Check if String is a Positive Integer in JavaScript

Check if String is a Positive Integer #

To check if a string is a positive integer:

1. Convert the string to a number and pass it to the Number.isInteger() method. The method returns true if the provided value is an integer and false otherwise.
2. Check if the number is greater than 0.
3. If both conditions are met, then the string is a valid positive integer.

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function isPositiveInteger(str) {
  if (typeof str !== 'string') {
    return false;
  }

  const num = Number(str);

  if (Number.isInteger(num) && num > 0) {
    return true;
  }

  return false;
}

console.log(isPositiveInteger('0')); // 👉️ false
console.log(isPositiveInteger('1')); // true
console.log(isPositiveInteger('-1')); // 👉️ false
console.log(isPositiveInteger('1.5')); // 👉️ false
console.log(isPositiveInteger('test')); // false
console.log(isPositiveInteger('')); // 👉️ false
console.log(isPositiveInteger('   ')); // 👉️ false

We first check if the passed in value isn't a string. In this case, we return false straight away because the function only handles strings.

We then convert the string to a number and store the value into a variable.

Here are some examples of converting a string to a number.

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console.log(Number('1')); // 👉️ 1
console.log(Number('1.5')); // 👉️ 1.5
console.log(Number('')); // 👉️ 0
console.log(Number('       ')); // 👉️ 0
console.log(Number('hello')); // 👉️ NaN

If it's an empty string or contains spaces, we get 0 back.

The first condition in our second if statement checks if the value is a valid integer.

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function isPositiveInteger(str) {
  if (typeof str !== 'string') {
    return false;
  }

  const num = Number(str);

  if (Number.isInteger(num) && num > 0) {
    return true;
  }

  return false;
}

The Number.isInteger method returns true if the provided value is an integer and false otherwise.

Here are some examples of using the Number.isInteger method directly.

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console.log(Number.isInteger(1)); // 👉️ true
console.log(Number.isInteger(1.5)); // 👉️ false
console.log(Number.isInteger('1')); // 👉️ false
console.log(Number.isInteger(-1)); // 👉️ true

We used the logical AND (&&) operator to chain another condition. The if block is only run if both conditions are true.

The second condition checks if the number is greater than 0.

However, there is a caveat with the Number.isInteger method. The method returns true if the passed in value:

• is an integer
• is a float that can be represented as an integer

Here are examples of floats that can be represented as integers:

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// 👇️ true
console.log(Number.isInteger(10.0));

// 👇️ true
console.log(Number.isInteger(10.000000000000000123));

This shouldn't matter as most applications are OK with considering numbers such as 1.0 and 2.0 to be integers.

Further Reading #

• Extract a Number from a String in JavaScript
• Check if String is a Valid Number in JavaScript
• Check if a Character is a Number using JavaScript
• Get the First Number in a String using JavaScript
• Replace all Numbers in a String using JavaScript
• Remove all Numbers from a String in JavaScript
• Parse a String with Commas to a Number in JavaScript
• Check if a Number is a Negative Integer in JavaScript