Fix ?: operator returning wrong result when LHS has array predicate

[DevLeoko]

Quick Note

We are using JSONata quite extensively over at Ambivio and love it!
But we have noticed some issues like this one.

Please review these changes carefully as they have been AI-assisted and I don't have a deep understanding of JSONatas parsing logic.

Summary

The ?: (elvis/default) operator returns the wrong value whenever its left-hand side is an array predicate or a negative numeric literal. For example:

Expression	Expected	Actual (before fix)
[true][-1] ?: "no"	true	"no"
[1,2,3][-1] ?: "no"	3	2
array[1] ?: "no" (with array=[10,20])	20	"no"
-5 ?: 99	-5	5

Root cause

In src/parser.js, the ?: infix handler stored the same left AST node reference into both condition and then:

infix("?:", operators['?:'], function (left) {
    this.type = 'condition';
    this.condition = left;
    this.then = left;        // SAME REFERENCE
    this.else = expression(0);
    return this;
});

The post-parse processAST step then walks both branches, but processAST mutates the AST it receives:

• The [ (predicate) handler pushes a filter stage onto the LHS step. Run twice on the same node, you get two filters — so array[1] becomes effectively array[1][1].
• The unary-- handler folds -N into the underlying number literal by mutating its value in place. Run twice, the negation is undone — so -5 becomes 5.

This is the exact same class of bug that was fixed for the ?? operator in #774 / commit fd47061 (closing #773). The fix was never propagated to ?:.

Fix

Deep-clone left for the condition branch so the two branches have independent AST nodes — mirroring the ?? fix:

infix("?:", operators['?:'], function (left) {
    this.type = 'condition';
    this.condition = JSON.parse(JSON.stringify(left));
    this.then = left;
    this.else = expression(0);
    return this;
});

Tests

Added 5 new test cases under test/test-suite/groups/default-operator/:

• case014.json — array predicate on LHS exists, returns LHS value
• case015.json — array predicate on LHS does not exist, returns RHS value
• case016.json — negative array index on multi-element array
• case017.json — negative array index on single-element array
• case018.json — unary minus on number literal LHS is not double-negated

Full suite passes locally: 1768 passing, 100% coverage maintained.

Test plan

• npm test passes (1768 tests)
• 100% coverage maintained
• All cases from the linked issue return correct values
• Existing default-operator tests still pass

Signed-off-by: Leo Garbe leo@respark.dev

[andrew-coleman]

Looks good. Thank you!