reaches the`address`object, it doesn’t go deeper to compare the contents and relies on the two objects having the same reference.

### refer to the same object

```javascript
user1 = {
    name: "John",
    address: {
        line1: "55 Green Park Road",
        line2: "Purple Valley"  
    }
}

user2 = user1;
console.log("user1 === user2", user1 === user2);
console.log("shallowEqual(user1, user2)", shallowEqual(user1, user2));
console.log("deepEqual(user1, user2)", deepEqual(user1, user2));

// user1 === user2 true // reference equality
// shallowEqual(user1, user2) true
// deepEqual(user1, user2) true
```

### same data but are not referentially equal.

```javascript
user2 = {
    name: "John",
    address: user1.address
}
console.log("user1 === user2", user1 === user2);
console.log("shallowEqual(user1, user2)", shallowEqual(user1, user2));
console.log("deepEqual(user1, user2)", deepEqual(user1, user2));

// user1 === user2 false
// shallowEqual(user1, user2) true
// deepEqual(user1, user2) true
```

### same content

```javascript
user2 = {
    name: "John",
    address: {
        line1: "55 Green Park Road",
        line2: "Purple Valley" 
    }
}
console.log("user1 === user2", user1 === user2);
console.log("shallowEqual(user1, user2)", shallowEqual(user1, user2));
console.log("deepEqual(user1, user2)", deepEqual(user1, user2));

// user1 === user2 false
// shallowEqual(user1, user2) false
// deepEqual(user1, user2) true
```

## equality with JSON.stringify <a href="#checking-for-equality-with-jsonstringify" id="checking-for-equality-with-jsonstringify"></a>

```javascript
function jsonEqual(a,b) {
    return JSON.stringify(a) === JSON.stringify(b);
}
jsonEqual(user1, user2) // true in all three cases above

jsonEqual({a: 1, b: 2}, {b: 2, a: 1}); // false

jsonEqual({a: 5, b: function(){}}, {a: 5}); // true
```

JSON.stringify rely on \[1]the ordering of the properties, \[2]does not serialize functions, \[3]still generate the two complete JSON strings.

deepEqual is much faster at finding out False. As soon as it finds two properties that don’t match up it returns false.

***Prefer deepEqual***


---

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