If var isnumber, for script

If you want to do number stuff if the value is a number or a string representation of a number you can do the following:

if (!isNaN(myVar)){//myVar is either a number or a string representing a number
    myNumericVar = +myVar;//convert myVar to number if necessary
    //do number stuff
} else {
   //do non-numeric stuff
}

See:

(Built-in) way in JavaScript to check if a string is a valid number

This will check if A1 is a number:

function ifIsNumber() {
    var ss = SpreadsheetApp.getActiveSpreadsheet();
    var s = ss.getActiveSheet();
    var substring1 = s.getRange("A1").getValue();
    if (!isNaN(parseFloat(substring1)) && isFinite(substring1)) {
        s.getRange("B1").setValue("Is a number");
    } else {
        s.getRange("B1").setValue("Not a number");
    }
}

Google Apps Script could use most of the JavaScript methods and operators.

One alternative to check if an object is a number is to use typeof.

The following code will return the type of the value of the cell specified by the reference.

function typeofcellvalue(reference) {
  var ss = SpreadsheetApp.getActive();
  var rng = ss.getRange(reference);
  var value = rng.getValue();
  return typeof value;
}

Example of use as a custom function

=typeofcellvalue("A1")

If cell A1

• has a number, the result will be number.
• is empty, the result will be string.