Disallow unnecessary escape usage (no-useless-escape)

Escaping non-special characters in strings, template literals, and regular expressions doesn't have any effect, as demonstrated in the following example:

let foo = "hol\a"; // > foo = "hola"
let bar = `${foo}\!`; // > bar = "hola!"
let baz = /\:/ // same functionality with /:/

Rule Details

This rule flags escapes that can be safely removed without changing behavior.

Examples of incorrect code for this rule:

/*eslint no-useless-escape: "error"*/

"\'";
'\"';
"\#";
"\e";
`\"`;
`\"${foo}\"`;
`\#{foo}`;
/\!/;
/\@/;

Examples of correct code for this rule:

/*eslint no-useless-escape: "error"*/

"\"";
'\'';
"\x12";
"\u00a9";
"\371";
"xs\u2111";
`\``;
`\${${foo}\}`;
`$\{${foo}\}`;
/\\/g;
/\t/g;
/\w\$\*\^\./;

When Not To Use It

If you don't want to be notified about unnecessary escapes, you can safely disable this rule. Source: http://eslint.org/docs/rules/

Enforce require() on the top-level module scope (global-require)

In Node.js, module dependencies are included using the require() function, such as:

var fs = require("fs");

While require() may be called anywhere in code, some style guides prescribe that it should be called only in the top level of a module to make it easier to identify dependencies. For instance, it's arguably harder to identify dependencies when they are deeply nested inside of functions and other statements:

function foo() {

    if (condition) {
        var fs = require("fs");
    }
}

Since require() does a synchronous load, it can cause performance problems when used in other locations.

Further, ES6 modules mandate that import and export statements can only occur in the top level of the module's body.

Rule Details

This rule requires all calls to require() to be at the top level of the module, similar to ES6 import and export statements, which also can occur only at the top level.

Examples of incorrect code for this rule:

/*eslint global-require: "error"*/
/*eslint-env es6*/

// calling require() inside of a function is not allowed
function readFile(filename, callback) {
    var fs = require('fs');
    fs.readFile(filename, callback)
}

// conditional requires like this are also not allowed
if (DEBUG) { require('debug'); }

// a require() in a switch statement is also flagged
switch(x) { case '1': require('1'); break; }

// you may not require() inside an arrow function body
var getModule = (name) => require(name);

// you may not require() inside of a function body as well
function getModule(name) { return require(name); }

// you may not require() inside of a try/catch block
try {
    require(unsafeModule);
} catch(e) {
    console.log(e);
}

Examples of correct code for this rule:

/*eslint global-require: "error"*/

// all these variations of require() are ok
require('x');
var y = require('y');
var z;
z = require('z').initialize();

// requiring a module and using it in a function is ok
var fs = require('fs');
function readFile(filename, callback) {
    fs.readFile(filename, callback)
}

// you can use a ternary to determine which module to require
var logger = DEBUG ? require('dev-logger') : require('logger');

// if you want you can require() at the end of your module
function doSomethingA() {}
function doSomethingB() {}
var x = require("x"),
    z = require("z");

When Not To Use It

If you have a module that must be initialized with information that comes from the file-system or if a module is only used in very rare situations and will cause significant overhead to load it may make sense to disable the rule. If you need to require() an optional dependency inside of a try/catch, you can disable this rule for just that dependency using the // eslint-disable-line global-require comment. Source: http://eslint.org/docs/rules/

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/

Disallow unnecessary escape usage (no-useless-escape)

Escaping non-special characters in strings, template literals, and regular expressions doesn't have any effect, as demonstrated in the following example:

let foo = "hol\a"; // > foo = "hola"
let bar = `${foo}\!`; // > bar = "hola!"
let baz = /\:/ // same functionality with /:/

Rule Details

This rule flags escapes that can be safely removed without changing behavior.

Examples of incorrect code for this rule:

/*eslint no-useless-escape: "error"*/

"\'";
'\"';
"\#";
"\e";
`\"`;
`\"${foo}\"`;
`\#{foo}`;
/\!/;
/\@/;

Examples of correct code for this rule:

/*eslint no-useless-escape: "error"*/

"\"";
'\'';
"\x12";
"\u00a9";
"\371";
"xs\u2111";
`\``;
`\${${foo}\}`;
`$\{${foo}\}`;
/\\/g;
/\t/g;
/\w\$\*\^\./;

When Not To Use It

If you don't want to be notified about unnecessary escapes, you can safely disable this rule. Source: http://eslint.org/docs/rules/

Suggest using template literals instead of string concatenation. (prefer-template)

In ES2015 (ES6), we can use template literals instead of string concatenation.

var str = "Hello, " + name + "!";

/*eslint-env es6*/

var str = `Hello, ${name}!`;

Rule Details

This rule is aimed to flag usage of + operators with strings.

Examples

Examples of incorrect code for this rule:

/*eslint prefer-template: "error"*/

var str = "Hello, " + name + "!";
var str = "Time: " + (12 * 60 * 60 * 1000);

Examples of correct code for this rule:

/*eslint prefer-template: "error"*/
/*eslint-env es6*/

var str = "Hello World!";
var str = `Hello, ${name}!`;
var str = `Time: ${12 * 60 * 60 * 1000}`;

// This is reported by `no-useless-concat`.
var str = "Hello, " + "World!";

When Not To Use It

This rule should not be used in ES3/5 environments.

In ES2015 (ES6) or later, if you don't want to be notified about string concatenation, you can safely disable this rule.

Related Rules

• [no-useless-concat](no-useless-concat.md)
• [quotes](quotes.md) Source: http://eslint.org/docs/rules/

Disallow unnecessary escape usage (no-useless-escape)

Escaping non-special characters in strings, template literals, and regular expressions doesn't have any effect, as demonstrated in the following example:

let foo = "hol\a"; // > foo = "hola"
let bar = `${foo}\!`; // > bar = "hola!"
let baz = /\:/ // same functionality with /:/

Rule Details

This rule flags escapes that can be safely removed without changing behavior.

Examples of incorrect code for this rule:

/*eslint no-useless-escape: "error"*/

"\'";
'\"';
"\#";
"\e";
`\"`;
`\"${foo}\"`;
`\#{foo}`;
/\!/;
/\@/;

Examples of correct code for this rule:

/*eslint no-useless-escape: "error"*/

"\"";
'\'';
"\x12";
"\u00a9";
"\371";
"xs\u2111";
`\``;
`\${${foo}\}`;
`$\{${foo}\}`;
/\\/g;
/\t/g;
/\w\$\*\^\./;

When Not To Use It

If you don't want to be notified about unnecessary escapes, you can safely disable this rule. Source: http://eslint.org/docs/rules/

Disallow unnecessary escape usage (no-useless-escape)

Escaping non-special characters in strings, template literals, and regular expressions doesn't have any effect, as demonstrated in the following example:

let foo = "hol\a"; // > foo = "hola"
let bar = `${foo}\!`; // > bar = "hola!"
let baz = /\:/ // same functionality with /:/

Rule Details

This rule flags escapes that can be safely removed without changing behavior.

Examples of incorrect code for this rule:

/*eslint no-useless-escape: "error"*/

"\'";
'\"';
"\#";
"\e";
`\"`;
`\"${foo}\"`;
`\#{foo}`;
/\!/;
/\@/;

Examples of correct code for this rule:

/*eslint no-useless-escape: "error"*/

"\"";
'\'';
"\x12";
"\u00a9";
"\371";
"xs\u2111";
`\``;
`\${${foo}\}`;
`$\{${foo}\}`;
/\\/g;
/\t/g;
/\w\$\*\^\./;

When Not To Use It

If you don't want to be notified about unnecessary escapes, you can safely disable this rule. Source: http://eslint.org/docs/rules/

Suggest using template literals instead of string concatenation. (prefer-template)

In ES2015 (ES6), we can use template literals instead of string concatenation.

var str = "Hello, " + name + "!";

/*eslint-env es6*/

var str = `Hello, ${name}!`;

Rule Details

This rule is aimed to flag usage of + operators with strings.

Examples

Examples of incorrect code for this rule:

/*eslint prefer-template: "error"*/

var str = "Hello, " + name + "!";
var str = "Time: " + (12 * 60 * 60 * 1000);

Examples of correct code for this rule:

/*eslint prefer-template: "error"*/
/*eslint-env es6*/

var str = "Hello World!";
var str = `Hello, ${name}!`;
var str = `Time: ${12 * 60 * 60 * 1000}`;

// This is reported by `no-useless-concat`.
var str = "Hello, " + "World!";

When Not To Use It

This rule should not be used in ES3/5 environments.

In ES2015 (ES6) or later, if you don't want to be notified about string concatenation, you can safely disable this rule.

Related Rules

• [no-useless-concat](no-useless-concat.md)
• [quotes](quotes.md) Source: http://eslint.org/docs/rules/

Suggest using template literals instead of string concatenation. (prefer-template)

In ES2015 (ES6), we can use template literals instead of string concatenation.

var str = "Hello, " + name + "!";

/*eslint-env es6*/

var str = `Hello, ${name}!`;

Rule Details

This rule is aimed to flag usage of + operators with strings.

Examples

Examples of incorrect code for this rule:

/*eslint prefer-template: "error"*/

var str = "Hello, " + name + "!";
var str = "Time: " + (12 * 60 * 60 * 1000);

Examples of correct code for this rule:

/*eslint prefer-template: "error"*/
/*eslint-env es6*/

var str = "Hello World!";
var str = `Hello, ${name}!`;
var str = `Time: ${12 * 60 * 60 * 1000}`;

// This is reported by `no-useless-concat`.
var str = "Hello, " + "World!";

When Not To Use It

This rule should not be used in ES3/5 environments.

In ES2015 (ES6) or later, if you don't want to be notified about string concatenation, you can safely disable this rule.

Related Rules

• [no-useless-concat](no-useless-concat.md)
• [quotes](quotes.md) Source: http://eslint.org/docs/rules/

Require braces in arrow function body (arrow-body-style)

Arrow functions have two syntactic forms for their function bodies. They may be defined with a block body (denoted by curly braces) () => { ... } or with a single expression () => ..., whose value is implicitly returned.

Rule Details

This rule can enforce or disallow the use of braces around arrow function body.

Options

The rule takes one or two options. The first is a string, which can be:

• "always" enforces braces around the function body
• "as-needed" enforces no braces where they can be omitted (default)
• "never" enforces no braces around the function body (constrains arrow functions to the role of returning an expression)

The second one is an object for more fine-grained configuration when the first option is "as-needed". Currently, the only available option is requireReturnForObjectLiteral, a boolean property. It's false by default. If set to true, it requires braces and an explicit return for object literals.

"arrow-body-style": ["error", "always"]

always

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

/*eslint arrow-body-style: ["error", "always"]*/
/*eslint-env es6*/
let foo = () => 0;

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

let foo = () => {
    return 0;
};
let foo = (retv, name) => {
    retv[name] = true;
    return retv;
};

as-needed

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

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

let foo = () => {
    return 0;
};
let foo = () => {
    return {
       bar: {
            foo: 1,
            bar: 2,
        }
    };
};

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

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

let foo = () => 0;
let foo = (retv, name) => {
    retv[name] = true;
    return retv;
};
let foo = () => ({
    bar: {
        foo: 1,
        bar: 2,
    }
});
let foo = () => { bar(); };
let foo = () => {};
let foo = () => { /* do nothing */ };
let foo = () => {
    // do nothing.
};
let foo = () => ({ bar: 0 });

requireReturnForObjectLiteral

This option is only applicable when used in conjunction with the "as-needed" option.

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

/*eslint arrow-body-style: ["error", "as-needed", { "requireReturnForObjectLiteral": true }]*/
/*eslint-env es6*/
let foo = () => ({});
let foo = () => ({ bar: 0 });

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

/*eslint arrow-body-style: ["error", "as-needed", { "requireReturnForObjectLiteral": true }]*/
/*eslint-env es6*/

let foo = () => {};
let foo = () => { return { bar: 0 }; };

never

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

/*eslint arrow-body-style: ["error", "never"]*/
/*eslint-env es6*/

let foo = () => {
    return 0;
};
let foo = (retv, name) => {
    retv[name] = true;
    return retv;
};

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

/*eslint arrow-body-style: ["error", "never"]*/
/*eslint-env es6*/

let foo = () => 0;
let foo = () => ({ foo: 0 });

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

Suggest using template literals instead of string concatenation. (prefer-template)

In ES2015 (ES6), we can use template literals instead of string concatenation.

var str = "Hello, " + name + "!";

/*eslint-env es6*/

var str = `Hello, ${name}!`;

Rule Details

This rule is aimed to flag usage of + operators with strings.

Examples

Examples of incorrect code for this rule:

/*eslint prefer-template: "error"*/

var str = "Hello, " + name + "!";
var str = "Time: " + (12 * 60 * 60 * 1000);

Examples of correct code for this rule:

/*eslint prefer-template: "error"*/
/*eslint-env es6*/

var str = "Hello World!";
var str = `Hello, ${name}!`;
var str = `Time: ${12 * 60 * 60 * 1000}`;

// This is reported by `no-useless-concat`.
var str = "Hello, " + "World!";

When Not To Use It

This rule should not be used in ES3/5 environments.

In ES2015 (ES6) or later, if you don't want to be notified about string concatenation, you can safely disable this rule.

Related Rules

• [no-useless-concat](no-useless-concat.md)
• [quotes](quotes.md) Source: http://eslint.org/docs/rules/