no-unused-expressions

禁止未使用的表达式

对程序状态没有影响的未使用表达式表示逻辑错误。

例如，n + 1; 不是语法错误，但它可能是一个打字错误，程序员的意思是赋值语句 n += 1;。有时，这些未使用的表达式可能会被生产环境中的某些构建工具消除，这可能会破坏应用程序逻辑。

规则详情

该规则旨在消除对程序状态没有影响的未使用表达式。

此规则不适用于带有 new 运算符的函数调用或构造函数调用，因为它们可能对程序的状态产生副作用。

var i = 0;
function increment() { i += 1; }
increment(); // return value is unused, but i changed as a side effect

var nThings = 0;
function Thing() { nThings += 1; }
new Thing(); // constructed object is unused, but nThings changed as a side effect

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选项

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/*eslint no-unused-expressions: "error"*/

0

if(0) 0

{0}

f(0), {}

a && b()

a, b()

c = a, b;

a() && function namedFunctionInExpressionContext () {f();}

(function anIncompleteIIFE () {});

injectGlobal`body{ color: red; }`

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/*eslint no-unused-expressions: "error"*/

{} // In this context, this is a block statement, not an object literal

{myLabel: someVar} // In this context, this is a block statement with a label and expression, not an object literal

function namedFunctionDeclaration () {}

(function aGenuineIIFE () {}());

f()

a = 0

new C

delete a.b

void a

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/*eslint no-unused-expressions: "error"*/

"use strict";
"use asm"
"use stricter";
"use babel"
"any other strings like this in the directive prologue";
"this is still the directive prologue";

function foo() {
    "bar";
}

class Foo {
    someMethod() {
        "use strict";
    }
}

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/*eslint no-unused-expressions: "error"*/

doSomething();
"use strict"; // this isn't in a directive prologue, because there is a non-directive statement before it

function foo() {
    "bar" + 1;
}

class Foo {
    static {
        "use strict"; // class static blocks do not have directive prologues
    }
}

allowShortCircuit

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/*eslint no-unused-expressions: ["error", { "allowShortCircuit": true }]*/

a || b

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/*eslint no-unused-expressions: ["error", { "allowShortCircuit": true }]*/

a && b()
a() || (b = c)

allowTernary

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/*eslint no-unused-expressions: ["error", { "allowTernary": true }]*/

a ? b : 0
a ? b : c()

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/*eslint no-unused-expressions: ["error", { "allowTernary": true }]*/

a ? b() : c()
a ? (b = c) : d()

allowShortCircuit 和 allowTernary

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/*eslint no-unused-expressions: ["error", { "allowShortCircuit": true, "allowTernary": true }]*/

a ? b() || (c = d) : e()

allowTaggedTemplates

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/*eslint no-unused-expressions: ["error", { "allowTaggedTemplates": true }]*/

`some untagged template string`;

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/*eslint no-unused-expressions: ["error", { "allowTaggedTemplates": true }]*/

tag`some tagged template string`;

enforceForJSX

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/*eslint no-unused-expressions: ["error", { "enforceForJSX": true }]*/

<MyComponent />;

<></>;

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/*eslint no-unused-expressions: ["error", { "enforceForJSX": true }]*/

var myComponentPartial = <MyComponent />;

var myFragment = <></>;