How to Check if a Value is a Number in JavaScript

# Check if a Value is a Number in JavaScript

You can check if a value is a number in three ways:

1. typeof - if the value is a number, the string "number" is returned.
2. Number.isFinite() - if the value is a number, true is returned.
3. isNaN() - if the value is a number, false is returned.

# Check if a Value is a Number using the typeof operator

Use the typeof operator to check if a value is a number in JavaScript.

The typeof operator will return the string "number" if the value is of type number.

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const myVar = 5;

if (typeof myVar === 'number') {
  // 👇️ this runs
  console.log('✅ value is a number');
} else {
  console.log('⛔️ value is NOT a number');
}

We used the typeof operator to check if a value is a number.

The typeof operator returns a string that indicates the type of a value. Here are some examples.

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console.log(typeof 5); // 👉️ "number"
console.log(typeof '5'); // 👉️ "string"
console.log(typeof NaN); // 👉️ "number"
console.log(typeof {}); // 👉️ "object"
console.log(typeof []); // 👉️ "object"
console.log(typeof null); // 👉️ "object"

If you look closely, you'll notice that NaN (Not a Number) is also of type number.

Use the Number.isNaN() method to make sure NaN values don't meet the condition.

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const myVar = 5;

if (typeof myVar === 'number' && !Number.isNaN(myVar)) {
  // 👇️ this runs
  console.log('✅ value is a number');
} else {
  console.log('⛔️ value is NOT a number');
}

We used the logical AND (&&) operator to chain a second condition.

Both conditions have to be met for our if block to run.

Notice that we used the logical NOT (!) operator to negate the call to Number.isNaN method.

The NOT (!) operator takes the return value of the method, converts it to a boolean and then flips it.

The Number.isNaN method checks if the passed-in value is NaN (not a number) AND its type is number.

Here are some examples:

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console.log(Number.isNaN(NaN)); // 👉️ true
console.log(Number.isNaN(Number.NaN)); // 👉️ true
console.log(Number.isNaN(0 / 0)); // 👉️ true
console.log(Number.isNaN(5)); // 👉️ false
console.log(Number.isNaN('5')); // 👉️ false
console.log(Number.isNaN({})); // 👉️ false

Only NaN, Number.NaN and 0 / 0 return true.

# Check if a Value is a Number using Number.isFinite()

Alternatively, you can use the Number.isFinite().

The method returns true if the provided value is of type number and is not positive or negative Infinity or NaN.

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const myVar = 5;

if (Number.isFinite(myVar)) {
  // 👇️ this runs
  console.log('✅ value is a number');
} else {
  console.log('⛔️ value is NOT a number');
}

We used the Number.isFinite() method to combine the conditions from the previous example.

The Number.isFinite() method checks that the provided value:

• is of type number
• is not positive or negative Infinity
• is not NaN

Here are some examples:

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console.log(Number.isFinite(5)); // 👉️ true
console.log(Number.isFinite(5.5)); // 👉️ true
console.log(Number.isFinite('5')); // 👉️ false
console.log(Number.isFinite(NaN)); // 👉️ false
console.log(Number.isFinite(null)); // 👉️ false

# Check if a Value is a Number using NaN

There are 2 isNaN methods in JavaScript:

• Number.isNaN() - checks if the passed-in value is of type number and is NaN (no coercion)
• isNaN() - coerces the value to a number and checks if the value is NaN

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// ⛔️ Don't do this

const myVar = 5;

if (!isNaN(myVar)) {
  // 👇️ this runs
  console.log('value might be a number');
} else {
  console.log('value is NOT a number');
}

We negated a call to the isNaN() function to check if the passed-in value can be coerced to a number.

Here are some examples of using isNaN.

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console.log(isNaN(5)); // 👉️ false
console.log(isNaN('5')); // 👉️ false
console.log(isNaN(3.5)); // 👉️ false
console.log(isNaN('3.5')); // 👉️ false
console.log(isNaN('3.5abc')); // 👉️ true
console.log(isNaN({})); // 👉️ true
console.log(isNaN([])); // 👉️ false
console.log(isNaN(true)); // 👉️ false
console.log(isNaN(false)); // 👉️ false
console.log(isNaN(null)); // 👉️ false
console.log(isNaN(undefined)); // 👉️ true

This goes wrong in many cases because many non-numeric values convert to valid numbers. Here are some examples:

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console.log(Number(null)); // 👉️ 0
console.log(Number([])); // 👉️ 0
console.log(Number('')); // 👉️ 0
console.log(Number(true)); // 👉️ 1
console.log(Number(false)); // 👉️ 0

For any of these values, you would get a false positive.

You should never use this approach, as it leads to confusing behavior and difficult to track bugs.

# Check if a Variable is a Number using lodash

First, make sure you have lodash installed by running the following command from your terminal.

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# 👇️ initialize package.json if you don't have one
npm init -y

npm install lodash

Now you can import and use the isNumber method to check if a value is a number.

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import _ from 'lodash';

console.log(_.isNumber(NaN)); // 👉️ true
console.log(_.isNumber(5)); // 👉️ true
console.log(_.isNumber(-100)); // 👉️ true

console.log(_.isNumber('hello')); // 👉️ false
console.log(_.isNumber(null)); // 👉️ false
console.log(_.isNumber([])); // 👉️ false

const variable = 100;

if (_.isNumber(variable)) {
  // 👇️ this runs
  console.log('The value is a number');
} else {
  console.log('The value is NOT a number');
}

The lodash.isNumber() method returns true if the supplied value is a number and false otherwise.

Even if you already use lodash in your project, I would advise against using the isNumber() method as the typeof operator is sufficient and doesn't require you to use an external library.

# Don't use the instanceof operator to check if a value is a number

You should never use the instanceof operator to check if a value is of type number.

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import _ from 'lodash';

function isNumber(value) {
  return value instanceof Number;
}

console.log(isNumber(100)); // 👉️ false
console.log(isNumber(-1000)); // 👉️ false
console.log(isNumber(new Number(10))); // 👉️ true
console.log(isNumber(new Number(-100))); // 👉️ true
console.log(isNumber(null)); // 👉️ false
console.log(isNumber('')); // 👉️ false

The syntax for the instanceof operator is object instanceof constructor.

The operator returns true if the prototype property of the constructor appears anywhere in the prototype chain of the object.

Notice that the operator returns true only for object-wrapped numbers.

These are numbers that were created with the new Number() constructor.

You should never use the new Number() constructor in your code, so you should never have to use the instanceof operator to check if a value is a number.

The typeof operator is all you need in order to check if a value stores a number in JavaScript.

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const myVar = 5;

if (typeof myVar === 'number') {
  // 👇️ this runs
  console.log('✅ value is a number');
} else {
  console.log('⛔️ value is NOT a number');
}

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

• Check if a Value is Falsy or Truthy in JavaScript
• Check if a value is a Float or an Integer in JavaScript
• Check if Value is Negative or Positive Number in JavaScript