From 6f682fa91782daa3be09c0f1243ffa72fbe4fd27 Mon Sep 17 00:00:00 2001
From: maxtaran2010 <ocotifuzo727@gmail.com>
Date: Fri, 10 Jul 2026 00:06:55 +0300
Subject: [PATCH] fix(string): use nullish check in template() to preserve
 falsy variable values

`vars[key] || fallback` incorrectly falls through to the fallback when
a variable is explicitly set to a falsy value like 0, false, or "".
Switch to `key in vars ? vars[key] : fallback` so only missing keys
trigger the fallback, not legitimate falsy values.

Adds test cases for 0, false, and "" to prevent regression.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
---
 src/string.test.ts | 13 +++++++++++++
 src/string.ts      |  2 +-
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/src/string.test.ts b/src/string.test.ts
index 6821e6d..6ca9988 100644
--- a/src/string.test.ts
+++ b/src/string.test.ts
@@ -94,6 +94,19 @@ it('namedTemplate', () => {
       k => String(+k * 2),
     ),
   ).toEqual('2 4 6 known key')
+
+  // Falsy values that are explicitly set should not fall back to the fallback
+  expect(
+    template('{count}', { count: 0 }, 'N/A'),
+  ).toEqual('0')
+
+  expect(
+    template('{active}', { active: false }, 'unknown'),
+  ).toEqual('false')
+
+  expect(
+    template('{name}', { name: '' }, 'anonymous'),
+  ).toEqual('')
 })
 
 it('slash', () => {
diff --git a/src/string.ts b/src/string.ts
index 1dc048c..cb0284a 100644
--- a/src/string.ts
+++ b/src/string.ts
@@ -67,7 +67,7 @@ export function template(str: string, ...args: any[]): string {
 
   if (isObject(firstArg)) {
     const vars = firstArg as Record<string, any>
-    return str.replace(/\{(\w+)\}/g, (_, key) => vars[key] || ((typeof fallback === 'function' ? fallback(key) : fallback) ?? key))
+    return str.replace(/\{(\w+)\}/g, (_, key) => key in vars ? vars[key] : ((typeof fallback === 'function' ? fallback(key) : fallback) ?? key))
   }
   else {
     return str.replace(/\{(\d+)\}/g, (_, key) => {