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To prevent a page refresh on form submit in React, call event.preventDefault() in the submit event listener.

App.jsxCopied!

import React, { useState } from 'react';

function LoginForm() {
  const [email, setEmail] = useState('');
  const [password, setPassword] = useState('');

  const handleEmailChange = (event) => {
    setEmail(event.target.value);
  };

  const handlePasswordChange = (event) => {
    setPassword(event.target.value);
  };

  const handleSubmit = (event) => {
    event.preventDefault();
    
    // Simulate form submission by clearing input fields
    setEmail('');
    setPassword('');
  };

  return (
    <form onSubmit={handleSubmit}>
      <label>
        Email
        <br />
        <input
          type="email"
          value={email}
          onChange={handleEmailChange}
          required
        />
      </label>
      <br />
      <label>
        Password
        <br />
        <input
          type="password"
          value={password}
          onChange={handlePasswordChange}
          required
        />
      </label>
      <br />
      <button type="submit" style={{ marginTop: 16 }}>
        Submit
      </button>
    </form>
  );
}

export default function App() {
  return <LoginForm />;
}

We use the Event preventDefault() method to stop the default action of the event from happening. In this case, that action was a page refresh, so preventDefault() prevented the page refresh on the form submission.

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const handleSubmit = (event) => {
   event.preventDefault();
   
   // Simulate form submission by clearing input fields
   setEmail('');
   setPassword('');
 };

preventDefault() is a property of an Event object, which lets us access and modify event-related data.

We use the onSubmit prop to add a submit event listener to the form; this listener runs when the user clicks the button to submit the form.

All we did here was reset the form, but of course in the real world, you probably want to make an AJAX request to a server with the form data.

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const handleSubmit = async (event) => {
  event.preventDefault();

  const user = {
    email: email,
    password: password,
  };

  try {
    const response = await axios.post('http://your-api-url.com', user);
    console.log(response.data);
  } catch (error) {
    console.error(error);
  }

  setEmail('');
  setPassword('');
};

For the button to submit the form, we must set its type prop to submit

App.jsxCopied!

<button type="submit" style={{ marginTop: 16 }}>
  Submit
</button>

With type="submit", the browser also submits the form when the user presses the Enter key in an input field.

Remove type="submit" from the button to prevent page refresh

To prevent page refresh on form submit in React, you can also remove the type="submit" from the button.

When is this useful? You may want to show an alert message or something before submission. In this case, we wouldn’t want type="submit" since it submits the form on button click.

App.jsxCopied!

import React, { useState } from 'react';

function LoginForm() {
  const [email, setEmail] = useState('');
  const [password, setPassword] = useState('');

  const handleEmailChange = (event) => {
    setEmail(event.target.value);
  };

  const handlePasswordChange = (event) => {
    setPassword(event.target.value);
  };

  const handleSubmit = () => {
    // Perform custom actions before form submission
    alert('Are you sure you want to submit the form?');

    // Simulate form submission by clearing input fields
    setEmail('');
    setPassword('');
  };

  return (
    <form>
      <label>
        Email
        <br />
        <input
          type="email"
          value={email}
          onChange={handleEmailChange}
          required
        />
      </label>
      <br />
      <label>
        Password
        <br />
        <input
          type="password"
          value={password}
          onChange={handlePasswordChange}
          required
        />
      </label>
      <br />
      <button type="button" onClick={handleSubmit} style={{ marginTop: 16 }}>
        Submit
      </button>
    </form>
  );
}

export default function App() {
  return <LoginForm />;
}

We used type="button" on the button, but that’s as good as not having a type attribute. It doesn’t do anything on click.

Key takeaways

• To prevent page refresh on form submit in React, call event.preventDefault() in the submit event listener.
• Set type="submit" to ensure the form submits on button click or Enter press. Remove it when you don’t want this.
• Remove type="submit" when you need to do something after the button click before submitting the form, e.g., show a dialog, make an initial request, etc.

The “unexpected reserved word (await)” error occurs in JavaScript when you use the await keyword in a function that is not specified as async. To fix it, add an async modifier to the function to mark it as async.

Here’s an example of the error occurring:

index.jsCopied!

function getName() {
  // ❌ SyntaxError: Unexpected reserved word
  const str = await Promise.resolve('Coding Beauty');

  return str;
}

Note: As this is a syntax error, the function doesn’t need to be invoked for it to be detected, and no part of the code runs until it is resolved.

The async and await keywords work together in JavaScript (hence the commonly used term, async/await); to use the await keyword in a function, you must add the async modifier to the function to specify that it is an async function.

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// ✅ Use "async" keyword modifier
async function getName() {
  // ✅ Successful assignment - no error
  const str = await Promise.resolve('Coding Beauty');

  return str;
}

Fix “Unexpected reserved word (await)” error in nested function

If you’re using the await keyword, it’s likely that you already know that it has to be in an async function. What probably happened is that you nested functions and mistakenly ommited the async modifier from the innermost function containing the await keyword.

For example:

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// ❌ SyntaxError: Unexpected reserved word
export const createTask =
  async ({ description }) =>
  // ❌ "async" keyword missing from innermost function
  (dispatch) => {
    await fetch('https://example.com/tasks', {
      method: 'POST',
      headers: {
        'Content-Type': 'application/json'
      },
      body: JSON.stringify({ description }),
    });

    dispatch({ type: 'taskCreated', description });
  };

For await to work, the deepest function in the nesting hierarchy is required to be specified as async. It won’t work even if any or all of the outer functions are marked as async.

So we resolve the error in this case by adding the async modifier to the innermost function:

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// ✅ No error
export const createTask =
  async ({ description }) =>
  // ✅ Innermost function marked as async
  async (dispatch) => {
    await fetch('https://example.com/tasks', {
      method: 'POST',
      headers: {
        'Content-Type': 'application/json',
      },
      body: JSON.stringify({ description }),
    });

    dispatch({ type: 'taskCreated', description });
  };

In this example, we should be able to remove the async keyword from the outer function, as it isn’t performing any asynchronous operations with await, but this depends on whether the caller of createTask() is expecting it to return a Promise or not.

Here’s another example where this mistake frequently happens; using await in an array looping method:

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// ❌ SyntaxError: Unexpected reserved word
async function processPhotos(photoIds) {
  const data = await Promise.all(photoIds.map((photoId) => {
    const res = await fetch(`http://example.com/photos/${photoId}`);
    return await res.json();
  }));
  
  // process data...
}

Like in the previous example, the error occurs because the async keyword modifier is absent from the map() callback, even though it’s present in the function that calls map(). The fix is the same, add async to the map() callback.

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// ✅ No error
async function processPhotos(photoIds) {
  const data = await Promise.all(
    photoIds.map(async (photoId) => {
      const res = await fetch(`http://example.com/photos/${photoId}`);
      return await res.json();
    })
  );
  // processing...
}

Use await at top level

If you’re trying to use await at the top level of your file, you’ll need to set the type attribute to "module" in the script tag referencing the file in the HTML. This works when your code runs in browser environments.

For example:

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<script type="module" src="index.js"></script>

Now you’ll be able to use await at the global scope in your file, e.g.:

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console.log(await Promise.resolve('Coding Beauty'));

If you’re using Node.js, you’ll need to set the type attribute to "module" in your package.json file.

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{
  "name": "cb-js",
  "type": "module",
  "version": "1.0.0",
  "main": "index.js",
  "license": "MIT",
  // other fields...
}

If there’s no package.json in your project directory, you can create one with the following command

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# NPM
npm init -y

# Yarn
yarn init -y

The await keyword will now work at the top level of your project files.

To convert hours and minutes to seconds in JavaScript:

1. Multiply the hours by 3600 to convert it to seconds.
2. Multiply the minutes by 60 to convert it to seconds.
3. Add these seconds to the seconds time value to get the total seconds.

For example:

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function toSeconds(hours, minutes, seconds) {
  return hours * 3600 + minutes * 60 + seconds;
}

console.log(toSeconds(2, 15, 10)); // 8110

console.log(toSeconds(1, 5, 30)); // 3930

We create a reusable toSeconds() function to easily convert the hours and minutes to seconds.

The function is quite easy to understand; 1 hour equals 3600 seconds, so we multiply the hour value by 3600 to get the seconds equivalent. Similarly, 1 minute equals 60 seconds, so we multiply the minute value by 60 to get the seconds equivalent.

After this, we add the equivalent seconds values to the seconds argument to get the total seconds.

We could rewrite the function to accept the time values as named properties of an object, instead of as multiple parameters.

JavaScriptCopied!

function toSeconds({ h: hours, m: minutes, s: seconds }) {
  return hours * 3600 + minutes * 60 + seconds;
}

console.log(toSeconds({ h: 2, m: 15, s: 10 })); // 8110

console.log(toSeconds({ h: 1, m: 5, s: 30 })); // 3930

This approach makes it easier to understand the role of each value passed to the function, a

Convert HH:mm:ss to seconds in JavaScript

Sometimes the time input to convert to seconds is a string in a time format, like HH:mm:ss. To convert to seconds in this case, we’ll separate the individual time values by the separator (: in this case), cast them each to numbers, and perform the same time conversion steps done in our previous examples.

For example:

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function toSeconds(timeStr) {
  const [hours, minutes, seconds] = timeStr.split(':').map(Number);
  return hours * 3600 + minutes * 60 + seconds;
}

console.log(toSeconds('02:15:10')); // 8110

console.log(toSeconds('01:05:30')); // 3930

The input string here is in the HH:mm:ss format; the hour, minute, and second values are separated by a colon (:) and are each represented by a minimum of 2 digits.

The String split() method splits a string into an array of substrings separated by a given separator in the original string. We pass a colon as the separator to get an array of the individual time values.

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const timeStr = '02:15:10';

console.log(timeStr.split(':')); // [ '02', '15', '10' ]

After getting this array, we use the map() method to transform each time value into a number. The map() method takes a callback and calls it on each element of an array and uses the result to populate a new array. For our scenario, the callback is simply the Number() constructor, so each time value in the array is converted to a number.

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console.log(['02', '15', '10'].map(Number)); // [ 2, 15, 10 ]

Note

This:

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const [hours, minutes, seconds] = timeStr.split(':').map(Number);

is equivalent to this:

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  const [hours, minutes, seconds] = timeStr
    .split(':')
    .map((str) => Number(str));

The second one is longer, but it makes it clear exactly what arguments are passed to the map() callback. In the first one, map() automatically passes 3 arguments to its callback, which could be problematic if the callback returns a different result depending on the number of arguments it receives, for instance, a parseInt() callback.

We then use a destructuring assignment to unpack the number array values into separate hour, minute, and second variables.

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const [hour, minute, seconds] = [2, 15, 10];

console.log(hour); // 2

console.log(minute); // 15

console.log(seconds); // 10

After doing this, we perform the same multiplication and addition we did in the first example, to convert the hours and minutes to seconds and get the total seconds.

To get the width and height of the window in React, use window.innerWidth and window.innerHeight.

App.jsCopied!

import { useRef } from 'react';

export default function App() {
  const windowSize = useRef([window.innerWidth, window.innerHeight]);

  return (
    <div>
      <h2>Width: {windowSize.current[0]}</h2>

      <h2>Height: {windowSize.current[1]}</h2>
    </div>
  );
}


The innerWidth property returns the interior width of the window in pixels, including the width of the vertical scrollbar, if it’s present.

Similarly, innerHeight property returns the interior height of the window in pixels, including the height of the horizontal scrollbar, if it is present.

Since we are getting the window width and height only once – when the component mounts – we use the useRef() hook to store them.

useRef returns a mutable ref object that doesn’t change its value when a component is updated. Also, modifying the value of this object’s current property does not cause a re-render. This is in contrast to the setState update function returned from useState.

Get window width and height on resize in React

To get the window width and height on resize in React:

1. Get the window width and height with window.innerWidth and window.innerHeight.
2. Add an event listener for the resize event on the window object.
3. Update the window width and height in this event listener.

In the previous example, we needed to get the window width and height only once, and we used a ref to store it.

If you instead want to get the window’s width and height when it is resized, you’ll need to add a resize event listener to the window object and create a state variable to track changes to the width and height.

App.jsCopied!

import { useState, useEffect } from 'react';

export default function App() {
  const [windowSize, setWindowSize] = useState([
    window.innerWidth,
    window.innerHeight,
  ]);

  useEffect(() => {
    const handleWindowResize = () => {
      setWindowSize([window.innerWidth, window.innerHeight]);
    };

    window.addEventListener('resize', handleWindowResize);

    return () => {
      window.removeEventListener('resize', handleWindowResize);
    };
  }, []);

  return (
    <div>
      <h2>Width: {windowSize[0]}</h2>

      <h2>Height: {windowSize[1]}</h2>
    </div>
  );
}

We use the useState React hook to create a state variable that will be updated whenever the height or width of the window changes.

The useState hook returns an array of two values. This first is a variable that stores the state, and the second is a function that updates the state when it is called.

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 const [windowSize, setWindowSize] = useState([
    window.innerWidth,
    window.innerHeight,
  ]);

The useEffect hook is used to perform an action when a component first renders, and when one or more specified dependencies change. In our example, the action is to add the event listener for the resize hook with the addEventListener() method.

We pass an empty dependencies array to the useEffect, so that it gets called only once in the component’s lifetime, and the resize event listener is only registered once – when the component first renders.

App.jsCopied!

  useEffect(() => {
    const handleWindowResize = () => {
      setWindowSize([window.innerWidth, window.innerHeight]);
    };

    window.addEventListener('resize', handleWindowResize);

    return () => {
      window.removeEventListener('resize', handleWindowResize);
    };
  }, []);

In the resize event listener, we update the state variable with the new height and width of the window.

The function we return in useEffect is a function that performs clean-up operations in the component. We use the removeEventListener() method to remove the resize event listener in this clean-up function and prevent a memory leak.

To handle the onScroll event on a React element, assign a function to the onScroll prop of the element and use the event object to perform an action. That action will occur whenever the user scrolls up or down on the page.

For example:

App.jsx

import { useState } from 'react';

export default function App() {
  const [scrollTop, setScrollTop] = useState(0);

  const handleScroll = (event) => {
    setScrollTop(event.currentTarget.scrollTop);
  };

  return (
    <div>
      Scroll top: <b>{scrollTop}</b>
      <br />
      <br />
      <div
        style={{
          border: '1px solid black',
          width: '400px',
          height: '200px',
          overflow: 'auto',
        }}
        onScroll={handleScroll}
      >
        {[...Array(10)].map((_, i) => (
          <p key={i}>Content</p>
        ))}
      </div>
    </div>
  );
}

The function (event listener) passed to the onScroll prop is invoked whenever the viewport is scrolled. It is called with an event object, which you can use to perform actions and access information related to the scroll event.

The currentTarget property of this event object returns the element that the onScroll listener was attached to.

Tip: If you’re not sure of when to use the event object’s currentTarget or target properties, this article might help: Event target vs currentTarget in JavaScript: The Important Difference.

We use the element’s scrollTop property to get how far the element’s scrollbar is from its topmost position. Then we update the state variable with the new value, and this reflects on the page.

Note: We used the useState hook to manage the state. This hook returns an array of two values, where the first is a variable that stores the state, and the second is a function that updates the state when it is called.

Handle onScroll event on window object in React

We can also handle the onScroll event on the global window object, to perform an action when the viewport is scrolled. We can do this with the addEventListener() method:

App.js

import { useState, useEffect } from 'react';

export default function App() {
  const [scrollTop, setScrollTop] = useState(0);

  useEffect(() => {
    const handleScroll = (event) => {
      setScrollTop(window.scrollY);
    };

    window.addEventListener('scroll', handleScroll);

    return () => {
      window.removeEventListener('scroll', handleScroll);
    };
  }, []);

  return (
    <div>
      <div
        style={{
          position: 'fixed',
          padding: '10px 0',
          top: '0',
          backgroundColor: 'white',
          borderBottom: '1px solid #c0c0c0',
          width: '100%',
        }}
      >
        Scroll top: <b>{scrollTop}</b>
      </div>
      <div style={{ marginTop: '50px' }}>
        {[...Array(30)].map((_, i) => (
          <p key={i}>Content</p>
        ))}
      </div>
    </div>
  );
}

The addEventListener() method takes up to two arguments:

1. type: a string representing the event type to listen for.
2. listener: the function called when the event fires.

It also takes some optional arguments, which you can learn more about here.

We call addEventListener() in the useEffect hook to register the listener once the component renders as the page loads. We pass an empty dependencies array to useEffect so this registration happens only once. In the cleanup function, we call the removeEventListener() method to unregister the event listener and prevent a memory leak.

In the onScroll listener, we access the Window scrollY property to display the number of pixels that the page has currently scrolled horizontally.