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Fix string/number concatenation with add operator #23

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Merged
Sander-Toonen merged 3 commits into from
Jan 23, 2026
Merged

Fix string/number concatenation with add operator #23

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9cd9850
Fix string/number concatenation with add operator
Jan 23, 2026
c214647
Add unit tests for sub, mul, div, mod, and pow operators (#25)
Copilot Jan 23, 2026
0215fb8
Restrict string concatenation to reject array/object/boolean operands…
Copilot Jan 23, 2026
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17 changes: 9 additions & 8 deletions src/operators/binary/arithmetic.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -14,15 +14,21 @@ export function add(a: any, b: any): any {
return a + b;
}

// If both values are strings and at least one of them is a non-number
// then we want to concatenate the strings.
if (typeof a === 'string' && typeof b === 'string') {
// If one of the values is a string and both are either string or number, try to add or else concatenate.
if (
(typeof a === 'string' || typeof b === 'string') &&
(typeof a === 'string' || typeof a === 'number') &&
(typeof b === 'string' || typeof b === 'number')
) {
const numA = Number(a);
const numB = Number(b);

if (isNaN(numA) || isNaN(numB)) {
return `${a}${b}`;
}

// If both values can be converted to numbers then we want to add the numbers.
return numA + numB;
}

// If both values are arrays then we want to concatenate the arrays.
Expand All @@ -40,11 +46,6 @@ export function add(a: any, b: any): any {
return { ...a, ...b };
}

Copilot AI Jan 23, 2026

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By removing the numeric-coercion fallback (previously adding when both operands could be converted via Number()), expressions like true + 1 / 1 + true will now hit the final throw. This breaks existing behavior covered by tests (test/functions/functions-binary-ops.ts:52-57). Reintroduce the numeric conversion branch (after the array/object cases so null + null still merges to {}) to keep boolean/number (and similar) additions working.

Suggested change
// As a fallback, if both values can be converted to numbers then add the numbers.
const numA = Number(a);
const numB = Number(b);
if (!isNaN(numA) && !isNaN(numB)) {
return numA + numB;
}

Copilot uses AI. Check for mistakes.
// If both values can be converted to numbers then we want to add the numbers.
if (!isNaN(Number(a)) && !isNaN(Number(b))) {
return Number(a) + Number(b);
}

// Otherwise return an error indicating that the values of mixed types cannot be added.
throw new Error(`Cannot add values of incompatible types: ${typeof a} and ${typeof b}`);
}
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8 changes: 4 additions & 4 deletions test/functions/functions-binary-ops.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -49,11 +49,11 @@ describe('Binary Operators TypeScript Test', function () {
const parser = new Parser();
assert.deepStrictEqual(parser.evaluate('null + null'), {});
});
it('should convert numeric values to numbers before adding', function () {
it('should throw error for boolean operands', function () {
const parser = new Parser();
assert.strictEqual(parser.evaluate('true + 1'), 2);
assert.strictEqual(parser.evaluate('false + 5'), 5);
assert.strictEqual(parser.evaluate('1 + true'), 2);
assert.throws(() => parser.evaluate('true + 1'), /Cannot add values of incompatible types/);
assert.throws(() => parser.evaluate('false + 5'), /Cannot add values of incompatible types/);
assert.throws(() => parser.evaluate('1 + true'), /Cannot add values of incompatible types/);
});
it('should throw error for incompatible types', function () {
const parser = new Parser();
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