语法错误：相等测试 (==) 被错误地输入为赋值 (=)？

¥Message

Warning: SyntaxError: test for equality (==) mistyped as assignment (=)?

¥Error type

（仅限 Firefox）仅当 javascript.options.strict 首选项设置为 true 时才会报告 SyntaxError 警告。

¥(Firefox only) SyntaxError warning which is reported only if javascript.options.strict preference is set to true.

¥What went wrong?

当你通常期望进行相等性测试（==）时，有一个作业（=）。为了帮助调试，JavaScript（启用严格警告）会对此模式发出警告。

¥There was an assignment (=) when you would normally expect a test for equality (==). To help debugging, JavaScript (with strict warnings enabled) warns about this pattern.

¥Examples

¥Assignment within conditional expressions

建议不要在条件表达式中使用简单的赋值（例如 if...else），因为浏览代码时赋值可能会与相等相混淆。例如，请勿使用以下代码：

¥It is advisable to not use simple assignments in a conditional expression (such as if...else), because the assignment can be confused with equality when glancing over the code. For example, do not use the following code:

if (x = y) {
  // do the right thing
}

如果需要在条件表达式中使用赋值，常见的做法是在赋值周围添加额外的括号。例如：

¥If you need to use an assignment in a conditional expression, a common practice is to put additional parentheses around the assignment. For example:

if ((x = y)) {
  // do the right thing
}

否则，你可能打算使用比较运算符（例如 == 或 ===）：

¥Otherwise, you probably meant to use a comparison operator (e.g. == or ===):

if (x === y) {
  // do the right thing
}

¥See also

• if...else
• 相等运算符