JavaScript Pure Functions
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Questions and Answers

What characterizes a pure function in computer science?

  • It minds its own business and does not change external state (correct)
  • It changes objects or variables that existed before it was called
  • It is a type of object-oriented programming
  • It always returns a different result for the same inputs

What is a benefit of writing components as pure functions?

  • It helps you avoid bugs and unpredictable behavior (correct)
  • It makes your code more complex
  • It slows down your code
  • It allows you to change external state

Why is it important to keep changes out of the render phase?

  • To avoid bugs and unpredictable behavior (correct)
  • To improve performance
  • To make your code slower
  • To make your code more complex

What is an example of a pure function in math?

<p>y = 2x (D)</p> Signup and view all the answers

What is a requirement for a pure function?

<p>It must have the same output for the same inputs (C)</p> Signup and view all the answers

What assumption does React make about the components you write?

<p>They are pure functions (C)</p> Signup and view all the answers

What is the benefit of using Strict Mode to find mistakes in your components?

<p>It helps you find mistakes in your components (C)</p> Signup and view all the answers

What is a way to think of your components?

<p>As recipes (B)</p> Signup and view all the answers

What is the main issue with the component that reads and writes a guest variable declared outside of it?

<p>It is not predictable (A)</p> Signup and view all the answers

What is the purpose of passing the guest variable as a prop instead of declaring it outside the component?

<p>To make the component more predictable (B)</p> Signup and view all the answers

What are the three kinds of inputs that you can read while rendering in React?

<p>Props, state, and context (B)</p> Signup and view all the answers

What is the purpose of React's Strict Mode?

<p>To detect impure calculations (B)</p> Signup and view all the answers

What happens when you mutate a variable or object outside of a component's scope while rendering?

<p>The component becomes impure (D)</p> Signup and view all the answers

What is local mutation in the context of React components?

<p>Changing a variable created inside the component's scope (B)</p> Signup and view all the answers

Why does React care about purity in functions?

<p>To improve performance by skipping rendering components whose inputs have not changed. (C)</p> Signup and view all the answers

Where do side effects typically belong in React?

<p>In event handlers (B)</p> Signup and view all the answers

What is a key characteristic of a pure function in React?

<p>It always returns the same result for the same inputs. (B)</p> Signup and view all the answers

What is the main difference between pure and impure functions?

<p>Pure functions are predictable, impure functions are not predictable (B)</p> Signup and view all the answers

When should you use useEffect to attach an event handler?

<p>As a last resort, when you can't find the right event handler for your side effect. (B)</p> Signup and view all the answers

Why does calling a component's function twice in Strict Mode help detect impure calculations?

<p>Because it reveals the component's dependency on external variables (A)</p> Signup and view all the answers

What is the result of calling a pure function twice with the same inputs?

<p>The function returns the same output (D)</p> Signup and view all the answers

What is the benefit of having components that can run in a different environment?

<p>It enables one component to serve many user requests. (D)</p> Signup and view all the answers

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

<p>To unlock the power of React's paradigm and take advantage of its features. (A)</p> Signup and view all the answers

What is a key rule to keep in mind when working with props, state, and context in React?

<p>You should not mutate any of the inputs that your components use for rendering. (C)</p> Signup and view all the answers

What is the benefit of having components that are safe to cache?

<p>It improves performance by skipping rendering components whose inputs have not changed. (C)</p> Signup and view all the answers

Why should you avoid using event handlers to change things during rendering?

<p>Because event handlers should not be used to change things during rendering, instead use 'set' state. (C)</p> Signup and view all the answers

Flashcards

Pure function

A function that always produces the same output for the same input and has no side effects.

Impure function

A function that can produce different outputs for the same input or has side effects such as modifying external state.

Side effect free function

A function that does not modify any data outside its own scope. It does not interact with external state, such as databases or files.

Pure React component

A component that encapsulates its logic and renders the same output for the same input. These components are predictable and testable.

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Impure React component

A component that relies on external state or modifies data outside its own scope. These components are less predictable and more difficult to test.

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Strict Mode

A mechanism in React that helps developers identify impure calculations by calling components twice and comparing the output.

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Local mutation

The process of modifying data within a component's own scope, without affecting external state.

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External mutation

The process of modifying data outside a component's own scope, which can lead to unpredictable behavior and side effects.

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Rendering inputs

Inputs that React components can read while rendering, including props, state, and context.

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Deterministic function

A function that always returns the same output for the same inputs.

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Non-deterministic function

A function that can produce different outputs for the same inputs, even if the environment is the same.

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useEffect hook

A hook in React that allows developers to perform side effects, such as fetching data or setting up event listeners.

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State updates

The process of changing the state of a component in response to user interaction or other events.

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Event handling

The process of changing the state of a component in response to events, such as user interactions or data updates.

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Child component

A component that receives data from its parent component through props.

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Parent component

A component that passes data to its child components through props.

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Props

Values passed from a parent component to its child component.

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Context

A way to share data and state between different components without explicitly passing it down the component tree.

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Lifecycle method

A mechanism that allows a React component to run a function after rendering, such as fetching data or updating the DOM.

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Component reusability

Creating a component that can be reused in different parts of an application, improving code organization and maintainability.

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Component isolation

Ensuring that a component functions as intended, regardless of its environment or context. Focus on its logic and behaviour.

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Server-side rendering

The ability of a component to be rendered on the server-side before being sent to the browser, improving the initial load time of a web page.

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Performance optimization

A component that is optimised for performance, avoiding unnecessary re-renders and reducing the amount of work needed to update the UI.

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Client-side rendering

The ability for a component to be rendered on the client-side after the initial load, improving the user experience and interactivity.

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Component testing

The process of testing a component's functionality and behavior to ensure that it meets the desired requirements.

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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

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Learn about the benefits of pure functions in JavaScript and how to avoid bugs by following specific rules. Discover how to keep components pure and use Strict Mode to find mistakes.

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