Cognitive Complexity

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"

Further reading

• Cognitive Complexity docs
• Cognitive Complexity: A new way of measuring understandability

Enforces return statements in callbacks of array's methods (array-callback-return)

Array has several methods for filtering, mapping, and folding. If we forget to write return statement in a callback of those, it's probably a mistake.

// example: convert ['a', 'b', 'c'] --> {a: 0, b: 1, c: 2}
var indexMap = myArray.reduce(function(memo, item, index) {
  memo[item] = index;
}, {}); // Error: cannot set property 'b' of undefined

This rule enforces usage of return statement in callbacks of array's methods.

Rule Details

This rule finds callback functions of the following methods, then checks usage of return statement.

• Array.from
• Array.prototype.every
• Array.prototype.filter
• Array.prototype.find
• Array.prototype.findIndex
• Array.prototype.map
• Array.prototype.reduce
• Array.prototype.reduceRight
• Array.prototype.some
• Array.prototype.sort
• And above of typed arrays.

Examples of incorrect code for this rule:

/*eslint array-callback-return: "error"*/

var indexMap = myArray.reduce(function(memo, item, index) {
    memo[item] = index;
}, {});

var foo = Array.from(nodes, function(node) {
    if (node.tagName === "DIV") {
        return true;
    }
});

var bar = foo.filter(function(x) {
    if (x) {
        return true;
    } else {
        return;
    }
});

Examples of correct code for this rule:

/*eslint array-callback-return: "error"*/

var indexMap = myArray.reduce(function(memo, item, index) {
    memo[item] = index;
    return memo;
}, {});

var foo = Array.from(nodes, function(node) {
    if (node.tagName === "DIV") {
        return true;
    }
    return false;
});

var bar = foo.map(node => node.getAttribute("id"));

Known Limitations

This rule checks callback functions of methods with the given names, even if the object which has the method is not an array.

When Not To Use It

If you don't want to warn about usage of return statement in callbacks of array's methods, then it's safe to disable this rule. Source: http://eslint.org/docs/rules/