Cognitive Complexity is a measure of how difficult a unit of code is to intuitively understand. Unlike Cyclomatic Complexity, which determines how difficult your code will be to test, Cognitive Complexity tells you how difficult your code will be to read and comprehend.
A method's cognitive complexity is based on a few simple rules:
Code is not considered more complex when it uses shorthand that the language provides for collapsing multiple statements into one
Code is considered more complex for each "break in the linear flow of the code"
Code is considered more complex when "flow breaking structures are nested"
Cognitive Complexity is a measure of how difficult a unit of code is to intuitively understand. Unlike Cyclomatic Complexity, which determines how difficult your code will be to test, Cognitive Complexity tells you how difficult your code will be to read and comprehend.
A method's cognitive complexity is based on a few simple rules:
Code is not considered more complex when it uses shorthand that the language provides for collapsing multiple statements into one
Code is considered more complex for each "break in the linear flow of the code"
Code is considered more complex when "flow breaking structures are nested"
This rule can help you locate potential ReferenceErrors resulting from misspellings of variable and parameter names, or accidental implicit globals (for example, from forgetting the var keyword in a for loop initializer).
Rule Details
Any reference to an undeclared variable causes a warning, unless the variable is explicitly mentioned in a /*global ...*/ comment.
Examples of incorrect code for this rule:
/*eslint no-undef: "error"*/
var a =someFunction();
b =10;
Examples of correct code for this rule with global declaration:
/*global someFunction b:true*/
/*eslint no-undef: "error"*/
var a =someFunction();
b =10;
The b:true syntax in /*global */ indicates that assignment to b is correct.
Examples of incorrect code for this rule with global declaration:
/*global b*/
/*eslint no-undef: "error"*/
b =10;
By default, variables declared in /*global */ are read-only, therefore assignment is incorrect.
Options
typeof set to true will warn for variables used inside typeof check (Default false).
typeof
Examples of correct code for the default { "typeof": false } option:
/*eslint no-undef: "error"*/
if(typeof UndefinedIdentifier ==="undefined"){
// do something ...
}
You can use this option if you want to prevent typeof check on a variable which has not been declared.
Examples of incorrect code for the { "typeof": true } option:
For convenience, ESLint provides shortcuts that pre-define global variables exposed by popular libraries and runtime environments. This rule supports these environments, as listed in Specifying Environments. A few examples are given below.
browser
Examples of correct code for this rule with browser environment:
/*eslint no-undef: "error"*/
/*eslint-env browser*/
setTimeout(function(){
alert("Hello");
});
node
Examples of correct code for this rule with node environment:
/*eslint no-undef: "error"*/
/*eslint-env node*/
var fs =require("fs");
module.exports=function(){
console.log(fs);
};
When Not To Use It
If explicit declaration of global variables is not to your taste.
This rule can help you locate potential ReferenceErrors resulting from misspellings of variable and parameter names, or accidental implicit globals (for example, from forgetting the var keyword in a for loop initializer).
Rule Details
Any reference to an undeclared variable causes a warning, unless the variable is explicitly mentioned in a /*global ...*/ comment.
Examples of incorrect code for this rule:
/*eslint no-undef: "error"*/
var a =someFunction();
b =10;
Examples of correct code for this rule with global declaration:
/*global someFunction b:true*/
/*eslint no-undef: "error"*/
var a =someFunction();
b =10;
The b:true syntax in /*global */ indicates that assignment to b is correct.
Examples of incorrect code for this rule with global declaration:
/*global b*/
/*eslint no-undef: "error"*/
b =10;
By default, variables declared in /*global */ are read-only, therefore assignment is incorrect.
Options
typeof set to true will warn for variables used inside typeof check (Default false).
typeof
Examples of correct code for the default { "typeof": false } option:
/*eslint no-undef: "error"*/
if(typeof UndefinedIdentifier ==="undefined"){
// do something ...
}
You can use this option if you want to prevent typeof check on a variable which has not been declared.
Examples of incorrect code for the { "typeof": true } option:
For convenience, ESLint provides shortcuts that pre-define global variables exposed by popular libraries and runtime environments. This rule supports these environments, as listed in Specifying Environments. A few examples are given below.
browser
Examples of correct code for this rule with browser environment:
/*eslint no-undef: "error"*/
/*eslint-env browser*/
setTimeout(function(){
alert("Hello");
});
node
Examples of correct code for this rule with node environment:
/*eslint no-undef: "error"*/
/*eslint-env node*/
var fs =require("fs");
module.exports=function(){
console.log(fs);
};
When Not To Use It
If explicit declaration of global variables is not to your taste.
This rule can help you locate potential ReferenceErrors resulting from misspellings of variable and parameter names, or accidental implicit globals (for example, from forgetting the var keyword in a for loop initializer).
Rule Details
Any reference to an undeclared variable causes a warning, unless the variable is explicitly mentioned in a /*global ...*/ comment.
Examples of incorrect code for this rule:
/*eslint no-undef: "error"*/
var a =someFunction();
b =10;
Examples of correct code for this rule with global declaration:
/*global someFunction b:true*/
/*eslint no-undef: "error"*/
var a =someFunction();
b =10;
The b:true syntax in /*global */ indicates that assignment to b is correct.
Examples of incorrect code for this rule with global declaration:
/*global b*/
/*eslint no-undef: "error"*/
b =10;
By default, variables declared in /*global */ are read-only, therefore assignment is incorrect.
Options
typeof set to true will warn for variables used inside typeof check (Default false).
typeof
Examples of correct code for the default { "typeof": false } option:
/*eslint no-undef: "error"*/
if(typeof UndefinedIdentifier ==="undefined"){
// do something ...
}
You can use this option if you want to prevent typeof check on a variable which has not been declared.
Examples of incorrect code for the { "typeof": true } option:
For convenience, ESLint provides shortcuts that pre-define global variables exposed by popular libraries and runtime environments. This rule supports these environments, as listed in Specifying Environments. A few examples are given below.
browser
Examples of correct code for this rule with browser environment:
/*eslint no-undef: "error"*/
/*eslint-env browser*/
setTimeout(function(){
alert("Hello");
});
node
Examples of correct code for this rule with node environment:
/*eslint no-undef: "error"*/
/*eslint-env node*/
var fs =require("fs");
module.exports=function(){
console.log(fs);
};
When Not To Use It
If explicit declaration of global variables is not to your taste.
This rule can help you locate potential ReferenceErrors resulting from misspellings of variable and parameter names, or accidental implicit globals (for example, from forgetting the var keyword in a for loop initializer).
Rule Details
Any reference to an undeclared variable causes a warning, unless the variable is explicitly mentioned in a /*global ...*/ comment.
Examples of incorrect code for this rule:
/*eslint no-undef: "error"*/
var a =someFunction();
b =10;
Examples of correct code for this rule with global declaration:
/*global someFunction b:true*/
/*eslint no-undef: "error"*/
var a =someFunction();
b =10;
The b:true syntax in /*global */ indicates that assignment to b is correct.
Examples of incorrect code for this rule with global declaration:
/*global b*/
/*eslint no-undef: "error"*/
b =10;
By default, variables declared in /*global */ are read-only, therefore assignment is incorrect.
Options
typeof set to true will warn for variables used inside typeof check (Default false).
typeof
Examples of correct code for the default { "typeof": false } option:
/*eslint no-undef: "error"*/
if(typeof UndefinedIdentifier ==="undefined"){
// do something ...
}
You can use this option if you want to prevent typeof check on a variable which has not been declared.
Examples of incorrect code for the { "typeof": true } option:
For convenience, ESLint provides shortcuts that pre-define global variables exposed by popular libraries and runtime environments. This rule supports these environments, as listed in Specifying Environments. A few examples are given below.
browser
Examples of correct code for this rule with browser environment:
/*eslint no-undef: "error"*/
/*eslint-env browser*/
setTimeout(function(){
alert("Hello");
});
node
Examples of correct code for this rule with node environment:
/*eslint no-undef: "error"*/
/*eslint-env node*/
var fs =require("fs");
module.exports=function(){
console.log(fs);
};
When Not To Use It
If explicit declaration of global variables is not to your taste.
This rule can help you locate potential ReferenceErrors resulting from misspellings of variable and parameter names, or accidental implicit globals (for example, from forgetting the var keyword in a for loop initializer).
Rule Details
Any reference to an undeclared variable causes a warning, unless the variable is explicitly mentioned in a /*global ...*/ comment.
Examples of incorrect code for this rule:
/*eslint no-undef: "error"*/
var a =someFunction();
b =10;
Examples of correct code for this rule with global declaration:
/*global someFunction b:true*/
/*eslint no-undef: "error"*/
var a =someFunction();
b =10;
The b:true syntax in /*global */ indicates that assignment to b is correct.
Examples of incorrect code for this rule with global declaration:
/*global b*/
/*eslint no-undef: "error"*/
b =10;
By default, variables declared in /*global */ are read-only, therefore assignment is incorrect.
Options
typeof set to true will warn for variables used inside typeof check (Default false).
typeof
Examples of correct code for the default { "typeof": false } option:
/*eslint no-undef: "error"*/
if(typeof UndefinedIdentifier ==="undefined"){
// do something ...
}
You can use this option if you want to prevent typeof check on a variable which has not been declared.
Examples of incorrect code for the { "typeof": true } option:
For convenience, ESLint provides shortcuts that pre-define global variables exposed by popular libraries and runtime environments. This rule supports these environments, as listed in Specifying Environments. A few examples are given below.
browser
Examples of correct code for this rule with browser environment:
/*eslint no-undef: "error"*/
/*eslint-env browser*/
setTimeout(function(){
alert("Hello");
});
node
Examples of correct code for this rule with node environment:
/*eslint no-undef: "error"*/
/*eslint-env node*/
var fs =require("fs");
module.exports=function(){
console.log(fs);
};
When Not To Use It
If explicit declaration of global variables is not to your taste.
This rule can help you locate potential ReferenceErrors resulting from misspellings of variable and parameter names, or accidental implicit globals (for example, from forgetting the var keyword in a for loop initializer).
Rule Details
Any reference to an undeclared variable causes a warning, unless the variable is explicitly mentioned in a /*global ...*/ comment.
Examples of incorrect code for this rule:
/*eslint no-undef: "error"*/
var a =someFunction();
b =10;
Examples of correct code for this rule with global declaration:
/*global someFunction b:true*/
/*eslint no-undef: "error"*/
var a =someFunction();
b =10;
The b:true syntax in /*global */ indicates that assignment to b is correct.
Examples of incorrect code for this rule with global declaration:
/*global b*/
/*eslint no-undef: "error"*/
b =10;
By default, variables declared in /*global */ are read-only, therefore assignment is incorrect.
Options
typeof set to true will warn for variables used inside typeof check (Default false).
typeof
Examples of correct code for the default { "typeof": false } option:
/*eslint no-undef: "error"*/
if(typeof UndefinedIdentifier ==="undefined"){
// do something ...
}
You can use this option if you want to prevent typeof check on a variable which has not been declared.
Examples of incorrect code for the { "typeof": true } option:
For convenience, ESLint provides shortcuts that pre-define global variables exposed by popular libraries and runtime environments. This rule supports these environments, as listed in Specifying Environments. A few examples are given below.
browser
Examples of correct code for this rule with browser environment:
/*eslint no-undef: "error"*/
/*eslint-env browser*/
setTimeout(function(){
alert("Hello");
});
node
Examples of correct code for this rule with node environment:
/*eslint no-undef: "error"*/
/*eslint-env node*/
var fs =require("fs");
module.exports=function(){
console.log(fs);
};
When Not To Use It
If explicit declaration of global variables is not to your taste.
This rule can help you locate potential ReferenceErrors resulting from misspellings of variable and parameter names, or accidental implicit globals (for example, from forgetting the var keyword in a for loop initializer).
Rule Details
Any reference to an undeclared variable causes a warning, unless the variable is explicitly mentioned in a /*global ...*/ comment.
Examples of incorrect code for this rule:
/*eslint no-undef: "error"*/
var a =someFunction();
b =10;
Examples of correct code for this rule with global declaration:
/*global someFunction b:true*/
/*eslint no-undef: "error"*/
var a =someFunction();
b =10;
The b:true syntax in /*global */ indicates that assignment to b is correct.
Examples of incorrect code for this rule with global declaration:
/*global b*/
/*eslint no-undef: "error"*/
b =10;
By default, variables declared in /*global */ are read-only, therefore assignment is incorrect.
Options
typeof set to true will warn for variables used inside typeof check (Default false).
typeof
Examples of correct code for the default { "typeof": false } option:
/*eslint no-undef: "error"*/
if(typeof UndefinedIdentifier ==="undefined"){
// do something ...
}
You can use this option if you want to prevent typeof check on a variable which has not been declared.
Examples of incorrect code for the { "typeof": true } option:
For convenience, ESLint provides shortcuts that pre-define global variables exposed by popular libraries and runtime environments. This rule supports these environments, as listed in Specifying Environments. A few examples are given below.
browser
Examples of correct code for this rule with browser environment:
/*eslint no-undef: "error"*/
/*eslint-env browser*/
setTimeout(function(){
alert("Hello");
});
node
Examples of correct code for this rule with node environment:
/*eslint no-undef: "error"*/
/*eslint-env node*/
var fs =require("fs");
module.exports=function(){
console.log(fs);
};
When Not To Use It
If explicit declaration of global variables is not to your taste.
Variables that are declared and not used anywhere in the code are most likely an error due to incomplete refactoring. Such variables take up space in the code and can lead to confusion by readers.
Rule Details
This rule is aimed at eliminating unused variables, functions, and parameters of functions.
A variable is considered to be used if any of the following are true:
It represents a function that is called (doSomething())
It is read (var y = x)
It is passed into a function as an argument (doSomething(x))
It is read inside of a function that is passed to another function (doSomething(function() { foo(); }))
A variable is not considered to be used if it is only ever assigned to (var x = 5) or declared.
Examples of incorrect code for this rule:
/*eslint no-unused-vars: "error"*/
/*global some_unused_var*/
// It checks variables you have defined as global
some_unused_var =42;
var x;
// Write-only variables are not considered as used.
var y =10;
y =5;
// A read for a modification of itself is not considered as used.
var z =0;
z = z +1;
// By default, unused arguments cause warnings.
(function(foo){
return5;
})();
// Unused recursive functions also cause warnings.
functionfact(n){
if(n <2)return1;
return n *fact(n -1);
}
// When a function definition destructures an array, unused entries from the array also cause warnings.
functiongetY([x, y]){
return y;
}
Examples of correct code for this rule:
/*eslint no-unused-vars: "error"*/
var x =10;
alert(x);
// foo is considered used here
myFunc(functionfoo(){
// ...
}.bind(this));
(function(foo){
return foo;
})();
var myFunc;
myFunc =setTimeout(function(){
// myFunc is considered used
myFunc();
},50);
// Only the second argument from the descructured array is used.
functiongetY([, y]){
return y;
}
exported
In environments outside of CommonJS or ECMAScript modules, you may use var to create a global variable that may be used by other scripts. You can use the /* exported variableName */ comment block to indicate that this variable is being exported and therefore should not be considered unused.
Note that /* exported */ has no effect for any of the following:
when the environment is node or commonjs
when parserOptions.sourceType is module
when ecmaFeatures.globalReturn is true
The line comment // exported variableName will not work as exported is not line-specific.
Examples of correct code for /* exported variableName */ operation:
/* exported global_var */
var global_var =42;
Options
This rule takes one argument which can be a string or an object. The string settings are the same as those of the vars property (explained below).
By default this rule is enabled with all option for variables and after-used for arguments.
The varsIgnorePattern option specifies exceptions not to check for usage: variables whose names match a regexp pattern. For example, variables whose names contain ignored or Ignored.
Examples of correct code for the { "varsIgnorePattern": "[iI]gnored" } option:
after-used - only the last argument must be used. This allows you, for instance, to have two named parameters to a function and as long as you use the second argument, ESLint will not warn you about the first. This is the default setting.
all - all named arguments must be used.
none - do not check arguments.
args: after-used
Examples of incorrect code for the default { "args": "after-used" } option:
The ignoreRestSiblings option is a boolean (default: false). Using a Rest Property it is possible to "omit" properties from an object, but by default the sibling properties are marked as "unused". With this option enabled the rest property's siblings are ignored.
Examples of correct code for the { "ignoreRestSiblings": true } option:
// 'type' is ignored because it has a rest property sibling.
var{ type,...coords }= data;
argsIgnorePattern
The argsIgnorePattern option specifies exceptions not to check for usage: arguments whose names match a regexp pattern. For example, variables whose names begin with an underscore.
Examples of correct code for the { "argsIgnorePattern": "^_" } option:
The caughtErrorsIgnorePattern option specifies exceptions not to check for usage: catch arguments whose names match a regexp pattern. For example, variables whose names begin with a string 'ignore'.
Examples of correct code for the { "caughtErrorsIgnorePattern": "^ignore" } option:
If you don't want to be notified about unused variables or function arguments, you can safely turn this rule off.
Source: http://eslint.org/docs/rules/
Disallow Undeclared Variables (no-undef)
This rule can help you locate potential ReferenceErrors resulting from misspellings of variable and parameter names, or accidental implicit globals (for example, from forgetting the var keyword in a for loop initializer).
Rule Details
Any reference to an undeclared variable causes a warning, unless the variable is explicitly mentioned in a /*global ...*/ comment.
Examples of incorrect code for this rule:
/*eslint no-undef: "error"*/
var a =someFunction();
b =10;
Examples of correct code for this rule with global declaration:
/*global someFunction b:true*/
/*eslint no-undef: "error"*/
var a =someFunction();
b =10;
The b:true syntax in /*global */ indicates that assignment to b is correct.
Examples of incorrect code for this rule with global declaration:
/*global b*/
/*eslint no-undef: "error"*/
b =10;
By default, variables declared in /*global */ are read-only, therefore assignment is incorrect.
Options
typeof set to true will warn for variables used inside typeof check (Default false).
typeof
Examples of correct code for the default { "typeof": false } option:
/*eslint no-undef: "error"*/
if(typeof UndefinedIdentifier ==="undefined"){
// do something ...
}
You can use this option if you want to prevent typeof check on a variable which has not been declared.
Examples of incorrect code for the { "typeof": true } option:
For convenience, ESLint provides shortcuts that pre-define global variables exposed by popular libraries and runtime environments. This rule supports these environments, as listed in Specifying Environments. A few examples are given below.
browser
Examples of correct code for this rule with browser environment:
/*eslint no-undef: "error"*/
/*eslint-env browser*/
setTimeout(function(){
alert("Hello");
});
node
Examples of correct code for this rule with node environment:
/*eslint no-undef: "error"*/
/*eslint-env node*/
var fs =require("fs");
module.exports=function(){
console.log(fs);
};
When Not To Use It
If explicit declaration of global variables is not to your taste.
This rule can help you locate potential ReferenceErrors resulting from misspellings of variable and parameter names, or accidental implicit globals (for example, from forgetting the var keyword in a for loop initializer).
Rule Details
Any reference to an undeclared variable causes a warning, unless the variable is explicitly mentioned in a /*global ...*/ comment.
Examples of incorrect code for this rule:
/*eslint no-undef: "error"*/
var a =someFunction();
b =10;
Examples of correct code for this rule with global declaration:
/*global someFunction b:true*/
/*eslint no-undef: "error"*/
var a =someFunction();
b =10;
The b:true syntax in /*global */ indicates that assignment to b is correct.
Examples of incorrect code for this rule with global declaration:
/*global b*/
/*eslint no-undef: "error"*/
b =10;
By default, variables declared in /*global */ are read-only, therefore assignment is incorrect.
Options
typeof set to true will warn for variables used inside typeof check (Default false).
typeof
Examples of correct code for the default { "typeof": false } option:
/*eslint no-undef: "error"*/
if(typeof UndefinedIdentifier ==="undefined"){
// do something ...
}
You can use this option if you want to prevent typeof check on a variable which has not been declared.
Examples of incorrect code for the { "typeof": true } option:
For convenience, ESLint provides shortcuts that pre-define global variables exposed by popular libraries and runtime environments. This rule supports these environments, as listed in Specifying Environments. A few examples are given below.
browser
Examples of correct code for this rule with browser environment:
/*eslint no-undef: "error"*/
/*eslint-env browser*/
setTimeout(function(){
alert("Hello");
});
node
Examples of correct code for this rule with node environment:
/*eslint no-undef: "error"*/
/*eslint-env node*/
var fs =require("fs");
module.exports=function(){
console.log(fs);
};
When Not To Use It
If explicit declaration of global variables is not to your taste.
This rule can help you locate potential ReferenceErrors resulting from misspellings of variable and parameter names, or accidental implicit globals (for example, from forgetting the var keyword in a for loop initializer).
Rule Details
Any reference to an undeclared variable causes a warning, unless the variable is explicitly mentioned in a /*global ...*/ comment.
Examples of incorrect code for this rule:
/*eslint no-undef: "error"*/
var a =someFunction();
b =10;
Examples of correct code for this rule with global declaration:
/*global someFunction b:true*/
/*eslint no-undef: "error"*/
var a =someFunction();
b =10;
The b:true syntax in /*global */ indicates that assignment to b is correct.
Examples of incorrect code for this rule with global declaration:
/*global b*/
/*eslint no-undef: "error"*/
b =10;
By default, variables declared in /*global */ are read-only, therefore assignment is incorrect.
Options
typeof set to true will warn for variables used inside typeof check (Default false).
typeof
Examples of correct code for the default { "typeof": false } option:
/*eslint no-undef: "error"*/
if(typeof UndefinedIdentifier ==="undefined"){
// do something ...
}
You can use this option if you want to prevent typeof check on a variable which has not been declared.
Examples of incorrect code for the { "typeof": true } option:
For convenience, ESLint provides shortcuts that pre-define global variables exposed by popular libraries and runtime environments. This rule supports these environments, as listed in Specifying Environments. A few examples are given below.
browser
Examples of correct code for this rule with browser environment:
/*eslint no-undef: "error"*/
/*eslint-env browser*/
setTimeout(function(){
alert("Hello");
});
node
Examples of correct code for this rule with node environment:
/*eslint no-undef: "error"*/
/*eslint-env node*/
var fs =require("fs");
module.exports=function(){
console.log(fs);
};
When Not To Use It
If explicit declaration of global variables is not to your taste.