export function newInstance(name, initialProps = {}, mountTo = undefined) {
  // clean all left over components
  cleanVirtualDom();
  // validate inputs
  const definition = getDefinition(name);

Close

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

  return newInstance;

Close

require return statements to either always or never specify values (consistent-return)

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

function doSomething(condition) {
    if (condition) {
        return true;
    } 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:

/*eslint consistent-return: "error"*/

function doSomething(condition) {
    if (condition) {
        return true;
    } else {
        return;
    }
}

function doSomething(condition) {
    if (condition) {
        return true;
    }
}

Examples of correct code for this rule:

/*eslint consistent-return: "error"*/

function doSomething(condition) {
    if (condition) {
        return true;
    } else {
        return false;
    }
}

function Foo() {
    if (!(this instanceof Foo)) {
        return new Foo();
    }

    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:

/*eslint consistent-return: ["error", { "treatUndefinedAsUnspecified": false }]*/

function foo(callback) {
    if (callback) {
        return void callback();
    }
    // no return statement
}

function bar(condition) {
    if (condition) {
        return undefined;
    }
    // no return statement
}

Examples of incorrect code for this rule with the { "treatUndefinedAsUnspecified": true } option:

/*eslint consistent-return: ["error", { "treatUndefinedAsUnspecified": true }]*/

function foo(callback) {
    if (callback) {
        return void callback();
    }
    return true;
}

function bar(condition) {
    if (condition) {
        return undefined;
    }
    return true;
}

Examples of correct code for this rule with the { "treatUndefinedAsUnspecified": true } option:

/*eslint consistent-return: ["error", { "treatUndefinedAsUnspecified": true }]*/

function foo(callback) {
    if (callback) {
        return void callback();
    }
    // no return statement
}

function bar(condition) {
    if (condition) {
        return undefined;
    }
    // no return statement
}

When Not To Use It

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/

  const blueprint = definition.blueprint;

Close

Prefer destructuring from arrays and objects (prefer-destructuring)

With JavaScript ES6, a new syntax was added for creating variables from an array index or object property, called destructuring. This rule enforces usage of destructuring instead of accessing a property through a member expression.

Rule Details

Options

This rule takes two sets of configuration objects. The first object parameter determines what types of destructuring the rule applies to.

The two properties, array and object, can be used to turn on or off the destructuring requirement for each of those types independently. By default, both are true.

Alternatively, you can use separate configurations for different assignment types. It accepts 2 other keys instead of array and object.

One key is VariableDeclarator and the other is AssignmentExpression, which can be used to control the destructuring requirement for each of those types independently. Each property accepts an object that accepts two properties, array and object, which can be used to control the destructuring requirement for each of array and object independently for variable declarations and assignment expressions. By default, array and object are set to true for both VariableDeclarator and AssignmentExpression.

The rule has a second object with a single key, enforceForRenamedProperties, which determines whether the object destructuring applies to renamed variables.

Examples of incorrect code for this rule:

// With `array` enabled
var foo = array[0];

// With `object` enabled
var foo = object.foo;
var foo = object['foo'];

Examples of correct code for this rule:

// With `array` enabled
var [ foo ] = array;
var foo = array[someIndex];

// With `object` enabled
var { foo } = object;

var foo = object.bar;

let foo;
({ foo } = object);

Examples of incorrect code when enforceForRenamedProperties is enabled:

var foo = object.bar;

Examples of correct code when enforceForRenamedProperties is enabled:

var { bar: foo } = object;

An example configuration, with the defaults array and object filled in, looks like this:

{
  "rules": {
    "prefer-destructuring": ["error", {
      "array": true,
      "object": true
    }, {
      "enforceForRenamedProperties": false
    }]
  }
}

The two properties, array and object, which can be used to turn on or off the destructuring requirement for each of those types independently. By default, both are true.

For example, the following configuration enforces only object destructuring, but not array destructuring:

{
  "rules": {
    "prefer-destructuring": ["error", {"object": true, "array": false}]
  }
}

An example configuration, with the defaults VariableDeclarator and AssignmentExpression filled in, looks like this:

{
  "rules": {
    "prefer-destructuring": ["error", {
      "VariableDeclarator": {
        "array": false,
        "object": true
      },
      "AssignmentExpression": {
        "array": true,
        "object": true
      }
    }, {
      "enforceForRenamedProperties": false
    }]
  }
}

The two properties, VariableDeclarator and AssignmentExpression, which can be used to turn on or off the destructuring requirement for array and object. By default, all values are true.

For example, the following configuration enforces object destructuring in variable declarations and enforces array destructuring in assignment expressions.

{
  "rules": {
    "prefer-destructuring": ["error", {
      "VariableDeclarator": {
        "array": false,
        "object": true
      },
      "AssignmentExpression": {
        "array": true,
        "object": false
      }
    }, {
      "enforceForRenamedProperties": false
    }]
  }
}

Examples of correct code when object destructuring in VariableDeclarator is enforced:

/* eslint prefer-destructuring: ["error", {VariableDeclarator: {object: true}}] */
var {bar: foo} = object;

Examples of correct code when array destructuring in AssignmentExpression is enforced:

/* eslint prefer-destructuring: ["error", {AssignmentExpression: {array: true}}] */
[bar] = array;

When Not To Use It

If you want to be able to access array indices or object properties directly, you can either configure the rule to your tastes or disable the rule entirely.

Additionally, if you intend to access large array indices directly, like:

var foo = array[100];

Then the array part of this rule is not recommended, as destructuring does not match this use case very well.

Or for non-iterable 'array-like' objects:

var $ = require('jquery');
var foo = $('body')[0];
var [bar] = $('body'); // fails with a TypeError

Further Reading

If you want to learn more about destructuring, check out the links below:

• Destructuring Assignment (MDN)
• Destructuring and parameter handling in ECMAScript 6 (2ality blog) Source: http://eslint.org/docs/rules/

    options.instances.filter((instance) => !!instance)

Close

Require parens in arrow function arguments (arrow-parens)

Arrow functions can omit parentheses when they have exactly one parameter. In all other cases the parameter(s) must be wrapped in parentheses. This rule enforces the consistent use of parentheses in arrow functions.

Rule Details

This rule enforces parentheses around arrow function parameters regardless of arity. For example:

/*eslint-env es6*/

// Bad
a => {}

// Good
(a) => {}

Following this style will help you find arrow functions (=>) which may be mistakenly included in a condition when a comparison such as >= was the intent.

/*eslint-env es6*/

// Bad
if (a => 2) {
}

// Good
if (a >= 2) {
}

The rule can also be configured to discourage the use of parens when they are not required:

/*eslint-env es6*/

// Bad
(a) => {}

// Good
a => {}

Options

This rule has a string option and an object one.

String options are:

• "always" (default) requires parens around arguments in all cases.
• "as-needed" allows omitting parens when there is only one argument.

Object properties for variants of the "as-needed" option:

• "requireForBlockBody": true modifies the as-needed rule in order to require parens if the function body is in an instructions block (surrounded by braces).

always

Examples of incorrect code for this rule with the default "always" option:

/*eslint arrow-parens: ["error", "always"]*/
/*eslint-env es6*/

a => {};
a => a;
a => {'\n'};
a.then(foo => {});
a.then(foo => a);
a(foo => { if (true) {} });

Examples of correct code for this rule with the default "always" option:

/*eslint arrow-parens: ["error", "always"]*/
/*eslint-env es6*/

() => {};
(a) => {};
(a) => a;
(a) => {'\n'}
a.then((foo) => {});
a.then((foo) => { if (true) {} });

If Statements

One of benefits of this option is that it prevents the incorrect use of arrow functions in conditionals:

/*eslint-env es6*/

var a = 1;
var b = 2;
// ...
if (a => b) {
 console.log('bigger');
} else {
 console.log('smaller');
}
// outputs 'bigger', not smaller as expected

The contents of the if statement is an arrow function, not a comparison.

If the arrow function is intentional, it should be wrapped in parens to remove ambiguity.

/*eslint-env es6*/

var a = 1;
var b = 0;
// ...
if ((a) => b) {
 console.log('truthy value returned');
} else {
 console.log('falsey value returned');
}
// outputs 'truthy value returned'

The following is another example of this behavior:

/*eslint-env es6*/

var a = 1, b = 2, c = 3, d = 4;
var f = a => b ? c: d;
// f = ?

f is an arrow function which takes a as an argument and returns the result of b ? c: d.

This should be rewritten like so:

/*eslint-env es6*/

var a = 1, b = 2, c = 3, d = 4;
var f = (a) => b ? c: d;

as-needed

Examples of incorrect code for this rule with the "as-needed" option:

/*eslint arrow-parens: ["error", "as-needed"]*/
/*eslint-env es6*/

(a) => {};
(a) => a;
(a) => {'\n'};
a.then((foo) => {});
a.then((foo) => a);
a((foo) => { if (true) {} });

Examples of correct code for this rule with the "as-needed" option:

/*eslint arrow-parens: ["error", "as-needed"]*/
/*eslint-env es6*/

() => {};
a => {};
a => a;
a => {'\n'};
a.then(foo => {});
a.then(foo => { if (true) {} });
(a, b, c) => a;
(a = 10) => a;
([a, b]) => a;
({a, b}) => a;

requireForBlockBody

Examples of incorrect code for the { "requireForBlockBody": true } option:

/*eslint arrow-parens: [2, "as-needed", { "requireForBlockBody": true }]*/
/*eslint-env es6*/

(a) => a;
a => {};
a => {'\n'};
a.map((x) => x * x);
a.map(x => {
  return x * x;
});
a.then(foo => {});

Examples of correct code for the { "requireForBlockBody": true } option:

/*eslint arrow-parens: [2, "as-needed", { "requireForBlockBody": true }]*/
/*eslint-env es6*/

(a) => {};
(a) => {'\n'};
a => ({});
() => {};
a => a;
a.then((foo) => {});
a.then((foo) => { if (true) {} });
a((foo) => { if (true) {} });
(a, b, c) => a;
(a = 10) => a;
([a, b]) => a;
({a, b}) => a;

Further Reading

• The "as-needed", { "requireForBlockBody": true } rule is directly inspired by the Airbnb JS Style Guide. Source: http://eslint.org/docs/rules/

  return !! getDefinition(name);

Close

Require or disallow spaces before/after unary operators (space-unary-ops)

Some style guides require or disallow spaces before or after unary operators. This is mainly a stylistic issue, however, some JavaScript expressions can be written without spacing which makes it harder to read and maintain.

Rule Details

This rule enforces consistency regarding the spaces after words unary operators and after/before nonwords unary operators.

Examples of unary words operators:

// new
var joe = new Person();

// delete
var obj = {
    foo: 'bar'
};
delete obj.foo;

// typeof
typeof {} // object

// void
void 0 // undefined

Examples of unary nonwords operators:

if ([1,2,3].indexOf(1) !== -1) {};
foo = --foo;
bar = bar++;
baz = !foo;
qux = !!baz;

Options

This rule has three options:

• words - applies to unary word operators such as: new, delete, typeof, void, yield
• nonwords - applies to unary operators such as: -, +, --, ++, !, !!
• overrides - specifies overwriting usage of spacing for each operator, word or non word. This is empty by default, but can be used to enforce or disallow spacing around operators. For example:

"space-unary-ops": [
        2, {
          "words": true,
          "nonwords": false,
          "overrides": {
            "new": false,
            "++": true
          }
    }]

In this case, spacing will be disallowed after a new operator and required before/after a ++ operator.

Examples of incorrect code for this rule with the default {"words": true, "nonwords": false} option:

/*eslint space-unary-ops: "error"*/

typeof!foo;

void{foo:0};

new[foo][0];

delete(foo.bar);

++ foo;

foo --;

- foo;

+ "3";

/*eslint space-unary-ops: "error"*/
/*eslint-env es6*/

function *foo() {
    yield(0)
}

/*eslint space-unary-ops: "error"*/

async function foo() {
    await(bar);
}

Examples of correct code for this rule with the {"words": true, "nonwords": false} option:

/*eslint space-unary-ops: "error"*/

// Word unary operator "delete" is followed by a whitespace.
delete foo.bar;

// Word unary operator "new" is followed by a whitespace.
new Foo;

// Word unary operator "void" is followed by a whitespace.
void 0;

// Unary operator "++" is not followed by whitespace.
++foo;

// Unary operator "--" is not preceded by whitespace.
foo--;

// Unary operator "-" is not followed by whitespace.
-foo;

// Unary operator "+" is not followed by whitespace.
+"3";

/*eslint space-unary-ops: "error"*/
/*eslint-env es6*/

function *foo() {
    yield (0)
}

/*eslint space-unary-ops: "error"*/

async function foo() {
    await (bar);
}

Source: http://eslint.org/docs/rules/

  Object.keys(registry).forEach((k) => (delete registry[k]));

Close

Require parens in arrow function arguments (arrow-parens)

Arrow functions can omit parentheses when they have exactly one parameter. In all other cases the parameter(s) must be wrapped in parentheses. This rule enforces the consistent use of parentheses in arrow functions.

Rule Details

This rule enforces parentheses around arrow function parameters regardless of arity. For example:

/*eslint-env es6*/

// Bad
a => {}

// Good
(a) => {}

Following this style will help you find arrow functions (=>) which may be mistakenly included in a condition when a comparison such as >= was the intent.

/*eslint-env es6*/

// Bad
if (a => 2) {
}

// Good
if (a >= 2) {
}

The rule can also be configured to discourage the use of parens when they are not required:

/*eslint-env es6*/

// Bad
(a) => {}

// Good
a => {}

Options

This rule has a string option and an object one.

String options are:

• "always" (default) requires parens around arguments in all cases.
• "as-needed" allows omitting parens when there is only one argument.

Object properties for variants of the "as-needed" option:

• "requireForBlockBody": true modifies the as-needed rule in order to require parens if the function body is in an instructions block (surrounded by braces).

always

Examples of incorrect code for this rule with the default "always" option:

/*eslint arrow-parens: ["error", "always"]*/
/*eslint-env es6*/

a => {};
a => a;
a => {'\n'};
a.then(foo => {});
a.then(foo => a);
a(foo => { if (true) {} });

Examples of correct code for this rule with the default "always" option:

/*eslint arrow-parens: ["error", "always"]*/
/*eslint-env es6*/

() => {};
(a) => {};
(a) => a;
(a) => {'\n'}
a.then((foo) => {});
a.then((foo) => { if (true) {} });

If Statements

One of benefits of this option is that it prevents the incorrect use of arrow functions in conditionals:

/*eslint-env es6*/

var a = 1;
var b = 2;
// ...
if (a => b) {
 console.log('bigger');
} else {
 console.log('smaller');
}
// outputs 'bigger', not smaller as expected

The contents of the if statement is an arrow function, not a comparison.

If the arrow function is intentional, it should be wrapped in parens to remove ambiguity.

/*eslint-env es6*/

var a = 1;
var b = 0;
// ...
if ((a) => b) {
 console.log('truthy value returned');
} else {
 console.log('falsey value returned');
}
// outputs 'truthy value returned'

The following is another example of this behavior:

/*eslint-env es6*/

var a = 1, b = 2, c = 3, d = 4;
var f = a => b ? c: d;
// f = ?

f is an arrow function which takes a as an argument and returns the result of b ? c: d.

This should be rewritten like so:

/*eslint-env es6*/

var a = 1, b = 2, c = 3, d = 4;
var f = (a) => b ? c: d;

as-needed

Examples of incorrect code for this rule with the "as-needed" option:

/*eslint arrow-parens: ["error", "as-needed"]*/
/*eslint-env es6*/

(a) => {};
(a) => a;
(a) => {'\n'};
a.then((foo) => {});
a.then((foo) => a);
a((foo) => { if (true) {} });

Examples of correct code for this rule with the "as-needed" option:

/*eslint arrow-parens: ["error", "as-needed"]*/
/*eslint-env es6*/

() => {};
a => {};
a => a;
a => {'\n'};
a.then(foo => {});
a.then(foo => { if (true) {} });
(a, b, c) => a;
(a = 10) => a;
([a, b]) => a;
({a, b}) => a;

requireForBlockBody

Examples of incorrect code for the { "requireForBlockBody": true } option:

/*eslint arrow-parens: [2, "as-needed", { "requireForBlockBody": true }]*/
/*eslint-env es6*/

(a) => a;
a => {};
a => {'\n'};
a.map((x) => x * x);
a.map(x => {
  return x * x;
});
a.then(foo => {});

Examples of correct code for the { "requireForBlockBody": true } option:

/*eslint arrow-parens: [2, "as-needed", { "requireForBlockBody": true }]*/
/*eslint-env es6*/

(a) => {};
(a) => {'\n'};
a => ({});
() => {};
a => a;
a.then((foo) => {});
a.then((foo) => { if (true) {} });
a((foo) => { if (true) {} });
(a, b, c) => a;
(a = 10) => a;
([a, b]) => a;
({a, b}) => a;

Further Reading

• The "as-needed", { "requireForBlockBody": true } rule is directly inspired by the Airbnb JS Style Guide. Source: http://eslint.org/docs/rules/

        newInstance.update(newPropsForUpdate);

Close

Disallow Early Use (no-use-before-define)

In JavaScript, prior to ES6, variable and function declarations are hoisted to the top of a scope, so it's possible to use identifiers before their formal declarations in code. This can be confusing and some believe it is best to always declare variables and functions before using them.

In ES6, block-level bindings (let and const) introduce a "temporal dead zone" where a ReferenceError will be thrown with any attempt to access the variable before its declaration.

Rule Details

This rule will warn when it encounters a reference to an identifier that has not yet been declared.

Examples of incorrect code for this rule:

/*eslint no-use-before-define: "error"*/
/*eslint-env es6*/

alert(a);
var a = 10;

f();
function f() {}

function g() {
    return b;
}
var b = 1;

{
    alert(c);
    let c = 1;
}

Examples of correct code for this rule:

/*eslint no-use-before-define: "error"*/
/*eslint-env es6*/

var a;
a = 10;
alert(a);

function f() {}
f(1);

var b = 1;
function g() {
    return b;
}

{
    let c;
    c++;
}

Options

{
    "no-use-before-define": ["error", { "functions": true, "classes": true }]
}

• functions (boolean) - The flag which shows whether or not this rule checks function declarations. If this is true, this rule warns every reference to a function before the function declaration. Otherwise, ignores those references. Function declarations are hoisted, so it's safe. Default is true.
• classes (boolean) - The flag which shows whether or not this rule checks class declarations of upper scopes. If this is true, this rule warns every reference to a class before the class declaration. Otherwise, ignores those references if the declaration is in upper function scopes. Class declarations are not hoisted, so it might be danger. Default is true.
• variables (boolean) - This flag determines whether or not the rule checks variable declarations in upper scopes. If this is true, the rule warns every reference to a variable before the variable declaration. Otherwise, the rule ignores a reference if the declaration is in an upper scope, while still reporting the reference if it's in the same scope as the declaration. Default is true.

This rule accepts "nofunc" string as an option. "nofunc" is the same as { "functions": false, "classes": true }.

functions

Examples of correct code for the { "functions": false } option:

/*eslint no-use-before-define: ["error", { "functions": false }]*/

f();
function f() {}

classes

Examples of incorrect code for the { "classes": false } option:

/*eslint no-use-before-define: ["error", { "classes": false }]*/
/*eslint-env es6*/

new A();
class A {
}

Examples of correct code for the { "classes": false } option:

/*eslint no-use-before-define: ["error", { "classes": false }]*/
/*eslint-env es6*/

function foo() {
    return new A();
}

class A {
}

variables

Examples of incorrect code for the { "variables": false } option:

/*eslint no-use-before-define: ["error", { "variables": false }]*/

console.log(foo);
var foo = 1;

Examples of correct code for the { "variables": false } option:

/*eslint no-use-before-define: ["error", { "variables": false }]*/

function baz() {
    console.log(foo);
}

var foo = 1;

Source: http://eslint.org/docs/rules/