isFinite() This method is used to determine whether the number passed as a parameter is any number within the range of number.MIN_VALUE (which is a very small number) and number.MAX_VALUE (which is a huge number). String values passed as the parameter will evaluate to false. syntax: isFinite(expression) EXAMPLE function someNumber() { var n = document.form1.text1.value; if(isFinite(n) == true) { alert(The number you entered was between MIN_VALUE and MAX_VALUE."); } } Basically, the function someNumber() shows an alert box when the user enters a number in the text box within a form that is between the MIN_VALUE and MAX_VALUE. If a string is entered, no alert box will be shown.