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"
Unlike statically-typed languages which enforce that a function returns a specified type of value, JavaScript allows different code paths in a function to return different types of values.
A confusing aspect of JavaScript is that a function returns undefined if any of the following are true:
it does not execute a return statement before it exits
it executes return which does not specify a value explicitly
it executes return undefined
it executes return void followed by an expression (for example, a function call)
it executes return followed by any other expression which evaluates to undefined
If any code paths in a function return a value explicitly but some code path do not return a value explicitly, it might be a typing mistake, especially in a large function. In the following example:
a code path through the function returns a Boolean value true
another code path does not return a value explicitly, therefore returns undefined implicitly
functiondoSomething(condition){
if(condition){
returntrue;
}else{
return;
}
}
Rule Details
This rule requires return statements to either always or never specify values. This rule ignores function definitions where the name begins with an uppercase letter, because constructors (when invoked with the new operator) return the instantiated object implicitly if they do not return another object explicitly.
Examples of incorrect code for this rule:
::: incorrect
/*eslint consistent-return: "error"*/
functiondoSomething(condition){
if(condition){
returntrue;
}else{
return;
}
}
functiondoSomething(condition){
if(condition){
returntrue;
}
}
:::
Examples of correct code for this rule:
::: correct
/*eslint consistent-return: "error"*/
functiondoSomething(condition){
if(condition){
returntrue;
}else{
returnfalse;
}
}
functionFoo(){
if(!(thisinstanceofFoo)){
returnnewFoo();
}
this.a =0;
}
:::
Options
This rule has an object option:
"treatUndefinedAsUnspecified": false (default) always either specify values or return undefined implicitly only.
"treatUndefinedAsUnspecified": true always either specify values or return undefined explicitly or implicitly.
treatUndefinedAsUnspecified
Examples of incorrect code for this rule with the default { "treatUndefinedAsUnspecified": false } option:
If you want to allow functions to have different return behavior depending on code branching, then it is safe to disable this rule.
Source: http://eslint.org/docs/rules/