Fix string/number concatenation with add operator (#23)

* Fix string/number concatenation with add operator
* Add unit tests for sub, mul, div, mod, and pow operators

Author: Sander-Toonen

src/operators/binary/arithmetic.ts

@@ -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.
@@ -40,11 +46,6 @@ export function add(a: any, b: any): any {
     return { ...a, ...b };
   }
 
-  // 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}`);
 }

test/functions/functions-binary-ops.ts

@@ -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();