How to Check if a Variable is False in JavaScript

Check if a Variable is False in JavaScript #

Use the strict equality (===) operator to check if a variable is equal to false - myVar === false. The strict equality operator will return true if the variable is equal to false, otherwise it will return false.

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const a = false;

if (a === false) {
  console.log('✅ a is equal to false');
} else {
  console.log('⛔️ a is not equal to false');
}

We used the strict equality (===) operator to check if the value, stored in the a variable is equal to false.

The operator returns a boolean value:

• true if the values are equal
• false if the values are not equal

The strict equality (===) operator considers two values of different types to be different, as opposed to the loose equality (==) operator.

This means that if we compare false with any other type, the strict equality operator (===) would return false.

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console.log(false === false); // 👉️ true
console.log(false === 'false'); // 👉️ false
console.log(false === 0); // 👉️ false

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const a = false;

if (!a) {
  console.log(`⛔️️ a is ONE OF
       false, 0, empty string, null, undefined, NaN`);
} else {
  console.log(`🚨 a is NOT
       false, 0, empty string, null, undefined, NaN`);
}

We use the logical NOT (!) operator to flip the value of the a variable.

Our if statement basically checks if the a variable is falsy. It's commonly read as "If not a".

The falsy values in JavaScript are: false, 0, "", null, undefined, NaN.

This means that for the if block to run, a could be either one of the 6 falsy, values and not necessarily false.

Things could go wrong in many different ways when writing code like this, here's an example:

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const a = 0;

if (!a) {
  console.log('✅ this runs');
} else {
  console.log("⛔️ this doesn't run");
}

In this example, the if block runs, because the variable a is set to a falsy value 0.

This is confusing to the person reading your code, because the if block would run if a is set to either of the 6 falsy values.

Instead you should be more explicit:

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const a = 0;

if (a !== 0) {
  console.log("⛔️ this doesn't run");
} else {
  console.log('✅ this runs');
}

Further Reading #

• Check if a Variable is equal to True in JavaScript
• Check if a Variable is Not NULL in JavaScript