语法错误：重复的形式参数 x

¥Message

SyntaxError: Duplicate parameter name not allowed in this context (V8-based)
SyntaxError: duplicate formal argument x (Firefox)
SyntaxError: duplicate argument names not allowed in this context (Firefox)
SyntaxError: Cannot declare a parameter named 'x' in strict mode as it has already been declared. (Safari)
SyntaxError: Duplicate parameter 'x' not allowed in function with default parameter values. (Safari)
SyntaxError: Duplicate parameter 'x' not allowed in function with a rest parameter. (Safari)
SyntaxError: Duplicate parameter 'x' not allowed in function with destructuring parameters. (Safari)

¥Error type

¥What went wrong?

有两个同名的形式参数很可能是一个错误 - 第二次出现会导致第一次出现无法通过参数名称访问。在旧版 JavaScript 中，这是允许的。因此，为了不破坏现有代码，这只有在保证代码不是遗留代码的情况下才会出错 - 因为它在 严格模式 中，或者它使用现代参数语法（rest、default 或 destructured 参数）。

¥Having two formal parameters of the same name is likely a mistake—the second occurrence would cause the first occurrence to be inaccessible through the parameter name. In legacy JavaScript, this was allowed. Therefore, to not break existing code, this is only an error if the code is guaranteed to not be legacy—either because it is in strict mode or it uses modern parameter syntax (rest, default, or destructured parameters).

¥Examples

¥Invalid cases

"use strict";

function add(x, x) {
  // How can you access both "x" parameters?
  // SyntaxError: duplicate formal argument x
}

function doSomething(name, { name }) {
  // How can you access both "name" parameters?
  // SyntaxError: duplicate argument names not allowed in this context
}

¥Valid cases

function doSomething(operationName, { name: userName }) {
  // You can access both "operationName" and "userName" parameters.
}

function doSomething(name, user) {
  // You can access both "name" and "user.name" parameters.
}

¥See also

• 函数
• 严格模式