Duplicate Parameters In JavaScript Function

Hi Devs,

In this article i will introduce you to one of the most confusing and common doubt for every newbie devs, Duplicate parameters in javascript functions

Table Of Contents

• duplicating parameter in regular JS function
• duplicating parameter in non strict mode
• duplicating parameter in strict mode
• How do arrow functions treat duplicate parameters

First we will see duplicating parameters in regular JavaScript function.

//this is syntax of duplicating parameter in js function
function Func(first, second, first) {
  console.log(first, second, first);
}

In non-strict mode, regular JavaScript functions allow duplicate named parameters

function Func(first, second, first) {
  console.log(first, second, first);
}
// first => 1
// second => 2
// first => 3
Func(1, 2, 3); // 3 2 3

// first => 1
// second => 2
// first => undefined
Func(1, 2); //undefined  [undefined, 2, undefined]

Lets check this in strict mode,

function Func(first, second, first) {
  "use strict";
  console.log(first, second, first);
}
//Throws an error because of duplicate parameters (Strict mode)

In Strict mode we cannot duplicate the parameter name.

How do arrow functions treat duplicate parameters?

Now here is something about arrow functions:

Unlike regular functions, arrow functions do not allow duplicate parameters, whether in strict or non-strict mode. Duplicate parameters will cause a Syntax Error to be thrown.

// Always throws a syntax error
const Func = (first, second, first) => {
  console.log(first, second);
};

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Thanks,
CapsCode