Function isDigit

Check that a string only consists of digits.

Example

    isDigit("abcdefghijklmnopqrstuvxyz") // => false
    isDigit("ABCDEFGHIJKLMNOPQRSTUVXYZ") // => false
    isDigit("1234567890")                // => true
    isDigit("=")                         // => false

Returns

True if it only contains alphanumeric characters.