Duplicate key '{a}'


This warning has existed in three forms in JSLint, JSHint and ESLint. It was introduced in the original version of JSLint and has remained in all three linters ever since.

  • In JSLint the warning given is the generic "Duplicate '{a}'"

  • In JSHint prior to version 1.0.0 the message used was "Duplicate member '{a}'"

  • In JSHint 1.0.0 and above and ESLint the message is "Duplicate key '{a}'"

When do I get this error?

The "Duplicate key '{a}'" error, and the alternatives "Duplicate member '{a}'" and "Duplicate '{a}'", is thrown when JSLint, JSHint or ESLint encounters an an object literal that contains more than one property with the same identifier. In the following example we attempt to assign an object containing two properties with the identifier y to a variable x:

var x = {
    y: 10,
    y: 20

Why do I get this error?

This error is raised to highlight code that may not work as you expect and could possibly cause a fatal JavaScript syntax error. In strict mode, your code will raise a syntax error. Otherwise, it will run without error but you will most likely get unexpected results.

The specification states the following, where "previous"" is the result of calling [[GetOwnProperty]] on the object in question with the identifier we are trying to add to it (ES5 §11.1.5):

If previous is not undefined then throw a SyntaxError exception if any of the following conditions are true
    a. This production is contained in strict code...


So, when your code is running in strict mode, a syntax error will be thrown if you attempt to define multiple properties with the same identifier. When not in strict mode, no error is thrown but usually the latest definition will override any earlier definitions and you may experience some strange bugs if the naming was unintentional. Another good reason to always run your code in strict mode.

In JSHint 1.0.0 and above you have the ability to ignore any warning with a special option syntax. The identifier of this warning is W075. This means you can tell JSHint to not issue this warning with the /*jshint -W075 */ directive.

In ESLint the rule that generates this warning is named no-dupe-keys. You can disable it by setting it to 0, or enable it by setting it to 1.

About the author

James Allardice

This article was written by James Allardice, Software engineer at Tesco and orangejellyfish in London. Passionate about React, Node and writing clean and maintainable JavaScript. Uses linters (currently ESLint) every day to help achieve this.