React.js 中声明式和命令式的区别？

Question

Recently I've been studying a lot about the functionality and the ways to use the Facebook JavaScript library React.js. When speaking of its differences to the rest of the JavaScript world often the two programming styles declarative and imperative are mentioned.

What's the difference between both?

Answer 1

A declarative style, like what react has, allows you to control flow and state in your application by saying "It should look like this". An imperative style turns that around and allows you to control your application by saying "This is what you should do".

The benefit of declarative is that you don't get bogged down in the implementation details of representing the state. You're delegating the organizational component of keeping your application views consistent so you just have to worry about state.

Imagine you have a butler, who is kind of a metaphor for a framework. And you would like to make dinner. In an imperative world, you would tell them step by step how to make dinner. You have to provide these instructions:

Go to the kitchen
Open fridge
Remove chicken from fridge
...
Bring food to the table

In a declarative world, you would simply describe what you want

I want dinner with chicken.

If your butler doesn't know how to make chicken, then you cannot operate in a declarative style. Just like if Backbone doesn't know how to mutate itself to do a certain task, you can't just tell it to do that task. React is able to be declarative because it "knows how to make chicken", for example. Compared to Backbone, which only knows how to interface with the kitchen.

Being able to describe the state reduces the surface area for bugs dramatically, which is a benefit. On the other hand, you might have less flexibility in how things occur because you're delegating or abstracting away how you implement the state.

Answer 2

Imagine a simple UI component, such as a "Like" button. When you tap it, it turns blue if it was previously grey, and grey if it was previously blue.

The imperative way of doing this would be:

if( user.likes() ) {
        if( hasBlue() ) {
            removeBlue();
            addGrey();
        } else {
            removeGrey();
            addBlue();
        }
    }

Basically, you have to check what is currently on the screen and handle all the changes necessary to redraw it with the current state, including undoing the changes from the previous state. You can imagine how complex this could be in a real-world scenario.

In contrast, the declarative approach would be:

return this.state.liked ? <blueLike /> : <greyLike />;

Because the declarative approach separates concerns, this part of it only needs to handle how the UI should look in a sepecific state, and is therefore much simpler to understand.

Answer 3

It is best to compare React (declarative) and JQuery (imperative) to show you the differences.

In React, you only need to describe the final state of your UI in the render() method, without worrying about how to transition from previous UI state to the new UI state. Eg,

render() {
  const { price, volume } = this.state;
  const totalPrice = price * volume;

  return (
    <div>
      <Label value={price} className={price > 100 ? 'expensive' : 'cheap'} ... />
      <Label value={volume} className={volume > 1000 ? 'high' : 'low'} ... />
      <Label value={totalPrice} ... />
      ...
    </div>
  )
}

On the other hand, JQuery requires you to transition your UI state imperatively, eg, selecting the label elements and update their text and CSS:

updatePrice(price) {
  $("#price-label").val(price);
  $("#price-label").toggleClass('expansive', price > 100);
  $("#price-label").toggleClass('cheap', price < 100);

  // also remember to update UI depending on price 
  updateTotalPrice();
  ... 
}

updateVolume(volume) {
  $("#volume-label").val(volume);
  $("#volume-label").toggleClass('high', volume > 1000);
  $("#volume-label").toggleClass('low', volume < 1000);
  
  // also remember to update UI depending on volume
  updateTotalPrice();
  ... 
}

updateTotalPrice() {
  const totalPrice = price * volume;
  $("#total-price-label").val(totalPrice);
  ...
}

In the real world scenario, there will be many more UI elements to be updated, plus their attributes (eg, CSS styles, and event listeners), etc. If you do this imperatively using JQuery, it will become complex and tedious; it is easy to forget to update some parts of the UI, or forget to remove old event handlers (cause memory leak or handler fires multiple times ), etc. This is where bugs happen, ie, UI state and the model state are out of sync.

States out of sync will never happen to React's declarative approach, because we only need to update the model state, and React is responsible to keep the UI and model states in sync.

• Under the hook, React will updates all changed DOM elements using imperative code.

You may also read my answer for What is the difference between declarative and imperative paradigm in programming? .

PS: from above jQuery example, you may think what if we put all the DOM manipulations in a updateAll() method, and call it every time when any of our model state changes, and the UI will never be out of sync. You are correct, and this is effectively what React does, the only difference is that jQuery updateAll() will cause many unnecessary DOM manipulations, but React will only update changed DOM elements using its Virtual DOM Diffing Algorithm .

Answer 4

This is great analogy:

*An imperative response : Go out of the north exit of the parking lot and take a left. Get on I-15 south until you get to the Bangerter Highway exit. Take a right off the exit like you're going to Ikea. Go straight and take a right at the first light. Continue through the next light then take your next left. My house is #298.

A declarative response : My address is 298 West Immutable Alley, Draper Utah 84020*

https://tylermcginnis.com/imperative-vs-declarative-programming/

Answer 5

Imperative code instructs JavaScript on how it should perform each step. With declarative code, we tell JavaScript what we want to be done, and let JavaScript take care of performing the steps.

React is declarative because we write the code that we want and React is in charge of taking our declared code and performing all of the JavaScript/DOM steps to get us to our desired result.

Answer 6

A real-life parallel in the imperative world would be entering a bar for a beer, and giving the following instructions to the bartender:

--Take a glass from the shelf

--Put the glass in front of the draft

--Pull down the handle until the glass is full

--Pass me the glass.

In the declarative world, instead, you would just say: "Beer, please."

The declarative approach of asking for a beer assumes that the bartender knows how to serve one, and that is an important aspect of the way declarative programming works.

In declarative programming, developers only describe what they want to achieve and there's no need to list all the steps to make it work.

The fact that React offers a declarative approach makes it easy to use, and consequently, the resulting code is simple, which often leads to fewer bugs and more maintainability.

Since React follows a declarative paradigm, and there's no need to tell it how to interact with the DOM; you just DECLARE what you want to see on the screen and React does the job for you.

Answer 7

Declarative programming is a style of programming where applications are structured in a way that prioritizes describing what should happen over defining how it should happen.

In order to understand declarative programming, Let's compare it with imperative programming (style of programming that's only concerned with how to achieve results with code).

Example: making a string URL-friendly. Typically, this can be accomplished by replacing all of the spaces in a string with hyphens, since spaces are not URL-friendly. First, An imperative approach to this task:

const string = "difference between declarative and imperative in react.js";
const urlFriendly = "";
for (var i = 0; i < string.length; i++) {
    if (string[i] === " ") {
      urlFriendly += "-";
    } else {
      urlFriendly += string[i];
    }
}
console.log(urlFriendly); // "difference-between-declarative-and-imperative-in-react-js"

In this example, we loop through every character in the string, replacing spaces as they occur. The structure of this program is only concerned with how such a task can be achieved. We use a for loop and an if statement and set values with an equality operator. Just looking at the code alone does not tell us much as imperative programs require lots of comments in order to understand what's going on.

Now let's look at a declarative approach to the same problem:

const string = "Difference between declarative and imperative in React.js?";
const urlFriendly = string.replace(/ /g, "-");
console.log(urlFriendly);

Here we are using string.replace along with a regular expression to replace all instances of spaces with hyphens. Using string.replace is a way of describing what's supposed to happen: spaces in the string should be replaced. The details of how spaces are dealt with are abstracted away inside the replace function.

In a declarative program, the syntax itself describes what should happen, and the details of how things happen are abstracted away.

Essentially, declarative programming produces applications that are easier to reason about, and when it's easier to reason about an application, that application is easier to scale. Additional details about the declarative programming paradigm can be found at the Declarative Programming wiki .

Now, let's consider the task of building a document object model. An imperative approach would be concerned with how the DOM is constructed:

const target = document.getElementById("target");
const wrapper = document.createElement("div");
const headline = document.createElement("h1");
wrapper.id = "welcome";
headline.innerText = "Hello World";
wrapper.appendChild(headline);
target.appendChild(wrapper);

This code is concerned with creating elements, setting elements, and adding them to the document. It would be very hard to make changes, add features, or scale 10,000 lines of code where the DOM is constructed imperatively.

Now let's take a look at how we can construct a DOM declaratively using a React component:

const { render } = ReactDOM;
const Welcome = () => (
   <div id="welcome">
      <h1>Hello World</h1>
   </div>
);

render(<Welcome />, document.getElementById("target"));

React is declarative. Here, the Welcome component describes the DOM that should be rendered. The render function uses the instructions declared in the component to build the DOM, abstracting away the details of how the DOM is to be rendered. We can clearly see that we want to render our Welcome component into the element with the ID of target

Source: Modern Patterns for Developing React Apps

Answer 8

Declarative programming is a programming paradigm … that expresses the logic of a computation without describing its control flow.

Imperative programming is a programming paradigm that uses statements that change a program's state.

ref link:- https://codeburst.io/declarative-vs-imperative-programming-a8a7c93d9ad2

Answer 9

• Declarative is allowed you to control all view. (like as state management)
• the Imperative is allowed you control around view. (like as $(this))

Answer 10

I'll start with an analogy: I have two cars, in my two cars I want the temperatue inside my car to be normal room temperature ~ 72°F. In the first (older) car, there's two knobs to control the temperature (1 knob to control the temperature and 1 knob to control the airflow). When it gets too hot, I have to adjust the first knob to lower the temperature and maybe change the airflow) and vice verse if it's too cold. This is imperative work! I have to manage the knobs myself. In my second (newer) car, I can set/declare the temperature. Which means I don't have to fiddle with the knobs to adjust the temperature my car knows I declare/set it to 72°F and my car will do the imperative work to get to that state.

React is the same, you declare the markup/template and stat then React does the imperative work to keep the DOM in sync with your app.

<button onClick={activateTeleporter}>Activate Teleporter</button>

Instead of using .addEventListener() to set up event handling, we declare what we want. When the button is clicked, it'll run the activateTeleporter function.

Answer 11

Declarative vs Imperative

Declarative Programming is like asking your friend to paint your House. You don't care how they clean it,what colour they use to paint,How many resources they used to complete it`.

//Declarative For Searching element from an array
  array.find(item)

The opposite of declarative is imperative. A common example of an imperative approach is is you told your friend exactly what to do to paint your House.

• Wash the house with detergent.
• Use Narolac paint or asian paint
• Paint the roof with Green color.
• Get 3 members to contract,etc.

//Imperative Algo

def imperative_search(array, item)
  for i in array do
    if i == item
      return item
    end
  end
  return false
end

Answer 12

This is my understanding so far:

Declarative code (almost?) always is a layer of abstraction above code that is more imperative in nature.

React allows you to write declarative code that is a layer of abstraction above the imperative code that interacts with the DOM directly (eg. the diffing algorithm ). If you need to write imperative code (ie. interact with the DOM directly) React provides Refs as an escape hatch .

Answer 13

Explaining every bit of steps is an Imperative approach eg We have to create a paragraph tag with Hello World! text inside.

//Imperative 

const para = document.createElement('p');
para.innerText = 'Hello World !';
document.querySelector('#root').appendChild(para);

Defining desired end target state without specifying exact procedure. ie p tag with text, without telling to createElement or innerText

 //Declarative 

 import React from "react"; 
 import ReactDOM from "react-dom"; 
 
 const App = () =>{
  return(<p>Hello World !</p>);
 }

 ReactDOM.render(<App />, document.getElementById("root"));