In non-strict mode, JavaScript allows a function to have several parameters with the same name, where later parameters shadow earlier parameters. This is an error-prone feature which makes code hard to understand and should be avoided.

Rename the parameters to have different names.

In the following example, function f has two parameters named x, the second one presumably being a typo for y. Since no parameter y is declared, the use of y in the body of f refers to a global variable of that name, if it exists.

Duplicate parameter names are illegal in strict mode.

  • Ecma International, ECMAScript Language Definition, 5.1 Edition, Annex C. ECMA, 2011.
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