Question

I am using isNan functionto check if entered value is a number:

if(isNaN(num)) {
    alert('I am not a number');
}
else {
//
     }

However, if entered value is, for example, “5748hh” it is understandable like a number and return false. What is wrong?

Answer 1

from Mozilla MDN

When the argument to the isNaN function is not a number, the value is first coerced to a number. The resulting value is then tested to determine whether it is NaN. Thus for non-numbers that when coerced to numeric type result in a valid non-NaN numeric value (notably the empty string and boolean primitives, which when coerced give numeric values zero or one), the “false” returned value may be unexpected; the empty string, for example, is surely “not a number.” The confusion stems from the fact that the term, “not a number”, has a specific meaning for numbers represented as IEEE-794 floating-point values. The function should be interpreted as answering the question, “is this value, when coerced to a numeric value, an IEEE-794 ‘Not A Number’ value?”

so alert(isNaN('45345ll')) returns true because its value coercion is different than a parseInt/parseFloat conversion.

e.g.

var num = false;
console.log(isNaN(num));        /* return false because the coerced value is 0 but */
console.log(parseInt(num, 10)); /* return isNaN */

Maybe you get an unexpected value due to a missing paren in your if statement