What is the reason behind my styled component only displaying the final state in its className logs?

Question

What is the reason behind my styled component only displaying the final state in its className logs?

Here is my implementation using styled components with the version

"@types/styled-components": "^5.1.26"

and I'll provide you with an example of my code.

// index.tsx
import React, { useEffect, useState } from 'react';
import { ProfileHomeContainer } from './styled';

const ProfileHome: React.FC = () => {
  const [test, setTest] = useState(false);

  useEffect(() => {
    const my = document.getElementById('test');
    console.log(my);
    console.log(test);
    setTest(true);
  }, [test]);

  return (
    <ProfileHomeContainer id='test' test={test ? 1 : 0}>
    </ProfileHomeContainer>
  )
}

// styled.ts
import styled from 'styled-components';

interface ProfileHomeContainerProps {
  test: number;
}

const ProfileHomeContainer = styled.div<ProfileHomeContainerProps>`
  display: flex;
  align-items: center;
  justify-content: center;
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: 100vh;
  background: rgba(0, 0, 0, 0.9);
  z-index: 1101;
  opacity: ${(props) => (props.test ? '1' : '0')};
  transition: 1s;
`;

In addition to that, there are two global CSS classes for ProfileHomeContainer.

// which has opacity: 0
.KBlzF{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-align-items:center;-webkit-box-align:center;-ms-flex-align:center;align-items:center;-webkit-box-pack:center;-webkit-justify-content:center;-ms-flex-pack:center;justify-content:center;position:fixed;top:0;left:0;width:100%;height:100vh;background:rgba(0,0,0,0.9);z-index:1101;opacity:0;-webkit-transition:1s;transition:1s;}


// has opacity: 1
.goFgwQ{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-align-items:center;-webkit-box-align:center;-ms-flex-align:center;align-items:center;-webkit-box-pack:center;-webkit-justify-content:center;-ms-flex-pack:center;justify-content:center;position:fixed;top:0;left:0;width:100%;height:100vh;background:rgba(0,0,0,0.9);z-index:1101;opacity:1;-webkit-transition:1s;transition:1s;}

Moreover, here are the console logs from my application:

<div id="test" class="sc-crXcEl goFgwQ"> // result of console.log(my)
false // result of console.log(test)
<div id="test" class="sc-crXcEl goFgwQ">
true

Now comes my question: Why does my component render with the class goFgwQ first? I expected it to be like this instead:

<div id="test" class="sc-crXcEl KBlzF">
false
<div id="test" class="sc-crXcEl goFgwQ">
true

This is because, during the initial run of useEffect, the state value for test is false.

If you want to see more of my actual code, check out the GitHub links below:

index.tsx

styled.ts

javascript css reactjs typescript styled-components

Answer 1

Answer №1

This situation is considered an "artifact" caused by the slight delay of the console.log function, which may be further aggravated by lazy access in its implementation.

You may have observed that your transition is functioning properly, indicating that your styled div (<ProfileHomeContainer>) initially rendered with opacity 0, then, after at least one animation frame, was updated through a state change to display the version with opacity 1 (as anticipated from your useEffect).

The puzzling part is that these two steps are not evident in the output of console.log(element), which only shows the final class name.

A live example can be found on CodeSandbox: https://codesandbox.io/s/mutable-surf-s1p4lt?file=/src/App.tsx

To better illustrate the effect of browser console lazy access, I purposely delayed the transition to test === true:

  useEffect(() => {
    const my = document.getElementById("test");
    console.log(my);
    console.log(test);
    // Delayed toggling of test state
    setTimeout(() => setTest(true), 1000);
    //setTest(true);
  }, [test]);

Make sure to view both CodeSandbox and the browser console simultaneously for potentially different behaviors:

With the delay, you will see both class names appear accurately in the consolehttps://i.sstatic.net/ubvFA.png
In the browser console, expand the element logged before the false state value is displayed
You'll notice the class name changing to the one with opacity 1! https://i.sstatic.net/UKZQC.png (I used Chrome browser for this demonstration)

This demonstrates how the browser console displays object states when expanded, showcasing potential discrepancies from the time of executing console.log, even compared to what's already previewed in the console!

Note that this behavior does not occur in the CodeSandbox console, as it operates slightly differently.

To address your initial concern, let's eliminate the delay (setTimeout):

Upon removal of the delay, we return to the original behavior where all element logs show only the "final" class name (with opacity 1). This suggests that by the time console.log accesses the element to present its details, React has likely executed a new rendering pass (triggered by the setTest(true) within useEffect).

This also highlights that the element remains the same across re-renders, indicating React effectively reuses DOM elements.

We can visualize this phenomenon by deliberately prompting React to use a distinct DOM element, for instance, by specifying a changing key prop:

<ProfileHomeContainer id="test" test={test ? 1 : 0} key={test.toString()}>

https://codesandbox.io/s/interesting-parm-mwgs9t?file=/src/App.tsx

As the value passed to the key prop alters, React refrains from reusing the DOM element, opting to create a new element instead. Consequently, the log now reflects the expected outcome: https://i.sstatic.net/jvB1Z.png

However, this modification disrupts the transition effect, necessitating application on the same element for proper functionality.

Answer 2

This situation is considered an "artifact" caused by the slight delay of the console.log function, which may be further aggravated by lazy access in its implementation.

You may have observed that your transition is functioning properly, indicating that your styled div (<ProfileHomeContainer>) initially rendered with opacity 0, then, after at least one animation frame, was updated through a state change to display the version with opacity 1 (as anticipated from your useEffect).

The puzzling part is that these two steps are not evident in the output of console.log(element), which only shows the final class name.

A live example can be found on CodeSandbox: https://codesandbox.io/s/mutable-surf-s1p4lt?file=/src/App.tsx

To better illustrate the effect of browser console lazy access, I purposely delayed the transition to test === true:

  useEffect(() => {
    const my = document.getElementById("test");
    console.log(my);
    console.log(test);
    // Delayed toggling of test state
    setTimeout(() => setTest(true), 1000);
    //setTest(true);
  }, [test]);

Make sure to view both CodeSandbox and the browser console simultaneously for potentially different behaviors:

With the delay, you will see both class names appear accurately in the consolehttps://i.sstatic.net/ubvFA.png
In the browser console, expand the element logged before the false state value is displayed
You'll notice the class name changing to the one with opacity 1! https://i.sstatic.net/UKZQC.png (I used Chrome browser for this demonstration)

This demonstrates how the browser console displays object states when expanded, showcasing potential discrepancies from the time of executing console.log, even compared to what's already previewed in the console!

Note that this behavior does not occur in the CodeSandbox console, as it operates slightly differently.

To address your initial concern, let's eliminate the delay (setTimeout):

Upon removal of the delay, we return to the original behavior where all element logs show only the "final" class name (with opacity 1). This suggests that by the time console.log accesses the element to present its details, React has likely executed a new rendering pass (triggered by the setTest(true) within useEffect).

This also highlights that the element remains the same across re-renders, indicating React effectively reuses DOM elements.

We can visualize this phenomenon by deliberately prompting React to use a distinct DOM element, for instance, by specifying a changing key prop:

<ProfileHomeContainer id="test" test={test ? 1 : 0} key={test.toString()}>

https://codesandbox.io/s/interesting-parm-mwgs9t?file=/src/App.tsx

As the value passed to the key prop alters, React refrains from reusing the DOM element, opting to create a new element instead. Consequently, the log now reflects the expected outcome: https://i.sstatic.net/jvB1Z.png

However, this modification disrupts the transition effect, necessitating application on the same element for proper functionality.

What is the reason behind my styled component only displaying the final state in its className logs?

Answer №1

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