JavaScript: how to check if a string variable is an integer value

In JavaScript, one way to check if a variable is of type integer, Number.isInteger() can be used:

Number.isInteger(22); // true
Number.isInteger(22.2); // false
Number.isInteger('22'); // false

Nevertheless, this solution has the disadvantage, that a string with integer value like '22' will result in false. But there are different solutions for this:

Use parseInt

You can use the parseInt function to parse a string and convert it into an integer. If the string signifies an integer parseInt will provide that value. Otherwise it will yield NaN (Not a Number). You can verify if the outcome is numeric or not to ascertain if the string signifies an integer. Here’s how you can accomplish this…

function isInteger(value) {
  return +value === parseInt(value);
}

isInteger(22); // true
isInteger(22.2); // false
isInteger('22'); // true

This will result in true for integer 22 as well as an integer “string” '22'.

Use Number()

Similar to the parseInt solution, you can also use Number() for this. Number values represent floating-point numbers like 37 or -9.25 (see its documentation). So to check for an integer value, you need to use Number.isInteger(). Otherwise, the result is also true for decimal values.

function isInteger(value) {
  return +value === Number(value) && Number.isInteger(Number(value));
}

isInteger(22); // true
isInteger(22.2); // false
isInteger('22'); // true

Use regular expression

function isInteger(value) {
  // Use regular expression to check if the value consists only of digits
  return /^\d+$/.test(value);
}

isInteger(22); // true
isInteger(22.2); // false
isInteger('22'); // true

This function isInteger verifies whether the string contains numbers (0 9) by utilizing a pattern matching technique. If it indeed contains digits it will return true signifying that the string denotes an integer value; otherwise it will return false.