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no-unreachable

Disallow unreachable code after return, throw, continue, and break statements

Recommended

The "extends": "eslint:recommended" property in a configuration file enables this rule

因为 returnthrowbreakcontinue 语句无条件地退出一个代码块,它们之后的任何语句都不能被执行。不会用到的语句通常是一个错误。

function fn() {
    x = 1;
    return x;
    x = 3; // this will never execute
}

另一种错误是在子类中定义实例字段,其构造函数没有调用 super()。子类的实例字段只有在 super() 之后才会被添加到实例中。如果没有 super() 的调用,它们的定义就永远不会被应用,因此是不可达的代码。

class C extends B {
    #x; // this will never be added to instances

    constructor() {
        return {};
    }
}

规则细节

使用此规则禁用 returnthrowcontinuebreak 后的不可达代码。此规则还标记了子类中实例字段的定义,其构造函数没有 super() 调用。

使用此规则的错误示例:

Open in Playground
/*eslint no-unreachable: "error"*/

function foo() {
    return true;
    console.log("done");
}

function bar() {
    throw new Error("Oops!");
    console.log("done");
}

while(value) {
    break;
    console.log("done");
}

throw new Error("Oops!");
console.log("done");

function baz() {
    if (Math.random() < 0.5) {
        return;
    } else {
        throw new Error();
    }
    console.log("done");
}

for (;;) {}
console.log("done");

因为 JavaScript 函数和变量的提升,导致的此规则的正确示例:

Open in Playground
/*eslint no-unreachable: "error"*/

function foo() {
    return bar();
    function bar() {
        return 1;
    }
}

function bar() {
    return x;
    var x;
}

switch (foo) {
    case 1:
        break;
        var x;
}

此规则的额外错误示例:

Open in Playground
/*eslint no-unreachable: "error"*/

class C extends B {
    #x; // unreachable
    #y = 1; // unreachable
    a; // unreachable
    b = 1; // unreachable

    constructor() {
        return {};
    }
}

此规则的额外正确示例:

Open in Playground
/*eslint no-unreachable: "error"*/

class D extends B {
    #x;
    #y = 1;
    a;
    b = 1;

    constructor() {
        super();
    }
}

class E extends B {
    #x;
    #y = 1;
    a;
    b = 1;

    // implicit constructor always calls `super()`
}

class F extends B {
    static #x;
    static #y = 1;
    static a;
    static b = 1;

    constructor() {
        return {};
    }
}

Handled by TypeScript

It is safe to disable this rule when using TypeScript because TypeScript's compiler enforces this check.

Version

This rule was introduced in ESLint v0.0.6.

Resources

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