react hook 'useeffect' is called conditionally error-ag捕鱼王app官网

the error “ react hook 'useeffect' is called conditionally ” occurs when we use a hook conditionally useeffector after a conditional that might return a value. to fix the error, move all react hooks above any conditional that might return a value.

below is sample code where the above error occurs.

import react, {useeffect, usestate} from 'react';

export default functionapp() {
  const [count, setcount] = usestate(0);

  if (count > 0) {
    // ⛔️ react hook "useeffect" is called conditionally.
    // react hooks must be called in the exact same order in every component render.
    useeffect(() => {
      console.log('count is greater than 0');
    }, [count]);
  }

  return (
    <div><h2>count: {count}h2><buttononclick={() => setcount(count   1)}>incrementbutton>div>
  );
}

the problem in your code sample is that we are calling useeffectthe hook conditionally.

to fix this error, we have to call react hooks only at the top level, so we can move the conditional useeffectinto the hook.

import react, {useeffect, usestate} from 'react';

export default functionapp() {
  const [count, setcount] = usestate(0);

  useeffect(() => {
    // 👇️ move condition in hook
    if (count > 0) {
      console.log('count is greater than 0');
    }
  }, [count]);

  return (
    <div><h2>count: {count}h2><buttononclick={() => setcount(count   1)}>incrementbutton>div>
  );
}

moving the if statement useeffectinto the hook helps, because the hook is now at the top level and has predictable behavior, allowing react to correctly preserve state between usestateand calls.useeffect

another reason for errors is when we call the hook after a condition that might return a value useeffect.

import react, {useeffect, usestate} from 'react';

export default functionapp() {
  const [count, setcount] = usestate(0);

  // 👇️ might return before useeffect is called
  if (count > 0) {
    return <h1>count is greater than 0h1>;
  }

  // ⛔️ react hook "useeffect" is called conditionally.
  // react hooks must be called in the exact same order in every component render.
  // did you accidentally call a react hook after an early return?
  useeffect(() => {
    console.log('count is greater than 0');
  }, [count]);

  return (
    <div><h2>count: {count}h2><buttononclick={() => setcount(count   1)}>incrementbutton>div>
  );
}

the problem here is that useeffectthe hook is called after the condition that might return a value.

to fix this bug, we have to move all of our hook calls above the conditional that can return.

import react, {useeffect, usestate} from 'react';

export default functionapp() {
  const [count, setcount] = usestate(0);

  // 👇️ moved hook above condition that may return
  useeffect(() => {
    console.log('count is greater than 0');
  }, [count]);

  // 👇️ condition that may return is below all hooks
  if (count > 0) {
    return <h1>count is greater than 0h1>;
  }

  return (
    <div><h2>count: {count}h2><buttononclick={() => setcount(count   1)}>incrementbutton>div>
  );
}

we useeffectmoved the hook above the conditions that might return a value.

this solves the error because we have to make sure that the react hooks are called in the same order every time the component renders.

this means that we are not allowed to use hooks in loops, conditionals or nested functions.

as the documentation states:

• only call hooks at the top level
• do not call hooks in loops, conditionals, or nested functions
• always use hooks at the top level of a react function, before any early returns
• hooks are only called from react function components or custom hooks.

this helps react preserve the hook state between multiple usestateand calls.useeffect