语法错误：不能在 `||` 和 `&&` 表达式中使用不带括号的 `??`

¥Message

SyntaxError: Unexpected token '??' (V8-based)
SyntaxError: cannot use `??` unparenthesized within `||` and `&&` expressions (Firefox)
SyntaxError: Unexpected token '??'. Coalescing and logical operators used together in the same expression; parentheses must be used to disambiguate. (Safari)

¥Error type

¥What went wrong?

运算符优先级 链如下所示：

¥The operator precedence chain looks like this:

|   >   &&   >   ||   >   =
|   >   ??   >   =

但是，故意未定义 ?? 和 &&/|| 之间的优先级，因为逻辑运算符的短路行为可能会使表达式的计算违反直觉。因此，以下组合都是语法错误，因为语言不知道如何给操作数加上括号：

¥However, the precedence between ?? and &&/|| is intentionally undefined, because the short circuiting behavior of logical operators can make the expression's evaluation counter-intuitive. Therefore, the following combinations are all syntax errors, because the language doesn't know how to parenthesize the operands:

a ?? b || c;
a || b ?? c;
a ?? b && c;
a && b ?? c;

相反，通过明确地在任一侧加上括号来明确你的意图：

¥Instead, make your intent clear by parenthesizing either side explicitly:

(a ?? b) || c;
a ?? (b && c);

¥Examples

当迁移使用 || 和 && 来防御 null 或 undefined 的旧代码时，你通常可能会对其进行部分转换：

¥When migrating legacy code that uses || and && for guarding against null or undefined, you may often convert it partially:

function getId(user, fallback) {
  // Previously: user && user.id || fallback
  return user && user.id ?? fallback; // SyntaxError: cannot use `??` unparenthesized within `||` and `&&` expressions
}

相反，请考虑将 && 添加括号：

¥Instead, consider parenthesizing the &&:

function getId(user, fallback) {
  return (user && user.id) ?? fallback;
}

更好的是，考虑使用 可选链接 而不是 &&：

¥Even better, consider using optional chaining instead of &&:

function getId(user, fallback) {
  return user?.id ?? fallback;
}

¥See also

• TC39 无效合并提案中的 关于无效合并优先级的问题
• 空合并运算符 (??)
• 运算符优先级