fix: implement strict type coercion in DataContext

renderers/web_core/src/v0_9/rendering/coercion.ts

@@ -0,0 +1,115 @@
+/*
+ * Copyright 2025 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+ * Strict Type Coercion Utilities
+ * 
+ * Implements the A2UI protocol's standard coercion rules to ensure
+ * consistent handling of null/undefined values and type conversions.
+ * 
+ * Without central enforcement, component authors must manually handle
+ * these edge cases, leading to bugs like [object Object] appearing
+ * in text labels.
+ */
+
+/**
+ * Coerces any value to a string following A2UI protocol rules:
+ * - null/undefined → ""
+ * - objects → localized string representation (not "[object Object]")
+ * - other types → String(value)
+ */
+export function coerceToString(value: unknown): string {
+  if (value === null || value === undefined) {
+    return "";
+  }
+  if (typeof value === "object") {
+    // Avoid "[object Object]" by using JSON.stringify for plain objects
+    // or calling toString for objects with custom implementations
+    if (Array.isArray(value)) {
+      return value.map(coerceToString).join(", ");
+    }
+    if (value instanceof Error) {
+      return value.message;
+    }
+    try {
+      return JSON.stringify(value);
+    } catch {
+      return String(value);
+    }
+  }
+  return String(value);
+}
+
+/**
+ * Coerces any value to a number following A2UI protocol rules:
+ * - null/undefined → 0
+ * - strings → parsed number (NaN becomes 0)
+ * - booleans → 1 for true, 0 for false
+ * - other types → Number(value)
+ */
+export function coerceToNumber(value: unknown): number {
+  if (value === null || value === undefined) {
+    return 0;
+  }
+  if (typeof value === "string") {
+    const parsed = parseFloat(value);
+    return isNaN(parsed) ? 0 : parsed;
+  }
+  if (typeof value === "boolean") {
+    return value ? 1 : 0;
+  }
+  const result = Number(value);
+  return isNaN(result) ? 0 : result;
+}
+
+/**
+ * Coerces any value to a boolean following A2UI protocol rules:
+ * - "true" (case-insensitive) → true
+ * - non-zero numbers → true
+ * - null/undefined → false
+ * - other types → Boolean(value)
+ */
+export function coerceToBoolean(value: unknown): boolean {
+  if (value === null || value === undefined) {
+    return false;
+  }
+  if (typeof value === "string") {
+    return value.toLowerCase() === "true" || value !== "";
+  }
+  if (typeof value === "number") {
+    return value !== 0;
+  }
+  return Boolean(value);
+}
+
+/**
+ * Coerces a value to a specific target type.
+ */
+export function coerceValue<T>(value: unknown, targetType: "string"): string;
+export function coerceValue<T>(value: unknown, targetType: "number"): number;
+export function coerceValue<T>(value: unknown, targetType: "boolean"): boolean;
+export function coerceValue<T>(value: unknown, targetType: string): unknown {
+  switch (targetType) {
+    case "string":
+      return coerceToString(value);
+    case "number":
+      return coerceToNumber(value);
+    case "boolean":
+      return coerceToBoolean(value);
+    default:
+      return value;
+  }
+}

renderers/web_core/src/v0_9/rendering/data-context.ts

@@ -23,6 +23,7 @@ import type {
 } from "../schema/common-types.js";
 import { A2uiExpressionError } from "../errors.js";
 import type { FunctionInvoker } from "../catalog/types.js";
+import { coerceToString, coerceToNumber, coerceToBoolean } from "./coercion.js";
 
 /**
  * A contextual view of the main DataModel, serving as the unified interface for resolving