Unexpected behavior: `undefined = undefined` evaluates to false

[benny779]

In JavaScript, both equality operators treat undefined as equal to itself:

undefined == undefined   // true
undefined === undefined  // true

However, in JSONata the following expression evaluates to false:

undefined = undefined

This behavior is surprising and may lead to unexpected results when comparing missing values.

Expected behavior

undefined = undefined should evaluate to true, or the documentation should clearly explain the rationale for the current behavior.

Actual behavior

The expression evaluates to false.

[mattbaileyuk]

This feels like a bug, but I'm also surprised to note that rolling back to early versions we have the same behaviour. You can either directly do undefined = undefined or having a reference to a value that doesn't exist (on one or both sides), and these undefined values are not equal; is this intentional @andrew-coleman?

[benny779]

It looks like this is done intentionally.

jsonata/src/jsonata.js

Line 740 in d4e4133

// if either side is undefined, the result is false

It even exists in an old version:

jsonata/src/jsonata.js

Line 2096 in 6886c01

function evaluateComparisonExpression(lhs, rhs, op) {

[mmkal]

Yeah I think this should be considered a bug. This same logic also means if you do something like this $type(request.status) = "number" OR $type(request.status) != "number" you get false either way, when request (or request.status) is undefined. Which will definitely lead to mistakes.

I think probably just removing that check would fix well enough. JS does the right thing with undefined===undefined, undefined!==undefined, undefined==="foo", undefined!=="foo" etc.

And probably it's fine to use the other operators with undefined too:

• undefined < undefined: false
• undefined < 1: false
• undefined > 1: false

Which is actually quite nice, because you can then do request.status < 400 type checks and they only evaluate to "true" when status is an actual number. So IMO validation could be skipped if either value is undefined too.