fix: formula 的加法支持字符串拼接 (#5251)

packages/amis-formula/__tests__/fomula.test.ts

@@ -138,6 +138,8 @@ test('formula:math', () => {
   expect(evalFormual('"3" - "3"')).toBe(0);
   expect(evalFormual('AVG(4, "6", "10", 10, 10)')).toBe(8);
   expect(evalFormual('MAX(4, "6", "10", 2, 3)')).toBe(10);
+
+  expect(evalFormual('"a" + "b"')).toBe('ab');
 });
 
 test('formula:text', () => {

packages/amis-formula/src/evalutor.ts

@@ -230,6 +230,10 @@ export class Evaluator {
   add(ast: {left: any; right: any}) {
     const left = this.evalute(ast.left);
     const right = this.evalute(ast.right);
+    // 如果有一个不是数字就变成字符串拼接
+    if (isNaN(left) || isNaN(right)) {
+      return left + right;
+    }
     return stripNumber(this.formatNumber(left) + this.formatNumber(right));
   }