JavaScript Pure Functions

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Questions and Answers

What is one of the characteristics of a pure function?

It minds its own business (correct)

It changes variables that existed before it was called

It returns different results for the same inputs

It is only used in object-oriented programming

What is the benefit of writing components as pure functions?

It is only used in functional programming

It makes the code more complex

It avoids an entire class of baffling bugs and unpredictable behavior (correct)

It only works in small codebases

What is an example of a pure function?

A function that returns different results for the same inputs

A function that is only used in object-oriented programming

The double function in JavaScript (correct)

A function that changes variables that existed before it was called

What does React assume about the components you write?

That they are pure functions Signup and view all the answers

What does a pure function always return given the same inputs?

The same result Signup and view all the answers

What is a benefit of using Strict Mode?

To find mistakes in your components Signup and view all the answers

What is a way to keep components pure?

By keeping changes out of the render phase Signup and view all the answers

What can be thought of as recipes?

React components Signup and view all the answers

Why does React care about purity in components?

Because it unlocks marvelous opportunities in React features Signup and view all the answers

What should you do if you can't find the right event handler for your side effect?

Attach it to your returned JSX with a useEffect call Signup and view all the answers

What is a characteristic of a pure function in React?

It always returns the same result for the same inputs Signup and view all the answers

Why is it safe to stop calculating at any time in React?

Because purity makes it safe to stop calculating at any time Signup and view all the answers

What should you do to update the screen in React?

Set state instead of mutating preexisting objects Signup and view all the answers

What is a benefit of writing pure functions in React?

It unlocks the power of the React paradigm Signup and view all the answers

Why should you strive to express your component's logic in the JSX you return?

Because it unlocks the power of the React paradigm Signup and view all the answers

What should you do when you need to 'change things' in React?

Use an event handler Signup and view all the answers

What does the component serve to React to render?

That 'dish' is the JSX Signup and view all the answers

What is the issue with calling the component multiple times?

It will produce different JSX Signup and view all the answers

What makes a component 'pure'?

It only depends on the guest prop Signup and view all the answers

What is the purpose of StrictMode in React?

To detect impure calculations Signup and view all the answers

What is the issue with the original component example?

It is not pure Signup and view all the answers

What is local mutation in React?

Changing variables and objects created during the same render Signup and view all the answers

Where do side effects usually belong in React?

Inside event handlers Signup and view all the answers

What is the result of calling a pure function twice?

Same outputs Signup and view all the answers

What is the issue with changing preexisting variables or objects while rendering?

It makes the component impure Signup and view all the answers

What is the purpose of treating inputs as read-only in React?

To make the component pure Signup and view all the answers

Study Notes

Pure Functions in React

A pure function has three characteristics:
- It minds its own business (does not change external variables or objects)
- Same inputs, same output
- Given the same inputs, the function always returns the same result
Writing pure functions in React helps avoid bugs and unpredictable behavior
React components should be written as pure functions, which means:
- They only perform calculations and do not change external variables or objects
- Same inputs, same JSX output
- Given the same inputs, the component always returns the same JSX

Benefits of Pure Functions

Allow components to be rendered in any order
Enable caching and skipping re-renders when inputs haven't changed
Allow React to safely stop calculating at any time
Enable server-side rendering
Unlock the power of the React paradigm for features like data fetching, animations, and performance

Detecting Impure Functions with Strict Mode

Strict Mode calls each component's function twice during development to detect impure functions
Impure functions can cause confusing bugs and unpredictable behavior
Pure functions are not affected by Strict Mode and will always return the same result

Local Mutation vs. External Mutation

Local mutation: changing variables or objects created within the component's scope
External mutation: changing variables or objects created outside the component's scope
Pure functions do not mutate external variables or objects

Side Effects and Event Handlers

Side effects: changes that occur outside of rendering, e.g. updating the screen, starting an animation, changing data
Event handlers: functions that React runs in response to user actions, e.g. clicking a button
Event handlers do not need to be pure functions and can cause side effects
useEffect can be used as a last resort to attach side effects to the returned JSX