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What is Shallow Comparison from Official dox

  • A> When shallow comparing scalar values (numbers, strings) it compares their values. When comparing objects, it does not compare their attributes - only their references are compared (e.g. "do they point to same object?).

  • B> Shallow comparison is when the properties of the objects being compared is done using "===" or strict equality and will not conduct comparisons deeper into the properties. So if you shallow compare a deep nested object it will just check the reference not the values inside that object.

  • C> shallowCompare performs a shallow equality check on the current props and nextProps objects as well as the current state and nextState objects. It does this by iterating on the keys of the objects being compared and returning true (i.e. the component SHOULD GET UPDATED ) when the values of a key in each object are not strictly equal.

    shallowCompare returns true if the shallow comparison for props or state fails and therefore the component should update. shallowCompare returns false if the shallow comparison for props and state both pass and therefore the component does not need to update. .

useEffect and shallow comparison

useEffect‘s primary goal is to encompass any side effect you might want to use. A side effect is essentially something that you do within your component which affects the world at large. Whether that’s a network request, setting the document title, or what have you.

When does it run? - when the component re-renders, useEffect will check dependencies. If the dependency values changed, useEffect will run the effect

What’s the catch? React does a shallow comparison. If you use an object or an array that you mutate, React will think nothing changed. Because objects are compared by reference.

Important features useEffect skips running the effect when things don’t change. You don’t actually have to use the dependency values in the effect. You can pass in a prop value as a dependency.

Now how does shallow comparison works in React

Shallow compare does check for equality. When comparing scalar values (numbers, strings) it compares their values. When comparing objects, it does not compare their's attributes - only their references are compared (e.g. "do they point to same object in memory ?).

Let's consider following shape of user object

user = {
	name: "John",
	surname: "Doe"
};

Example 1:

const user = this.state.user;
user.name = "Jane";

console.log(user === this.state.user); // true

Notice you changed users name. Even with this change objects are equal. They references are exactly same. Meaning no change and no re-render

Example 2:

const user = clone(this.state.user);
console.log(user === this.state.user); // false

Now, without any changes to object properties they are completely different. By cloning original object you create new copy, with different reference.

And the clone function might look as this (ES6 syntax)

const clone = obj => Object.assign({}, ...obj);

Shallow compare is efficient way to detect changes. It expect you don't mutate data.

shallowCompare() function in React actually works like this (just what the official doc says above) - iterating on the keys of the objects being compared and returning true (i.e. saying that the objects are different meaning a re-render is necessary ) when the values of a key in each object are not strictly equal.

Further Reading