ESLintのエラールール。日本語ざっくり解説[ベストプラクティス編] ref: https://qiita.com/M-ISO/items/4cd183e2496c2937a53e

file0.js

// Bad
var o = {
    set a(value) {
        this.val = value;
    }
};

// Good
var o = {
    set a(value) {
        this.val = value;
    },
    get a() {
        return this.val;
    }
};

file1.js

/*eslint block-scoped-var: 2*/
function doSomething() {
    if (true) {
        var build = true;
    }

    console.log(build); /*error "build" used outside of binding context.*/
}
////////////////////////////
//good

function doSomething() {
    var build;

    if (true) {
        build = true;
    }

    console.log(build);//ok
}

file10.js

alert("here!");                          /*error Unexpected alert.*/
confirm("Are you sure?");                /*error Unexpected confirm.*/
prompt("What's your name?", "John Doe"); /*error Unexpected prompt.*/

file11.js

/*eslint no-caller: 2*/
function foo(n) {
    if (n <= 0) {
        return;
    }

    arguments.callee(n - 1);   /*error Avoid arguments.callee.*/
}

[1,2,3,4,5].map(function(n) {
    return !(n > 1) ? 1 : arguments.callee(n - 1) * n; /*error Avoid arguments.callee.*/
});

file12.js

function bar() { return /=foo/; } /*error A regular expression literal can be confused with '/='.*/
//ok
function bar() { return /\=foo/; }

file13.js

function foo() {
    if (x) {
        return y;
    } else {            /*error Unexpected 'else' after 'return'.*/
        return z;
    }
}
//ok
function foo() {
    if (x) {
        return y;
    }

    return z;
}

file14.js

labeled:     /*error Unexpected label "labeled"*/
var x = 10;

file15.js

if (foo == null) {     /*error Use ‘===’ to compare with ‘null’.*/
  bar();
}
while (qux != null) {  /*error Use ‘===’ to compare with ‘null’.*/
  baz();
}
//ok
if (foo === null) {
  bar();
}
while (qux !== null) {
  baz();
}

file16.js

var obj = { x: "foo" },
    key = "x",
    value = eval("obj." + key); /*error eval can be harmful.*/
//ok
var obj = { x: "foo" },
    key = "x",
    value = obj[key];

file17.js

//disallow
Object.prototype.a = "a";
Object.defineProperty(Array.prototype, "times", {value: 999});

//これらはチェックされない
var x = Object; x.prototype.thing = a;
eval("Array.prototype.forEach = 'muhahaha'");
with(Array) { prototype.thing = 'thing'; };
window.Function.prototype.bind = 'tight';

file18.js

var boundGetName = (function getName() {
    return this.name;
}).bind({ name: "ESLint" });
console.log(boundGetName());      // "ESLint"

// useless bind
var boundGetName = (function getName() {
    return "ESLint";
}).bind({ name: "ESLint" });//とってオッケー

console.log(boundGetName());      // "ESLint"

file19.js

switch(foo) {
    case 1:
        doSomething();

    case 2:
        doSomethingElse();
}
//上の場合fooが1の場合2も通る
switch(foo) {
    case 1:
        doSomething();
        break;//good

    case 2:
        doSomethingElse();
}

file2.js

function a(x) {
    if (true) {
        return x; // 1st path
    } else if (false) {
        return x+1; // 2nd path
    } else {
        return 4; // 3rd path
    }
}

file20.js

var num = .5;  /*error A leading decimal point can be confused with a dot.*/
var num = 2.;  /*error A trailing decimal point can be confused with a dot.*/
var num = -.7; /*error A leading decimal point can be confused with a dot.*/

file21.js

//こういうやつ
var b = !!foo;
var b = ~foo.indexOf(".");
var n = +foo;
var n = 1 * foo;
var s = "" + foo;
//置き換えれる
var b = Boolean(foo);
var b = foo.indexOf(".") !== -1;
var n = Number(foo);
var n = Number(foo);
var s = String(foo);

file22.js

//こういうやつ。setTimeout(), setInterval() or execScript()
setTimeout("alert('Hi!');", 100);
//good
setTimeout(function() {
    alert("Hi!");
}, 100);

file23.js

/*eslint no-invalid-this: 2*/
this.a = 0;            /*error Unexpected `this`.*/
baz(() => this);       /*error Unexpected `this`.*/

(function() {
    this.a = 0;        /*error Unexpected `this`.*/
    baz(() => this);   /*error Unexpected `this`.*/
})();

function foo() {
    this.a = 0;        /*error Unexpected `this`.*/
    baz(() => this);   /*error Unexpected `this`.*/
}

var foo = function() {
    this.a = 0;        /*error Unexpected `this`.*/
    baz(() => this);   /*error Unexpected `this`.*/
};

foo(function() {
    this.a = 0;        /*error Unexpected `this`.*/
    baz(() => this);   /*error Unexpected `this`.*/
});

obj.foo = () => {
    // `this` of arrow functions is the outer scope's.
    this.a = 0;        /*error Unexpected `this`.*/
};

var obj = {
    aaa: function() {
        return function foo() {
            // There is in a method `aaa`, but `foo` is not a method.
            this.a = 0;      /*error Unexpected `this`.*/
            baz(() => this); /*error Unexpected `this`.*/
        };
    }
};

class Foo {
    static foo() {
        this.a = 0;      /*error Unexpected `this`.*/
        baz(() => this); /*error Unexpected `this`.*/
    }
}

foo.forEach(function() {
    this.a = 0;          /*error Unexpected `this`.*/
    baz(() => this);     /*error Unexpected `this`.*/
});

/////////////////////////////////////////////////////////////////////
//以下は問題ではない
////////////////////////////////////////////////////////////////////
function Foo() {
    // OK, this is in a legacy style constructor.
    this.a = 0;
    baz(() => this);
}

class Foo {
    constructor() {
        // OK, this is in a constructor.
        this.a = 0;
        baz(() => this);
    }
}

var obj = {
    foo: function foo() {
        // OK, this is in a method (this function is on object literal).
        this.a = 0;
    }
};

var obj = {
    foo() {
        // OK, this is in a method (this function is on object literal).
        this.a = 0;
    }
};

var obj = {
    get foo() {
        // OK, this is in a method (this function is on object literal).
        return this.a;
    }
};

var obj = Object.create(null, {
    foo: {value: function foo() {
        // OK, this is in a method (this function is on object literal).
        this.a = 0;
    }}
});

Object.defineProperty(obj, "foo", {
    value: function foo() {
        // OK, this is in a method (this function is on object literal).
        this.a = 0;
    }
});

Object.defineProperties(obj, {
    foo: {value: function foo() {
        // OK, this is in a method (this function is on object literal).
        this.a = 0;
    }}
});

function Foo() {
    this.foo = function foo() {
        // OK, this is in a method (this function assigns to a property).
        this.a = 0;
        baz(() => this);
    };
}

obj.foo = function foo() {
    // OK, this is in a method (this function assigns to a property).
    this.a = 0;
};

Foo.prototype.foo = function foo() {
    // OK, this is in a method (this function assigns to a property).
    this.a = 0;
};

class Foo {
    foo() {
        // OK, this is in a method.
        this.a = 0;
        baz(() => this);
    }
}

var foo = (function foo() {
    // OK, the `bind` method of this function is called directly.
    this.a = 0;
}).bind(obj);

foo.forEach(function() {
    // OK, `thisArg` of `.forEach()` is given.
    this.a = 0;
    baz(() => this);
}, thisArg);

/** @this Foo */
function foo() {
    // OK, this function has a `@this` tag in its JSDoc comment.
    this.a = 0;
}

file24.js

Foo.prototype.__iterator__ = function() { /*error Reserved name '__iterator__'.*/
    return new FooIterator(this);
};
foo.__iterator__ = function () {};        /*error Reserved name '__iterator__'.*/
foo["__iterator__"] = function () {};     /*error Reserved name '__iterator__'.*/

//ok
/*eslint no-iterator: 2*/

var __iterator__ = foo; // Not using the `__iterator__` property.

file25.js

label:                   /*error Unexpected labeled statement.*/
    while(true) {
        // ...
    }

label:                   /*error Unexpected labeled statement.*/
    while(true) {
        break label;     /*error Unexpected label in break statement.*/
    }

label:                  /*error Unexpected labeled statement.*/
    while(true) {
        continue label; /*error Unexpected label in continue statement.*/
    }
//ok
var f = {
    label: "foo"
};

while (true) {
    break;
}

while (true) {
    continue;
}

file26.js

for (var i=10; i; i--) {
    (function() { return i; })();     /*error Don't make functions within a loop*/
}
//ok
var a = function() {};
for (var i=10; i; i--) {
    a();
}

file27.js

if(foo  === "bar") {}//スペース複数
var a =  1;            /*error Multiple spaces found before '1'.*/

//ok
if(foo === "bar") {}
var a = 1;

file28.js

var x = "Line 1 \
         Line 2";//こういうのを許可しない

file29.js

String = new Object(); /*error String is a read-only native object.*/

file3.js

function doSomething(condition) {

    if (condition) {
        return true;
    } else {
        return;      /*error Expected a return value.*/
    }
}
//ok
function doSomething(condition) {

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

file30.js

var x = new Function("a", "b", "return a + b"); /*error The Function constructor is eval.*/
var x = Function("a", "b", "return a + b");     /*error The Function constructor is eval.*/

file31.js

var stringObject = new String("Hello world"); /*error Do not use String as a constructor.*/
var numberObject = new Number(33);            /*error Do not use Number as a constructor.*/
var booleanObject = new Boolean(false);       /*error Do not use Boolean as a constructor.*/
var stringObject = new String;                /*error Do not use String as a constructor.*/
var numberObject = new Number;                /*error Do not use Number as a constructor.*/
var booleanObject = new Boolean;              /*error Do not use Boolean as a constructor.*/

//OK
var text = String(someValue);
var num = Number(someValue);
var object = new MyString();

file32.js

new Thing(); /*error Do not use 'new' for side effects.*/

//OK
var thing = new Thing();
Thing();

file33.js

var foo = "Copyright \251"; /*error Don't use octal: '\251'. Use '\u....' instead.*/

//ok
var foo = "Copyright \u00A9";   // unicode
var foo = "Copyright \xA9";     // hexadecimal

file34.js

var num = 071;       /*error Octal literals should not be used.*/
//ok
var num  = "071";

file35.js

function foo(bar) {
    bar = 13;       /*error Assignment to function parameter 'bar'.*/
}
function foo(bar) {
    bar++;          /*error Assignment to function parameter 'bar'.*/
}

//ok
/*eslint no-param-reassign: 2*/
function foo(a) {
    var b = a;
}

file36.js

if(process.env.NODE_ENV === "development") { /*error Unexpected use of process.env.*/
    //...
}

//ok
var config = require("./config");

if(config.env === "development") {
    //...
}

file37.js

var a = obj.__proto__;    /*error The '__proto__' property is deprecated.*/
var a = obj["__proto__"]; /*error The '__proto__' property is deprecated.*/

//ok
/*eslint no-proto: 2*/
var a = Object.getPrototypeOf(obj);

file38.js

var a = 3;
var a = 10; /*error "a" is already defined*/

//ok
var a = 3;
// ...
a = 10;

file39.js

function doSomething() {
    return foo = bar + 2; /*error Return statement should not contain assignment.*/
}
function doSomething() {
    return foo += 2;      /*error Return statement should not contain assignment.*/
}

//ok
function doSomething() {
    return foo == bar + 2;
}
function doSomething() {
    return foo === bar + 2;
}
function doSomething() {
    return (foo = bar + 2);
}

file4.js

if (foo) foo++; /*error Expected { after 'if' condition.*/

while (bar)     /*error Expected { after 'while' condition.*/
    baz();
//ok
if (foo) {
    foo++;
}

while (bar) {
    baz();
}

file40.js

location.href = "javascript:void(0)"; /*error Script URL is a form of eval.*/

file41.js

/*eslint no-self-compare: 2*/
var x = 10;
if (x === x) { /*error Comparing to itself is potentially pointless.*/
    x = 20;
}

file42.js

foo = doSomething, val;              /*error Unexpected use of comma operator.*/
do {} while (doSomething(), !!test); /*error Unexpected use of comma operator.*/
for (; doSomething(), !!test; );     /*error Unexpected use of comma operator.*/
if (doSomething(), !!test);          /*error Unexpected use of comma operator.*/
switch (val = foo(), val) {}         /*error Unexpected use of comma operator.*/
while (val = foo(), val < 42);       /*error Unexpected use of comma operator.*/
with (doSomething(), val) {}         /*error Unexpected use of comma operator.*/

//ok
/*eslint no-sequences: 2*/
foo = (doSomething(), val);
(0,eval)("doSomething();");
do {} while ((doSomething(), !!test));
for (i = 0, j = 10; i < j; i++, j--);
if ((doSomething(), !!test));
switch ((val = foo(), val)) {}
while ((val = foo(), val < 42));
// with ((doSomething(), val)) {}

file43.js

throw "error";         /*error Expected an object to be thrown.*/
throw 0;               /*error Expected an object to be thrown.*/
throw undefined;       /*error Do not throw undefined.*/
throw null;            /*error Expected an object to be thrown.*/
var err = new Error();
throw "an " + err;     /*error Expected an object to be thrown.*/
// err is recast to a string literal
var err = new Error();
throw `${err}`         /*error Expected an object to be thrown.*/


//ok
throw new Error();
throw new Error("error");
var e = new Error("error");
throw e;
try {
    throw new Error("error");
} catch (e) {
    throw e;
}

file44.js

0        /*error Expected an assignment or function call and instead saw an expression.*/
if(0) 0  /*error Expected an assignment or function call and instead saw an expression.*/
{0}      /*error Expected an assignment or function call and instead saw an expression.*/
f(0), {} /*error Expected an assignment or function call and instead saw an expression.*/
a && b() /*error Expected an assignment or function call and instead saw an expression.*/
a, b()   /*error Expected an assignment or function call and instead saw an expression.*/
/////////////////////////////////////////////////////////////////////
//default//ok
{}
f()
a = 0
new C
delete a.b
void a
c = a, b;

file45.js

// foo(1, 2, 3);と同じ意味だし
foo.call(undefined, 1, 2, 3);     /*error unnecessary ".call()".*/
foo.apply(undefined, [1, 2, 3]);  /*error unnecessary ".apply()".*/
foo.call(null, 1, 2, 3);          /*error unnecessary ".call()".*/
foo.apply(null, [1, 2, 3]);       /*error unnecessary ".apply()".*/

// これと同じ意味だし、obj.foo(1, 2, 3);
obj.foo.call(obj, 1, 2, 3);       /*error unnecessary ".call()".*/
obj.foo.apply(obj, [1, 2, 3]);    /*error unnecessary ".apply()".*/
////////////////////////////////////////////////////////////////////
//ok
//thisのバインドは別.
foo.call(obj, 1, 2, 3);
foo.apply(obj, [1, 2, 3]);
obj.foo.call(null, 1, 2, 3);
obj.foo.apply(null, [1, 2, 3]);
obj.foo.call(otherObj, 1, 2, 3);
obj.foo.apply(otherObj, [1, 2, 3]);
// 引数リストのためはいい
foo.apply(undefined, args);
foo.apply(null, args);
obj.foo.apply(obj, args);

file46.js

//10と同じ
var a = `some` + `string`; /*error Unexpected string concatenation of literals.*/
var a = '1' + '0';         /*error Unexpected string concatenation of literals.*/
var a = '1' + `0`;         /*error Unexpected string concatenation of literals.*/
var a = `1` + '0';         /*error Unexpected string concatenation of literals.*/
var a = `1` + `0`;         /*error Unexpected string concatenation of literals.*/

/////////////////////////////////////////
//ok
// when a non string is included
var c = a + b;
var c = '1' + a;
var a = 1 + '1';
var c = 1 - 2;
// when the string concatenation is multiline
var c = "foo" +
    "bar";

file47.js

void foo              /*error Expected 'undefined' and instead saw 'void'.*/
var foo = void bar(); /*error Expected 'undefined' and instead saw 'void'.*/

file48.js

// TODO: this                          /*error Unexpected todo comment.*/
// todo: this too                      /*error Unexpected todo comment.*/
// Even this: TODO                     /*error Unexpected todo comment.*/
/*                                     /*error Unexpected todo comment.*/ /*error Unexpected fixme comment.*/ /*Unexpected any other term comment.*/ /*
 * The same goes for this TODO comment
 * Or a fixme
 * as well as any other term
 */
//////////////////////////////////////////////
//ok
// This is to do
// even not any other    term
// any other terminal
/*
 * The same goes for block comments
 * with any other interesting term
 * or fix me this
 */

file49.js

with (foo) { /*error Unexpected use of 'with' statement.*/

file5.js

switch (a) {       /*error Expected a default case.*/
    case 1:
        /* code */
        break;
}
//ok
switch (foo) {
    case 1:
        doSomething();
        break;

    case 2:
        doSomething();
        break;

    default:
        // do nothing
}

file50.js

var num = parseInt("071");     /*error Missing radix parameter.*/
var num = parseInt(someValue); /*error Missing radix parameter.*/
////////////////////////////////////
//ok
var num = parseInt("071", 10);
var num = parseFloat(someValue);

file51.js

// Variable declarations in a block:
function doSomething() {
    var first;
    if (true) {
        first = true;
    }
    var second;/*error All "var" declarations must be at the top of the function scope.*/
}
// Variable declaration in for initializer:
function doSomething() {
    for (var i=0; i<10; i++) {} /*error All "var" declarations must be at the top of the function scope.*/
}
// Variables after other statements:
f();
var a;/*error All "var" declarations must be at the top of the function scope.*/
//////////////////////////////////////////////////
///ok

function doSomething() {
    var first;//上
    var second; //multiple declarations are allowed at the top
    if (true) {
        first = true;
    }
}
function doSomething() {
    var i;//宣言とそれは別
    for (i=0; i<10; i++) {}
}

file52.js

var x = function () { return { y: 1 };}(); /*error Wrap an immediate function invocation in parentheses.*/

file53.js

if ("red" === color) {          /*error Expected literal to be on the right side of ===.*/
    // ...
}
if (true == flag) {             /*error Expected literal to be on the right side of ==.*/
    // ...
}
if (5 > count) {                /*error Expected literal to be on the right side of >.*/
    // ...
}
if (-1 < str.indexOf(substr)) { /*error Expected literal to be on the right side of <.*/
    // ...
}
if (0 <= x && x < 1) {          /*error Expected literal to be on the right side of <=.*/
    // ...
}
/////////////////////////////////////////////
//ok
if (5 & value) {
    // ...
}
if (value === "red") {
    // ...
}

file53.txt

var x = (function () { return { y: 1 };})(); /*error Move the invocation into the parens that contain the function.*/
//////////////////////////////////////////////////
//ok
var x = (function () { return { y: 1 };}());

file54.txt

var x = (function () { return { y: 1 };}()); /*error Wrap only the function expression in parens.*/
//////////////////////////////////////////////////
//ok
var x = (function () { return { y: 1 };})();

file55.js

if ("red" === color) {          /*error Expected literal to be on the right side of ===.*/
    // ...
}
if (true == flag) {             /*error Expected literal to be on the right side of ==.*/
    // ...
}
if (5 > count) {                /*error Expected literal to be on the right side of >.*/
    // ...
}
if (-1 < str.indexOf(substr)) { /*error Expected literal to be on the right side of <.*/
    // ...
}
if (0 <= x && x < 1) {          /*error Expected literal to be on the right side of <=.*/
    // ...
}
/////////////////////////////////////////////
//ok
if (5 & value) {
    // ...
}
if (value === "red") {
    // ...
}

file6.js

/*eslint dot-notation: 2*/
var x = foo.bar;

var x = foo[bar];    // Property name is a variable, square-bracket notation required

file7.js

var foo = object
.property;       /*error Expected dot to be on same line as object.*/

file8.js

if (x == 42) { }                     /*error Expected '===' and instead saw '=='.*/

if ("" == text) { }                  /*error Expected '===' and instead saw '=='.*/

if (obj.getStuff() != undefined) { } /*error Expected '!==' and instead saw '!='.*/
/* eslint eqeqeq: [2, "smart"] */
typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

file9.js

for (key in foo) {    /*error The body of a for-in should be wrapped in an if statement to filter unwanted properties from the prototype.*/
    doSomething(key);
}
//ok
for (key in foo) {
    if ({}.hasOwnProperty.call(foo, key)) {
        doSomething(key);
    }
}