Check if Value is a Negative Number in JavaScript

Check if Value is a Negative Number #

To check if a value is a negative number, call the Math.sign() method, passing it the value as a parameter. The Math.sign method returns -1 if the provided argument is a negative number or can be converted to one.

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function isNegative(num) {
  if (Math.sign(num) === -1) {
    return true;
  }

  return false;
}

console.log(isNegative(-5)); // 👉️ true
console.log(isNegative(5)); // 👉️ false
console.log(isNegative('-5')); // 👉️ true
console.log(isNegative('test')); // 👉️ false

In the code snippet, we use the Math.sign method to check if a value is a negative number.

The only argument the method takes is a number. If the provided value is not a number, it gets converted to a number.

If you want to make sure the passed in parameter is of type number, you can add an additional check to the if statement.

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function isNegative(num) {
  if (typeof num === 'number' && Math.sign(num) === -1) {
    return true;
  }

  return false;
}

We use the logical and (&&) operator to chain another condition. Before calling the Math.sign method we check if the provided argument is a number.

The Math.sign method has 5 possible return values:

• it returns 1 if the argument is positive
• it returns -1 if the argument is negative
• it returns 0 if the argument is 0
• it returns -0 if the argument is -0
• in all other cases it returns NaN (not a number)

Here are some examples of calling the Math.sign method directly.

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console.log(Math.sign(-5)); // 👉️ -1
console.log(Math.sign(5)); // 👉️ 1
console.log(Math.sign('-5')); // 👉️ -1
console.log(Math.sign('test')); // 👉️ NaN
console.log(Math.sign(0)); // 👉️ 0

If the provided value is not a number, the method attempts to convert it to one.

To check if a value is a negative number, compare it to 0, e.g. num < 0. If the value to the left hand side of the less-than operator is not already a number, it will get converted to one and compared to 0. If the expression returns true, the value is a negative number.

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// Supported in IE

function isNegative(num) {
  if (num < 0) {
    return true;
  }

  return false;
}

console.log(isNegative(-5)); // 👉️ true
console.log(isNegative(5)); // 👉️ false
console.log(isNegative('-5')); // 👉️ true
console.log(isNegative('test')); // 👉️ false

We use the less than (<) operator to compare the value on the left to 0.

If the value to the left is not a number, JavaScript will attempt to convert it to one.

If you only expect to take numbers as arguments, add another condition to the if statement.

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function isNegative(num) {
  if (typeof num === 'number' && num < 0) {
    return true;
  }

  return false;
}

Here are some more examples of using the less than operator.

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console.log(-1 < 0); // 👉️ true
console.log('-1' < 0); // 👉️ true
console.log('test' < 0); // 👉️ false
console.log('' < 0); // 👉️ false
console.log(null < 0); // 👉️ false

If you're interested to read more about how the less than operator converts values that are not of the same type when comparing them, check out this section of the MDN docs.

Further Reading #

• Check if String is a Valid Number in JavaScript
• Check if a Value is a Positive Number in JavaScript