Warnings

How to tell closure compiler which warnings you want

Introduction

Closure Compiler has a --warning_level flag that gives you a few high-level options to control what warnings you see. The --warning_level flag gives you an easy way to choose the most common options. But even --warning_level=VERBOSE doesn't give you all the warnings that Closure Compiler can emit, and sometimes you want more fine-grained control.

Warnings Level API

Closure Compiler has an API for configuring the errors and warnings that you would like to see, and what level they're emitted at.

Flag API	Java API	Effect
--jscomp_error=<group>	options.setWarningLevel("<group>", CheckLevel.ERROR);	Makes all warnings of the given group to build-breaking error.
--jscomp_warning=<group>	options.setWarningLevel("<group>", CheckLevel.WARNING);	Makes all warnings of the given group a non-breaking warning.
--jscomp_off=<group>	options.setWarningLevel("<group>", CheckLevel.OFF);	Silences all warnings of the given group.

Warnings Categories

In the examples above, <group> is a pre-defined category of warnings.

Run the compiler with the --help flag to get a comprehensive up-to-date list.

The following table describes the available diagnostic groups.

Name	Effect	Default Value
accessControls	Warnings when @deprecated, @private, or @protected are violated.	OFF
ambiguousFunctionDecl	Warnings about ambiguous definitions of functions	WARNING
checkDebuggerStatement	Check for the use of the debugger statement	OFF
checkRegExp	Check for regular expression literal problems	OFF
checkTypes	Type-checking	OFF by default, WARNING on --warning_level=VERBOSE
checkVars	Warnings when vars are not declared.	OFF by default, ERROR on --warning_level=VERBOSE
const	Warnings when @const annotated properties or variables are reassigned or deleted.	OFF
constantProperty	Warnings when @const annotated properties are reassigned or deleted.	OFF
deprecated	Warnings when non-deprecated code accesses code that's marked @deprecated	OFF by default, WARNING on --warning_level=VERBOSE
duplicate	Warnings when a variable is declared twice in the global scope	OFF by default, ERROR on --warning_level=VERBOSE
es5Strict	Warnings about code that is invalid in EcmaScript 5 Strict mode	WARNING
externsValidation	Warnings about malformed externs files	WARNING
fileoverviewTags	Warnings about duplicate @fileoverview tags	WARNING
globalThis	Warnings about improper use of the global this.	OFF by default, WARNING on --warning_level=VERBOSE
internetExplorerChecks	Warnings about syntax errors on Internet Explorer, like trailing commas.	ERROR
invalidCasts	Warnings about invalid type casts.	OFF by default, WARNING when type checking is on
missingProperties	Warnings about whether a property will ever be defined on an object. Part of type-checking.	OFF by default, WARNING on --warning_level=VERBOSE
nonStandardJsDocs	Warnings when JSDoc has annotations that the compiler thinks you misspelled.	WARNING
strictModuleDepCheck	Warnings about all references potentially violating module dependencies	OFF
suspiciousCode	Warning about things like missing missing semicolons and comparisons to NaN	WARNING
undefinedNames	Warnings when properties of global names are undefined.	OFF by default, WARNING on --warning_level=VERBOSE
undefinedVars	Warnings when variables are undefined.	OFF by default, ERROR on --warning_level=VERBOSE
unknownDefines	Warnings when unknown @define values are specified.	WARNING
uselessCode	Warnings when the compiler sees useless code that it plans to remove.	WARNING
visibility	Warnings when @private and @protected are violated.	OFF

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Defining Your Own Categories

If there is a category of warnings that you would like to configure but is not listed in the table above, it is easy to define a new one.

1. Find the unique identifier of the warnings that you'd like to configure.
2. Create a new DiagnosticGroup that gives this warning category a human-readable name, and add it to our central repository of warnings categories: https://github.com/google/closure-compiler/blob/master/src/com/google/javascript/jscomp/DiagnosticGroups.java
3. Send us a patch!

This is how we "canary" new warnings until we feel they're stable.

@suppress Tags

You can also silence warnings by adding JSDoc annotations to your code. All of the warnings categories above can be used in a @suppress tag in a @fileoverview JSDoc comment or a function JSDoc comment. For example,

/**
 * @fileoverview This is a file where deprecation checks are disabled.
 * @suppress {deprecated}
 */

/**
 * This is a function where type checking is disabled.
 * @suppress {checkTypes}
 */
function f() { /** ... */ }

There is also one warning category that you can suppress with @suppress tags, but is not available from the command-line.

/** @suppress {duplicate} */ foo.prop = ...

Suppresses warnings about a declaration of the same method or property twice in the global scope.