I'm not aware of any language that coerces the string `'null'` to the value `null`. Are you aware of any?

ColdFusion does. There was a famous stackoverflow thread on that "We have an employee whose last name is Null. Our employee lookup application is killed when that last name is used as the search term (which happens to be quite often now)"

http://stackoverflow.com/questions/4456438/how-to-pass-null-...

A badly coded website could go in the other direction, eg in JS, someone writes a poor string equality check:

    function normalise(s) {
        return (s + "").toLowerCase();
    }

    function isEqual(a, b) {
        return normalise(a) == normalise(b);
    }

And then someone else uses it in some form validation logic:

    if (isEqual(surname, null)) {
        return error;
    }

Then as the next step, some programmer somewhere notices that all the NULLs in the database have been converted into strings somewhere, and puts UPDATE table SET column = NULL WHERE column = "NULL" somewhere, maybe even in a trigger, and that's that.

The problem with data destruction is that only one thing in the entire data pipeline has to spuriously map two distinct values to the same output value, and the data is destroyed and can not be recovered. Very few programmers think carefully about whether a serialization or data transfer format maps every single possible input to a different output. It may only take one oversight to ruin your year.

It tends to not be in programming languages but in textual data formats (including many standard implementations of comma-separated files) and even databases that can only hold string values by nature but also want a way to represent null (and fail to fix this by escaping the hell out of everything with quotation marks, and yet it isn't that "code" is executing).

Rails at least used to do this when reading back from a database (eg if you had a nullable string field it would come back as nil if it happened to be the literal "nil")

Edit: IIRC there was a directive that would prevent the behavior though?

Java and JavaScript do the opposite: null values are represented by the string "null" ( http://stackoverflow.com/questions/24591212/why-the-coercion... ) - it's conceivable many programs processing strings would assume "null" in a string comes from a null value originally.

Your inaccurate description gave me a small heart attack. We all know JavaScript is horrible, but that is next-level horrible.

null is coerced to "null" when it's needed as a string. This behaviour is similar to printf in C. It's not the case that "null" === null like True == 1 in Python. (This would be my interpretation of "x is represented by y".)

If you want 'next-level horrible', open your browser's console and take a look at what you get back from "typeof null".

JavaScriptCore:

>>> typeof null

object

>>> typeof "null"

string

>>> typeof (null + "")

string

Something always bugs me about this criticism:

#1 arguably makes sense. Unless "null" were a "type" (which it isn't), the other possible options for this ("undefined", or "string", "number", "array", etc.) all make less sense.

#2 is expected/correct.

#3 makes sense if you accept that the "+" operator aggressively casts its arguments to strings when they are anything other than two numbers. Arguably that's a bad design, but it's not unexpected or unpredictable.

The one completely unexplainable "+" behavior I know of in JS (i.e., a recent node/v8) is the second of these:

  > [] + {}
  '[object Object]' // makes sense because the string representation of [] is ''
  > {} + []
  0 // ???
  > var x = {} + []
  undefined
  > x
  '[object Object]' // but assigning to a variable first makes sense again

> #1 arguably makes sense. Unless "null" were a "type" (which it isn't), the other possible options for this ("undefined", or "string", "number", "array", etc.) all make less sense.

`typeof` gets the primitive type of a value, and exactly like undefined null is a primitive value with its own primitive type, as `typeof undefined` returns `"undefined"` `typeof null` was supposed to return `"null"`, it doesn't due to a bug in the original implementation which was then spec-enshrined as fixing it would break too much stuff: http://www.2ality.com/2013/10/typeof-null.html

This is because:

> [] + {}

Becomes:

> [].toString() + {}.toString()

Empty array to a string is "" (empty string). Object to string is "[object Object]".

> {} + []

Becomes:

> +[]

The prefix turns the array into a number, same as Number([]). This seems to be because {} is interpreted as a code block, not an object, due to it being the first thing.

Put it in parentheses and it'll do toString like before.

And now the obligatory WAT video: https://www.destroyallsoftware.com/talks/wat

The first of which is just my point. Null cannot be treated safely as an object in any way. To attempt such elicits an exception. Yet its type is 'object' nonetheless, needlessly complicating every type check that has to do with objects. I've been working with Javascript, professionally and as a hobbyist, since about 1995, and have yet to encounter any satisfactory reason why this should be the case.

Edit: And a comment nearby explains that it is the way it is for the same reason Make barfs on leading spaces. Well, at least that's a reason! A terrible, terrible reason.