SyntaxError: duplicate formal argument x
The JavaScript exception "duplicate formal argument x" or "duplicate argument names not allowed in this context" occurs when a function creates two or more parameterbindings with the same name, and the function is not anon-strict function with only simple parameters.
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?
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 instrict mode or it uses modern parameter syntax (rest,default, ordestructured parameters).
Examples
Invalid cases
js
"use strict";function add(x, x) { // How can you access both "x" parameters? // SyntaxError: duplicate formal argument x}js
function doSomething(name, { name }) { // How can you access both "name" parameters? // SyntaxError: duplicate argument names not allowed in this context}Valid cases
js
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.}