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Orion 5.0 introduced ESLint as the validator that ships with Orion. This page captures ongoing issues.


The following sections describe all of the rules currently available in Orion and ones that we plan to add in the future.

To view this table in the same order as the Orion Settings page, hover over the Setting Message column title and press the down arrow to sort.

Potential Programming Problems

These rules report about problems with your code that might have undesired or incorrect results. Warnings from these rules should be addressed.

Rule Default Severity Available Since Setting Message Problem Message Details Quick Fix? Fix All?
no-comma-dangle Ignore 8.0 Trailing commas in object expressions Trailing commas in object expressions are discouraged. Flags trailing commas in object expressions.
no-cond-assign Error 8.0 Assignments in conditional expressions Expected a conditional expression and instead saw an assignment. Flags assignment in an if/while/do..while/for condition.
no-console Ignore 8.0 Discouraged console use in browser code Discouraged use of console in browser-based code. Flags the use of console in browser-based code
no-constant-condition Error 8.0 Constant as conditional expression ... is constant. Flags the use of a constant as a conditional expression
no-const-assign Error 13.0 Disallow modifying variables that are declared using const Discouraged use of constant as a conditional expression. Flags the assignment to a constant variable
no-control-regex Error 11.0 Disallow control characters in regular expressions Unexpected control character in regular expression Control characters are special, invisible characters in the ASCII range 0-31. These characters are rarely used in JavaScript strings so a regular expression containing these characters is most likely a mistake.
no-debugger Warning 6.0 Discouraged 'debugger' statement use debugger' statement use is discouraged. Warns when the debugger keyword is used
no-dupe-keys Error 6.0 Duplicate object keys Duplicate object key '{key}'. Flags object literals that contain the same key more than once.
no-duplicate-case Error 11.0 Duplicate case Duplicate case label. A switch statements with duplicate case labels is normally an indication of a programmer error.
no-empty-block Ignore 7.0 Undocumented empty block Empty block should be removed or commented. Flags an empty block like if (something) { }.
no-empty-character-class Error 11.0 Disallow empty character classes Empty class. Empty character classes in regular expressions do not match anything and can result in code that may not work as intended.
no-extra-boolean-cast Error 11.0 Discourage redundant double negation Redundant double negation in ... In contexts such as an if statement's test where the result of the expression will already be coerced to a Boolean, casting to a Boolean via double negation (!!) is unnecessary.
no-extra-parens Warning 11.0 Discourage redundant parentheses Gratuitous parentheses around expression. This rule restricts the use of parentheses to only where they are necessary. It may be restricted to report only function expressions.
no-extra-semi Warning 5.0 Unnecessary semicolons Unnecessary semicolon. Warns when a semicolon is found where it is not needed
no-fallthrough Error 6.0 Switch case fall-through Switch case may be entered by falling through previous case. If intended, add a new comment //$FALL-THROUGH$ on the line above Flags a fallthrough case within a switch statement, unless it is explicitly commented.
no-invalid-regexp Error 11.0 Invalid regular expressions Invalid flags supplied to RegExp constructor '{${0}}'. or the error message from calling the constructor with the regex This rule validates string arguments passed to the RegExp constructor.
no-irregular-whitespace Ignore 11.0 Report irregular whitespace 'Irregular whitespace not allowed This rule validates all whitespaces
no-negated-in-lhs Error 11.0 Disallow negated left operand of in operator The 'in' expression's left operand is negated. Disallow negated left operand of in operator
no-obj-calls Error 11.0 Disallow global object as function calls '{${0}}' is not a function. ECMAScript provides several global objects that are intended to be used as-is. Some of these objects look as if they could be constructors due their capitalization (such as Math and JSON) but will throw an error if you try to execute them as functions.
no-regex-spaces Error 8.0 Multiple spaces in regular expressions Avoid using multiple spaces in regular expressions. Use ' {${0}}' instead. Flags regular expressions that have two or more subsequent spaces.
no-reserved-keys Ignore 8.0 Reserved words used as property keys Reserved words should not be used as property keys. Checks to see if a JavaScript reserved word is being used as a property key
no-sparse-arrays Warning 8.0 Sparse array declarations Sparse array declarations should be avoided. Flags sparse arrays with null elements like var arr = [1, ,2]
no-unreachable Warning 6.0 Unreachable code Unreachable code. Flags statements that occur after a return, throw, etc.
type-checked-consistent-return Ignore 11.0 Discouraged inconsistent returns Inconsistent return types: (type1) (type2) Warns when a function returns two values with different types.
use-isnan Error 6.0 NaN not compared with isNaN() Use the isNaN function to compare with NaN. Warns when a comparison is done against NaN without using the isNaN function
valid-typeof Error 7.0 Invalid 'typeof' comparison Invalid typeof comparison. Warns when a typeof comparison uses an invalid type.

Best Practices

These rules are not necessarily problem with your code, but are warnings that you might be doing something you probably should not be.

Rule Default Severity Available Since Setting Message Problem Message Details Quick Fix? Fix All?
accessor-pairs Warning 11.0 Getter and setter accessors not in pairs Getter is not present or Setter is not present It's a common mistake in JavaScript to create an object with just a setter for a property but never have a corresponding getter defined for it. Without a getter, you cannot read the property, so it ends up not being used.
check-tern-plugin Warning 12.0 Missing .tern-project plugins entry for environment directive To work in the 'foo' environment, the 'bar' plugin must be running. Warn when the an eslint-env directive specifies a library Orion provides tooling support for, but the library plugin has been disabled in the .tern-project file.
curly Ignore 6.0 Statements not enclosed in braces Statements should be enclosed in braces. Warns when statements should be enclosed in braces, for example if(something) foo();
eqeqeq Warning 5.0 Discouraged '==' use
  • Expected '===' and saw '=='.
  • Expected '!==' and saw '!='.
Warns when == or != is used
missing-requirejs Warning 12.0 'requirejs' plugin missing in AMD environment To use AMD, the 'requirejs' plug-in needs to be running. Flags references to function calls from AMD when the Tern 'requirejs' plugin is not running
no-caller Error 9.0 Discouraged 'arguments.caller' or 'arguments.callee' use arguments.{callee, caller} is deprecated. Flags references to arguments.callee and arguments.caller. Future
no-else-return Warning 11.0 Unnecessary else after return Unexpected 'else' after 'return'. If an if block contains a return statement, the else block becomes unnecessary. Its contents can be placed outside of the block.
no-empty-label Error 11.0 No empty labels Unexpected label "{${0}}" Labeled statements are only used in conjunction with labeled break and continue statements. ECMAScript has no goto statement.
no-eq-null Error 11.0 Disallow null comparisons Use ‘===’ to compare with ‘null’. or Use ‘!==’ to compare with ‘null’. Comparing to null without a type-checking operator (== or !=), can have unintended results as the comparison will evaluate to true when comparing to not just a null, but also an undefined value.
no-extend-native Error 13.0 Disallow extending of native objects '..... prototype is read only, properties should not be added.' Disallows directly modifying the prototype of builtin objects.
no-extra-bind Warning 13.0 Unnecessary 'bind()' use The function binding is unnecessary. Warns about unnecessary use of bind()
no-eval Ignore 6.0 Discouraged 'eval()' use 'eval' function calls are discouraged. Warns when the eval() function is used
no-implicit-coercion Warning 13.0 Disallow the type conversion with shorter notations. use `Boolean(..)` instead., use `...indexOf(".") !== -1` instead., use `Number(..)` instead., use `String(foo)` instead. This rule is aimed to flag shorter notations for the type conversion, then suggest a more self-explanatory notation.
no-implied-eval Ignore 6.0 Discouraged implied 'eval()' use Implicit 'eval' function calls are discouraged. Flags calls to the string-argument form of setTimeout and setInterval, which implicitly perform eval. This rule logic was merged to be part of the no-eval rule.
no-invalid-this Warning 13.0 Discouraged 'this' use outside of classes or class-like objects Unexpected 'this' This rule aims to flag usage of 'this' keywords outside of classes or class-like objects
no-iterator Error 8.0 Discouraged __iterator__ property use Discouraged __iterator__ property use. Flags use of __iterator__ as an identifier name or property key.
no-lone-blocks Ignore 13.0 Discouraged lone block use Block is redundant. or Nested block is redundant. Flags use of unnecessary and potentially confusing blocks at the top level of a script or within other blocks.
no-native-reassign Error 13.0 Disallow reassignment of native objects ... is a read-only native object. This rule flags modifications to read-only global variables
no-new-array Warning 6.0 Discouraged 'new Array()' Use the array literal notation []. Flags new Array(). ESLint calls this rule no-array-constructor
no-new-func Warning 6.0 Discouraged 'new Function()' The Function constructor is eval. Flags new Function().
no-new-object Warning 6.0 Discouraged 'new Object()' Use the object literal notation {}. Flags new Object().
no-new-wrappers Warning 6.0 Discouraged wrapper objects Do not use {String, Number, Boolean, Math, JSON} as a constructor. Flags new applied to any of those.
no-param-reassign Ignore 13.0 Disallow Reassignment of Function Parameters
  • Assignment to function parameter 'bar'.
  • Assignment to property of function parameter 'bar'.
Report parameter reassignment inside functions. Can also report reassignment of parameter's properties.
no-proto Error 9.0 Discouraged __proto__ property use
  • Reserved name '__proto__' should not be assigned.
  • Reserved name '__proto__' should not be used as a key.
Flags use of the property __proto__ as object keys or in assignments
no-redeclare Warning 5.0 Variable re-declarations '{a}' is already defined. Usually results from having two for loops in the same function that share a loop variable declaration like var i=....
no-self-assign Error 11.0 Disallow self assignment '{a}' is assigned to itself. Warns when a variable is being assigned to itself
no-self-compare Error 11.0 Disallow self compare Comparing to itself is potentially pointless. Comparing a variable against itself is usually an error, either an typo or refactoring error. It is confusing to the reader and may potentially introduce a runtime error.
no-shadow Warning 8.0 Variable shadowing '{a}' is already declared in the upper scope. Flags variables that have the same name as a variable declared in an upper scope.
no-shadow-global Warning 9.0 Global shadowing '{a}' shadows a global member. Flags variables that have the same name as a variable declared in the global scope or specified environments.
no-throw-literal Warning 9.0 Literal used in 'throw' Throw an Error instead. Flags code that throws a literal, eg. throw "a problem occurred";
no-undef Error 5.0 Undeclared global reference '{variable}' is not defined.
  • Flags references to a global variable that is not listed in a /*global*/ or /*globals*/ block.
  • Predefined environments (eg. node, browser, amd) can be set in a /*eslint-env*/ block.
no-undef-expression Warning 12.0 Undeclared function reference '{member}' is undefined for '{object}' in {file}. Flags references to a function that is not known to be a property of an object. Will only warn if the object has known properties.
no-undef-init Warning 9.0 Undeclared function reference Avoid explicitly initializing variables to 'undefined'. Avoid initializing variables to undefined.
no-unused-expressions Ignore 13.0 Unused expressions Expected an assignment or function call and instead saw an expression. Flags expressions that have no effect on the running code
no-unused-params Warning 6.0 Unused parameters Parameter '{param}' is not used. Flags parameters in function declarations / expressions if they are not being used
no-unused-vars Warning 5.0 Unused variables
  • '{a}' is unread.
  • '{a}' is read-only.
  • '{a}' is unused.
  • Function '{a}' is unused
Warns when a variable is created and not used (read) or a variable that is read-only is assigned to.

Warns when a function is not used.

no-use-before-define Warning 5.0 Member used before definition '{a}' was used before it was defined. Warns when a variable, function or class is used before it is defined
no-void Ignore 13.0 Disallow use of the void operator. Expected 'undefined' and instead saw 'void'. Discourage the usage of the void operator.
no-with Warning 9.0 Discouraged 'with' statement use Expected an identifier and instead saw 'with'. Treated as fatal parse error in JSLint.
radix Warning 8.0 Missing radix parameter to parseInt() Missing radix parameter. Warns when parseInt() called without the 2nd parameter (radix).
unknown-require Warning 12.0 Validation of the tern settings This library could not be found, type information for it will be incomplete. Validation of the tern settings for libraries.
yoda Ignore 13.0 Require or disallow Yoda Conditions
  • Expected literal to be on the right side of ...
  • Expected literal to be on the left side of ...
Enforce consistent style of conditions which compare a variable to a literal value.

Code Style

These rules are not problems in any way but will warn based on certain accepted styles for coding.

Rule Default Severity Available Since Setting Message Problem Message Details Quick Fix? Fix All?
missing-doc Ignore 5.0 Missing JSDoc Missing documentation for function '{name}'. Flags missing documentation nodes on function declarations and function expressions when they appear as object properties
missing-nls Ignore 9.0 Non-externalized string literals (missing $NON-NLS$ tag) Non-externalized string literal '{a}'. Flags String literals that are not properly NLS'd
new-parens Error 6.0 Missing parentheses in constructor call Missing parentheses invoking constructor. Flags new Whatever
no-mixed-spaces-and-tabs Ignore 11.0 Mixed spaces and tabs Irregular whitespace not allowed. Invalid or irregular whitespace causes issues with ECMAScript 5 parsers and also makes code harder to debug in a similar nature to mixed tabs and spaces.
no-jslint Warning 7.0 Unsupported environment directive The 'jslint' directive is unsupported, please use eslint-env. Flags use of the /* jslint */ directive Future
no-trailing-spaces Ignore 13.0 Unsupported environment directive Trailing spaces not allowed. Flags any trailing spaces
quotes Ignore 13.0 Validate which quotes should be used (either single, double or backtick (es6 only)) Strings must use doublequote., Strings must use singlequote., Strings must use backtick. Warns when a quote is not the expected one
semi Warning 5.0 Missing semicolons Missing semicolon. Warns when a semicolon should be used but is not
unnecessary-nls Ignore 11.0 Unnecessary $NON-NLS$ tags Unnecessary $NON-NLS$ tag. Flags $NON-NLS$ comments that are not needed to exclude a string literal

Future Implementations

Rules that are being considered for inclusion or were implemented but removed.

Rule Default Severity Available Since Setting Message Problem Message Details Quick Fix? Fix All?
block-scoped-var Error - '{variable}' used outside of binding context.

Occurs when a function-scoped variable (var) is treated as if it was block scoped.

missing-doc-items Ignore - Missing '{a}' tag for '{b}'. Flags missing documentation for parameters, returns, throws, etc on function declarations and function expressions when they appear as object properties

Implementing a new rule

This section explains how to implement a new linting rule. For brevity, all paths are given relative to bundles/org.eclipse.orion.client.javascript/web/ in the client repo.

First, create the tests and rule:

  1. Create a new describe("{rule-name}") block in: /js-tests/javascript/eslintRuleTests.js. Add several tests to it.
  2. Run the JS bundle tests and ensure that your new tests fail.
  3. Implement the actual logic for your rule in /eslint/lib/load-rules-async.js.
  4. Run the tests again. This time they should all pass.

To get your rule running in the Orion product's validator, there are additional steps:

  1. Create strings for the validation message(s) generated by your rule in /javascript/nls/root/problems.js.
  2. Create a short string describing what your rule does in /javascript/nls/root/messages.js.
  3. Add an entry to the orion.core.setting declaration in /javascript/plugins/javascriptPlugin.js. This allows your rule to be configured from the JS validator settings page. Make sure it references the same nameKey used in messages.js.
  4. Add a config entry for your rule to /javascript/ruleData.js.
  5. Add your rule to the #updated() handler in /javascript/validator.js.

Reload the JS plugin, and your rule should now be configurable from the Settings page, and properly set up for translation too.


  • Every rule needs extensive unit tests.
  • Unit tests should use Mocha.

Running the tests

  • To run the ESLint tests as part of the main JavaScript tests suite load {orion-server-url}/javascript/js-tests/JsMochaSuite.html in your web browser.
  • To run just the ESLint rule tests, load {orion-server-url}/js-tests/javascript/JsMochaSuite.html?grep=ESLint%20Rule%20Tests in your browser.
  • To run just the ESLint core tests, load {orion-server-url}/js-tests/javascript/JsMochaSuite.html?grep=ESLint%20Core%20Tests in your browser.
  • To run only a subset of the tests, add ?grep=ESLint%20Rule%20Tests onto the test page URL.

See Mocha usage options for more options.

User interface

  • Should we have a UI for configuring what rules are active? bug 424268.
  • Should we try to support .eslintrc? This would be an ideal project-scope setting, and is being tracked in bug 461228.
  • Should we try to honor equivalent JSLint/JSHint flags when possible? For example /*jslint eqeqeq:false */ could disable the eqeqeq rule on a per-file basis.
    • ESLint now has its own syntax for this: /*eslint ..*/, which we are using instead.


We need to support i18n. Pre-req is bug 422278 (orion.edit.validator support for i18n).

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