语法错误：左侧赋值无效

¥Message

SyntaxError: Invalid left-hand side in assignment (V8-based)
SyntaxError: invalid assignment left-hand side (Firefox)
SyntaxError: Left side of assignment is not a reference. (Safari)

¥Error type

SyntaxError 或 ReferenceError，具体取决于语法。

¥SyntaxError or ReferenceError, depending on the syntax.

¥What went wrong?

某处有一个意想不到的任务。例如，这可能是由于 赋值运算符 和 相等运算符 不匹配造成的。虽然单个 = 符号将值分配给变量，但 == 或 === 运算符比较值。

¥There was an unexpected assignment somewhere. This might be due to a mismatch of an assignment operator and an equality operator, for example. While a single = sign assigns a value to a variable, the == or === operators compare a value.

¥Examples

¥Typical invalid assignments

if (Math.PI + 1 = 3 || Math.PI + 1 = 4) {
  console.log("no way!");
}
// SyntaxError: invalid assignment left-hand side

const str = "Hello, "
+= "is it me "
+= "you're looking for?";
// SyntaxError: invalid assignment left-hand side

在 if 语句中，你要使用相等运算符 (===)，而对于字符串连接，需要使用加号 (+) 运算符。

¥In the if statement, you want to use an equality operator (===), and for the string concatenation, the plus (+) operator is needed.

if (Math.PI + 1 === 3 || Math.PI + 1 === 4) {
  console.log("no way!");
}

const str = "Hello, "
  + "from the "
  + "other side!";

¥Assignments producing ReferenceErrors

无效的赋值并不总是会产生语法错误。有时语法几乎是正确的，但在运行时，左侧表达式的计算结果是一个值而不是引用，因此赋值仍然无效。此类错误会在执行稍后（即实际执行该语句时）发生。

¥Invalid assignments don't always produce syntax errors. Sometimes the syntax is almost correct, but at runtime, the left hand side expression evaluates to a value instead of a reference, so the assignment is still invalid. Such errors occur later in execution, when the statement is actually executed.

function foo() {
  return { a: 1 };
}
foo() = 1; // ReferenceError: invalid assignment left-hand side

函数调用、new 调用、super() 和 this 都是值而不是引用。如果你想在左侧使用它们，则赋值目标需要是它们生成值的属性。

¥Function calls, new calls, super(), and this are all values instead of references. If you want to use them on the left hand side, the assignment target needs to be a property of their produced values instead.

function foo() {
  return { a: 1 };
}
foo().a = 1;

注意：在 Firefox 和 Safari 中，第一个示例在非严格模式下生成 ReferenceError，在 严格模式 下生成 SyntaxError。Chrome 会为严格模式和非严格模式抛出运行时 ReferenceError。

¥Note: In Firefox and Safari, the first example produces a ReferenceError in non-strict mode, and a SyntaxError in strict mode. Chrome throws a runtime ReferenceError for both strict and non-strict modes.

¥Using optional chaining as assignment target

可选链接 不是有效的分配目标。

¥Optional chaining is not a valid target of assignment.

obj?.foo = 1; // SyntaxError: invalid assignment left-hand side

相反，你必须首先保护无效的情况。

¥Instead, you have to first guard the nullish case.

if (obj) {
  obj.foo = 1;
}

¥See also

• 赋值运算符
• 相等运算符