Hajime, the duck guy

Introduction

In this exercise, you will write a series of functions for validating user input. Input validation is a critical element of any program that deals with user input. It serves the purpose of both guiding the user towards successful completion of forms, and it also defends the application against intentionally malformed input that is designed to compromise your program. Keep this in mind while doing this exercise.

Skills you will acquire

• User input validation

Objective

Write functions to validate the following:

• Field is required and value is specified
• Field contains a valid email address
• Field contains a valid date in YYYY-MM-DD format.
• Field contains at least N characters
• Field contains at most N characters
• Field contains a numeric value (either an integer or a decimal number)
• Field contains a value that satisfies any combination of other validators

The validator should return a string that says what went wrong. If there is no error, it should return an empty string.

Check your solution

• Catch fields that are blank
• Catch fields that only comprise of whitespace as blank
• Catch fields that are filled in but aren't a valid email address
• Catch invalid month and date numbers in dates such as 2024-44-99
• Catch fields that are filled in but have more characters than specified
• Catch fields that are filled in but have less characters than specified
• Ignore trailing and leading whitespace when counting characters for length limits
• Correctly count emoji or characters written in some non-English languages (e.g., 🦆 as 1)
• Catch invalid numeric values such as 1.2.3 or .32
• When combining multiple validators, the the first failing validation is reported
• When combining multiple validators

Tools

• Regular Expressions 101

Keep in mind

All inputs are strings.

A validator could be written as a function that takes the input as its only argument and returns an empty string when there is no error, or an error message specific to what it validates.

When you are deciding what the error message should say, consider two possible patterns:

1. The string could represent a message intended for the end user
2. The string could represent an identifier that can be later mapped to a message for the end user

The second pattern is slightly more complex, but it gives the interface code more flexibility in how it will report the error (e.g., the same constraint may have different meanings depending on the context).

Validators cannot validate anything that is validated by another validator. For example, an email validator cannot also validate that field is missing a value.

Some validators have parameters (e.g., "contains at least N characters" has "N" as a parameter), and some do not.

A function that combines multiple validators can be seen as a validator that has other validators as parameters.

A standards-compliant email address may include ludicrous things that are not normally seen as an email address in practical everyday use. Research various ways to validate email addresses and pick a pragmatic approach.

When testing that something is in a valid format, do not forget that just because it can be used as an input in some cases, it doesn't mean it's valid. For example:

parseFloat('2a') // => 2

In the example, '2a' is not a valid number, but parseFloat() will let you treat it as a valid input. Your validator should fail this kind of input. In many cases, using regular expressions is the right approach to test input validity.

Try also playing with other ideas such as making each validator a class.

Reading list

• Closures (MDN)
• Regular expressions (MDN)

Hints & spoilers

function validateMaxLength(length) {
  return function (value) {
          // This is the actual validator function
  }
}