Check if a Variable is Null or Undefined in React

# Check if a Variable is Null or Undefined in React

To check if a variable is null or undefined in React, use the || (or) operator to check if either of the two conditions is met.

When used with two boolean values the || operator returns true if either of the conditions evaluates to true.

App.js

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import {useEffect, useState} from 'react';

export default function App() {
  const [message, setMessage] = useState(undefined);

  useEffect(() => {
    // 👇️ Check if NOT undefined or null
    if (message !== undefined && message !== null) {
      console.log('✅ variable is NOT undefined or null');
    }

    // 👇️ Check if undefined or null
    if (message === undefined || message === null) {
      console.log('✅ variable is undefined or null');
    } else {
      console.log('⛔️ variable is NOT undefined or null');
    }
  }, [message]);

  return (
    <div>
      <button onClick={() => setMessage('Hello world')}>Set message</button>

      <h2>{message}</h2>
    </div>
  );
}

check if undefined or null

The code for this article is available on GitHub

Our first if statement checks if the message state variable is not equal to undefined and null.

App.js

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if (message !== undefined && message !== null) {
  console.log('✅ variable is NOT undefined or null');
}

We used the logical && (and) operator to specify that both conditions have to be met for the if block to run.

Our second if statement uses the logical OR (||) operator to check if the message variable is either undefined or null.

App.js

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if (message === undefined || message === null) {
  console.log('✅ variable is undefined or null');
} else {
  console.log('⛔️ variable is NOT undefined or null');
}

If either of the conditions is satisfied the if block runs, otherwise, the else block runs.

We used the useEffect hook to track changes to the message state variable, however, you can perform the check anywhere in your code.

# Check if a Variable is Null or Undefined implicitly in React

We used the strict equality === operator for the examples, however, there is a more concise way to check if a variable is equal to null or undefined, using loose equality ==.

App.js

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import {useEffect, useState} from 'react';

export default function App() {
  const [message, setMessage] = useState(undefined);

  useEffect(() => {
    // 👇️ Check if NOT undefined or null
    // (using loose not equals)
    if (message != undefined) {
      console.log('✅ variable is NOT undefined or null');
    }

    // 👇️ Check if undefined or null
    // (using loose equals)
    if (message == undefined) {
      console.log('✅ variable is undefined or null');
    } else {
      console.log('⛔️ variable is NOT undefined or null');
    }
  }, [message]);

  return (
    <div>
      <button onClick={() => setMessage('bobbyhadz.com')}>
        Set message
      </button>

      <h2>{message}</h2>
    </div>
  );
}

check if undefined or null

The code for this article is available on GitHub

We used the loose inequality operator (!=) in the first example.

App.js

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if (message != undefined) {
  console.log('✅ variable is NOT undefined or null');
}

It checks for both null and undefined because when compared using loose equality null is equal to undefined.

App.js

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// 👇️ true
console.log(null == undefined);

The second example uses loose equality to check if the message state variable is equal to null or undefined.

App.js

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if (message == undefined) {
  console.log('✅ variable is undefined or null');
} else {
  console.log('⛔️ variable is NOT undefined or null');
}

This is a bit more implicit than using the strict equality operator and might be confusing to some developers reading your code.

It's generally best to be explicit and direct than to be implicit and write less intuitive code.

# Checking if a Variable stores a Truthy value in React.js

You can also check if a variable stores a truthy value.

The truthy values are all values that are not falsy.

The falsy values in JavaScript are: undefined, null, false, 0, "" (empty string), NaN (not a number).

App.js

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import {useEffect, useState} from 'react';

export default function App() {
  const [message, setMessage] = useState(undefined);

  useEffect(() => {
    if (message) {
      console.log('✅ variable is truthy');
    }

    if (!message) {
      console.log('⛔️ variable is falsy');
    }
  }, [message]);

  return (
    <div>
      <button onClick={() => setMessage('bobbyhadz.com')}>
        Set message
      </button>

      <h2>{message}</h2>
    </div>
  );
}

react check if variable truthy

The code for this article is available on GitHub

The first condition checks if the message variable stores a truthy value.

The variable could store any value other than the 7 falsy values for the condition to be met.

The second if statement checks if the variable stores one of the 7 falsy values - undefined, null, false, 0, "" (empty string), NaN (not a number).

All other values are truthy.

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Check if a Variable is Null or Undefined in React