How do devs make this mistake

it can happen many different ways if you're not explicitly watching out for these types of things

example let's say you have a csv file with a bunch of names

id, last_name
1, schaffer
2, thornton
3, NULL
4, smith
5, "NULL"

if you use the following to import into postgres

COPY user_data (id, last_name)
FROM '/path/to/data.csv'
WITH (FORMAT csv, HEADER true);

number 5 will be imported as a string "NULL" but number 3 will be imported as a NULL value. of course, this is why you sanitize the data (GIGO) but I can imagine this happening countless times at companies all over the country

there are easy fixes if you're paying attention

COPY user_data (id, last_name)
FROM '/path/to/data.csv'
WITH (FORMAT csv, HEADER true, NULL '');

sets the empty string to NULL value.

example with js

fetch('/api/user/1')
  .then(response => response.json())
  .then(data => {
    if (data.lastName == "null") {
      console.log("No last name found");
    } else {
      console.log("Last name is:", data.lastName);
    }
  });

if data is

data = {
  id: 5,
  lastName: "null"
};

then the if statement will trigger- as if there was no last name. that's why you gotta know the language you're using and the potential pitfalls

now you may ask -- why not just do

if (data.lastName === null)

instead? But what if the system you're working on uses JSON.parse(data) and that auto-converts everything to a string? it's a very natural move to check for the string "null"

obviously if you're paying attention and understand the pitfalls of certain languages (like javascript's type coercion and the particularities of JSON.parse()) it becomes easy but it's something that is honestly very easy to overlook