util: fix %s format behavior with Symbol.toPrimitive

This commit ensures `console.log("%s", obj)` correctly invokes `obj[Symbol.toPrimitive]` for string conversion, fixing unexpected object display issue. Fixes: #50909
nodejs · May 11, 2024 · 796a3ab · 796a3ab
1 parent c4d0229
commit 796a3ab
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diff --git a/lib/internal/util/inspect.js b/lib/internal/util/inspect.js
@@ -87,6 +87,7 @@ const {
   SymbolIterator,
   SymbolPrototypeToString,
   SymbolPrototypeValueOf,
+  SymbolToPrimitive,
   SymbolToStringTag,
   TypedArrayPrototypeGetLength,
   TypedArrayPrototypeGetSymbolToStringTag,
@@ -2103,6 +2104,11 @@ function hasBuiltInToString(value) {
     value = proxyTarget;
   }
 
+  // Check if value has a custom Symbol.toPrimitive transformation.
+  if (typeof value[SymbolToPrimitive] === 'function') {
+    return false;
+  }
+
   // Count objects that have no `toString` function as built-in.
   if (typeof value.toString !== 'function') {
     return true;

diff --git a/test/parallel/test-util-format.js b/test/parallel/test-util-format.js
@@ -269,6 +269,27 @@ assert.strictEqual(util.format('%s', -Infinity), '-Infinity');
   );
 }
 
+// Symbol.toPrimitive handling for string format specifier
+{
+  const objectWithToPrimitive = {
+    [Symbol.toPrimitive](hint) {
+      switch (hint) {
+        case 'number':
+          return 42;
+        case 'string':
+          return 'string representation';
+        case 'default':
+        default:
+          return 'default context';
+      }
+    }
+  };
+
+  assert.strictEqual(util.format('%s', +objectWithToPrimitive), '42');
+  assert.strictEqual(util.format('%s', objectWithToPrimitive), 'string representation');
+  assert.strictEqual(util.format('%s', objectWithToPrimitive + ''), 'default context');
+}
+
 // JSON format specifier
 assert.strictEqual(util.format('%j'), '%j');
 assert.strictEqual(util.format('%j', 42), '42');