'{a}' was used before it was defined

History

This warning has existed in two forms across the three main linters. It was introduced in the original version of JSLint and has remained in all three tools ever since.

  • In JSLint the warning given is "'{a}' was used before it was defined"

  • In JSHint and ESLint the message has always been "'{a}' is not defined"

  • In JSHint and ESLint the message "'{a}' was used before it was defined" is issued under closely related circumstances

The situations that produce the warning have not changed despite changes to the text of the warning itself.

When do I get this error?

The "'{a}' was used before it was defined" error (and the alternative "'{a}' is not defined" error) is thrown when JSLint, JSHint and ESLint encounter an identifier that has not been previously declared in a var statement or function declaration. Some very common examples of this error are those that refer to native environment objects:

  • "'document' was used before it was defined"
  • "'window' was used before it was defined"
  • "'alert' was used before it was defined"
  • "'console' was used before it was defined"
  • "'require' was used before it was defined"

In the following example we attempt to set the value of the undeclared variable x and then attempt to use some native browser environment objects:

x = 10;
alert("Errors...");
console.log("Errors everywhere");

In JSHint and ESLint the "'{a}' was used before it was defined" error (as opposed to the "'{a}' is not defined" error) is raised when a reference to an identifier occurs before the declaration of that identifier. In the following example we reference the variable a before we declare it:

/*jshint latedef: true */
function test() {
    "use strict";
    a = 1;
    var a;
    return a;
}

Why do I get this error?

This error is raised to highlight potentially dangerous code. Your code may run without error, depending on the identifier in question, but is likely to cause confusion to other developers and could in some cases cause a fatal error that will prevent the rest of your script from executing.

The example above is valid JavaScript when not running in strict mode. It will create a property of the global object (in the browser, the global object is window) with the given identifier. If you had accidentally omitted the var keyword, you could have ended up overwriting a variable declared in a parent scope, causing unexpected behaviour. If it does run in strict mode, it will generate a reference error as it's illegal to assign a value to an undefined variable under such conditions ([ES5 Annex C]es5-c]):

Assignment to an undeclared identifier or otherwise unresolvable reference does not create a property in the global object. When a simple assignment occurs within strict mode code, its LeftHandSide must not evaluate to an unresolvable Reference. If it does a ReferenceError exception is thrown.

If you are referring to an identifier that has been declared elsewhere (in another JavaScript file included in the page for example), you can tell JSLint, JSHint and ESLint about it by using the global directive:

/*global someFunction */
var x = someFunction();

If you have mistakenly omitted a var keyword, you can fix this error by simply adding it in. If you omitted the keyword on purpose (to allow access to a variable from other scope for example), declare the variable in the top-most scope in which it should be available:

/*global someFunction */
var x = 10;

In the case of environment-specific global identifiers (like window or document in the browser or module in Node.js) there are a few JSLint/JSHint options that you can set to let the linter know what environment the code is expected to run in:

  • browser - JSLint, JSHint and ESLint. Defines global variables available in the browser.

  • devel - JSLint and JSHint. Defines global variables that are only used in a development environment (such as alert and console).

  • node - JSLint, JSHint and ESLint. Defines global variables available in Node.js.

  • couch - JSLint and JSHint. Defines global variables available in CouchDB.

  • rhino - JSLint and JSHint. Defines global variables available in Rhino.

  • phantom - JSHint only. Defines global variables available in PhantomJS.

  • shelljs - JSHint only. Defines global variables available in ShellJS

  • typed - JSHint only. Defines global typed array variables (such as Int32Array and ArrayBuffer)

  • wsh - JSHint only. Defines global variables available in Windows Script Host

  • mocha - ESLint only. Defines global variables available in the Mocha test framework

JSHint also has a set options that tell it about libraries your script has access to. They can be set in the same way as the environment options listed above:

  • dojo - Defines global variables provided by the Dojo Toolkit

  • jquery - Defines global variables provided by jQuery

  • mootools - Defines global variables provided by Mootools

  • prototypejs - Defines global variables provided by PrototypeJS

  • yui - Defines global variables provided by YUI

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 W117. This means you can tell JSHint to not issue this warning with the /*jshint -W117 */ directive.

In ESLint the rule that generates this warning is named no-use-before-define. 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, JavaScript engineer at Mammal in London. Passionate about AngularJS, Node and writing clean and maintainable JavaScript and uses JSHint every day to help achieve this.