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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:

/*eslint no-undef: ["error", { "typeof": true }] */

if(typeof a === "string"){}

Examples of correct code for the { "typeof": true } option with global declaration:

/*global a*/
/*eslint no-undef: ["error", { "typeof": true }] */

if(typeof a === "string"){}

Environments

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.

Compatibility

This rule provides compatibility with treatment of global variables in JSHint and JSLint. Source: http://eslint.org/docs/rules/

    fetch(URL, {

Close

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:

/*eslint no-undef: ["error", { "typeof": true }] */

if(typeof a === "string"){}

Examples of correct code for the { "typeof": true } option with global declaration:

/*global a*/
/*eslint no-undef: ["error", { "typeof": true }] */

if(typeof a === "string"){}

Environments

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.

Compatibility

This rule provides compatibility with treatment of global variables in JSHint and JSLint. Source: http://eslint.org/docs/rules/

        obj: {item: input.value},

Close

enforce consistent spacing inside braces (object-curly-spacing)

While formatting preferences are very personal, a number of style guides require or disallow spaces between curly braces in the following situations:

// simple object literals
var obj = { foo: "bar" };

// nested object literals
var obj = { foo: { zoo: "bar" } };

// destructuring assignment (EcmaScript 6)
var { x, y } = y;

// import/export declarations (EcmaScript 6)
import { foo } from "bar";
export { foo };

Rule Details

This rule enforce consistent spacing inside braces of object literals, destructuring assignments, and import/export specifiers.

Options

This rule has two options, a string option and an object option.

String option:

• "never" (default) disallows spacing inside of braces
• "always" requires spacing inside of braces (except {})

Object option:

• "arraysInObjects": true requires spacing inside of braces of objects beginning and/or ending with an array element (applies when the first option is set to never)
• "arraysInObjects": false disallows spacing inside of braces of objects beginning and/or ending with an array element (applies when the first option is set to always)
• "objectsInObjects": true requires spacing inside of braces of objects beginning and/or ending with an object element (applies when the first option is set to never)
• "objectsInObjects": false disallows spacing inside of braces of objects beginning and/or ending with an object element (applies when the first option is set to always)

never

Examples of incorrect code for this rule with the default "never" option:

/*eslint object-curly-spacing: ["error", "never"]*/

var obj = { 'foo': 'bar' };
var obj = {'foo': 'bar' };
var obj = { baz: {'foo': 'qux'}, bar};
var obj = {baz: { 'foo': 'qux'}, bar};
var {x } = y;
import { foo } from 'bar';

Examples of correct code for this rule with the default "never" option:

/*eslint object-curly-spacing: ["error", "never"]*/

var obj = {'foo': 'bar'};
var obj = {'foo': {'bar': 'baz'}, 'qux': 'quxx'};
var obj = {
  'foo': 'bar'
};
var obj = {'foo': 'bar'
};
var obj = {
  'foo':'bar'};
var obj = {};
var {x} = y;
import {foo} from 'bar';

always

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

/*eslint object-curly-spacing: ["error", "always"]*/

var obj = {'foo': 'bar'};
var obj = {'foo': 'bar' };
var obj = { baz: {'foo': 'qux'}, bar};
var obj = {baz: { 'foo': 'qux' }, bar};
var obj = {'foo': 'bar'
};
var obj = {
  'foo':'bar'};
var {x} = y;
import {foo } from 'bar';

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

/*eslint object-curly-spacing: ["error", "always"]*/

var obj = {};
var obj = { 'foo': 'bar' };
var obj = { 'foo': { 'bar': 'baz' }, 'qux': 'quxx' };
var obj = {
  'foo': 'bar'
};
var { x } = y;
import { foo } from 'bar';

arraysInObjects

Examples of additional correct code for this rule with the "never", { "arraysInObjects": true } options:

/*eslint object-curly-spacing: ["error", "never", { "arraysInObjects": true }]*/

var obj = {"foo": [ 1, 2 ] };
var obj = {"foo": [ "baz", "bar" ] };

Examples of additional correct code for this rule with the "always", { "arraysInObjects": false } options:

/*eslint object-curly-spacing: ["error", "always", { "arraysInObjects": false }]*/

var obj = { "foo": [ 1, 2 ]};
var obj = { "foo": [ "baz", "bar" ]};

objectsInObjects

Examples of additional correct code for this rule with the "never", { "objectsInObjects": true } options:

/*eslint object-curly-spacing: ["error", "never", { "objectsInObjects": true }]*/

var obj = {"foo": {"baz": 1, "bar": 2} };

Examples of additional correct code for this rule with the "always", { "objectsInObjects": false } options:

/*eslint object-curly-spacing: ["error", "always", { "objectsInObjects": false }]*/

var obj = { "foo": { "baz": 1, "bar": 2 }};

When Not To Use It

You can turn this rule off if you are not concerned with the consistency of spacing between curly braces.

Related Rules

• [comma-spacing](comma-spacing.md)
• [space-in-parens](space-in-parens.md) Source: http://eslint.org/docs/rules/

        obj: {item: input.value},

Close

enforce consistent spacing inside braces (object-curly-spacing)

While formatting preferences are very personal, a number of style guides require or disallow spaces between curly braces in the following situations:

// simple object literals
var obj = { foo: "bar" };

// nested object literals
var obj = { foo: { zoo: "bar" } };

// destructuring assignment (EcmaScript 6)
var { x, y } = y;

// import/export declarations (EcmaScript 6)
import { foo } from "bar";
export { foo };

Rule Details

This rule enforce consistent spacing inside braces of object literals, destructuring assignments, and import/export specifiers.

Options

This rule has two options, a string option and an object option.

String option:

• "never" (default) disallows spacing inside of braces
• "always" requires spacing inside of braces (except {})

Object option:

• "arraysInObjects": true requires spacing inside of braces of objects beginning and/or ending with an array element (applies when the first option is set to never)
• "arraysInObjects": false disallows spacing inside of braces of objects beginning and/or ending with an array element (applies when the first option is set to always)
• "objectsInObjects": true requires spacing inside of braces of objects beginning and/or ending with an object element (applies when the first option is set to never)
• "objectsInObjects": false disallows spacing inside of braces of objects beginning and/or ending with an object element (applies when the first option is set to always)

never

Examples of incorrect code for this rule with the default "never" option:

/*eslint object-curly-spacing: ["error", "never"]*/

var obj = { 'foo': 'bar' };
var obj = {'foo': 'bar' };
var obj = { baz: {'foo': 'qux'}, bar};
var obj = {baz: { 'foo': 'qux'}, bar};
var {x } = y;
import { foo } from 'bar';

Examples of correct code for this rule with the default "never" option:

/*eslint object-curly-spacing: ["error", "never"]*/

var obj = {'foo': 'bar'};
var obj = {'foo': {'bar': 'baz'}, 'qux': 'quxx'};
var obj = {
  'foo': 'bar'
};
var obj = {'foo': 'bar'
};
var obj = {
  'foo':'bar'};
var obj = {};
var {x} = y;
import {foo} from 'bar';

always

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

/*eslint object-curly-spacing: ["error", "always"]*/

var obj = {'foo': 'bar'};
var obj = {'foo': 'bar' };
var obj = { baz: {'foo': 'qux'}, bar};
var obj = {baz: { 'foo': 'qux' }, bar};
var obj = {'foo': 'bar'
};
var obj = {
  'foo':'bar'};
var {x} = y;
import {foo } from 'bar';

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

/*eslint object-curly-spacing: ["error", "always"]*/

var obj = {};
var obj = { 'foo': 'bar' };
var obj = { 'foo': { 'bar': 'baz' }, 'qux': 'quxx' };
var obj = {
  'foo': 'bar'
};
var { x } = y;
import { foo } from 'bar';

arraysInObjects

Examples of additional correct code for this rule with the "never", { "arraysInObjects": true } options:

/*eslint object-curly-spacing: ["error", "never", { "arraysInObjects": true }]*/

var obj = {"foo": [ 1, 2 ] };
var obj = {"foo": [ "baz", "bar" ] };

Examples of additional correct code for this rule with the "always", { "arraysInObjects": false } options:

/*eslint object-curly-spacing: ["error", "always", { "arraysInObjects": false }]*/

var obj = { "foo": [ 1, 2 ]};
var obj = { "foo": [ "baz", "bar" ]};

objectsInObjects

Examples of additional correct code for this rule with the "never", { "objectsInObjects": true } options:

/*eslint object-curly-spacing: ["error", "never", { "objectsInObjects": true }]*/

var obj = {"foo": {"baz": 1, "bar": 2} };

Examples of additional correct code for this rule with the "always", { "objectsInObjects": false } options:

/*eslint object-curly-spacing: ["error", "always", { "objectsInObjects": false }]*/

var obj = { "foo": { "baz": 1, "bar": 2 }};

When Not To Use It

You can turn this rule off if you are not concerned with the consistency of spacing between curly braces.

Related Rules

• [comma-spacing](comma-spacing.md)
• [space-in-parens](space-in-parens.md) Source: http://eslint.org/docs/rules/

  function getID() {
    function s4() {
      return Math.floor((1 + Math.random()) * 0x10000)
          .toString(16)
          .substring(1);

Close

Duplicated Code

Duplicated code can lead to software that is hard to understand and difficult to change. The Don't Repeat Yourself (DRY) principle states:

Every piece of knowledge must have a single, unambiguous, authoritative representation within a system.

When you violate DRY, bugs and maintenance problems are sure to follow. Duplicated code has a tendency to both continue to replicate and also to diverge (leaving bugs as two similar implementations differ in subtle ways).

Tuning

This issue has a mass of 129.

We set useful threshold defaults for the languages we support but you may want to adjust these settings based on your project guidelines.

The threshold configuration represents the minimum mass a code block must have to be analyzed for duplication. The lower the threshold, the more fine-grained the comparison.

If the engine is too easily reporting duplication, try raising the threshold. If you suspect that the engine isn't catching enough duplication, try lowering the threshold. The best setting tends to differ from language to language.

See codeclimate-duplication's documentation for more information about tuning the mass threshold in your .codeclimate.yml.

Refactorings

• Extract Method
• Extract Class
• Form Template Method
• Introduce Null Object
• Pull Up Method
• Pull Up Field
• Substitute Algorithm

Further Reading

• Don't Repeat Yourself on the C2 Wiki
• Duplicated Code on SourceMaking
• Refactoring: Improving the Design of Existing Code by Martin Fowler. Duplicated Code, p76

  function renderList() {
    list.innerHTML = items.map((item) => `<li>${item}</li>`).join('');
  }

Close

Duplicated Code

Duplicated code can lead to software that is hard to understand and difficult to change. The Don't Repeat Yourself (DRY) principle states:

Every piece of knowledge must have a single, unambiguous, authoritative representation within a system.

When you violate DRY, bugs and maintenance problems are sure to follow. Duplicated code has a tendency to both continue to replicate and also to diverge (leaving bugs as two similar implementations differ in subtle ways).

Tuning

This issue has a mass of 48.

We set useful threshold defaults for the languages we support but you may want to adjust these settings based on your project guidelines.

The threshold configuration represents the minimum mass a code block must have to be analyzed for duplication. The lower the threshold, the more fine-grained the comparison.

If the engine is too easily reporting duplication, try raising the threshold. If you suspect that the engine isn't catching enough duplication, try lowering the threshold. The best setting tends to differ from language to language.

See codeclimate-duplication's documentation for more information about tuning the mass threshold in your .codeclimate.yml.

Refactorings

• Extract Method
• Extract Class
• Form Template Method
• Introduce Null Object
• Pull Up Method
• Pull Up Field
• Substitute Algorithm

Further Reading

• Don't Repeat Yourself on the C2 Wiki
• Duplicated Code on SourceMaking
• Refactoring: Improving the Design of Existing Code by Martin Fowler. Duplicated Code, p76