函数声明或函数表达式 [英] Function declaration or function expression

问题描述

在块作用域中定义函数时，我遇到了一个问题.请考虑以下程序:

I just ran into a problem when defining a function in a block scope. Consider the following program:

try {
    greet();

    function greet() {
        alert("Merry Christmas!");
    }
} catch (error) {
    alert(error);
}

我希望该程序向Merry Christmas!发出警报.但是在Firefox中，它提供了以下ReferenceError:

I expected this program to alert Merry Christmas!. However in Firefox is gives me the following ReferenceError:

ReferenceError: greet is not defined

在Opera和Chrome浏览器上，它会按我预期的方式警告问候语.

On Opera and Chrome it alerts the greeting as I expected it to.

很明显，Firefox将块作用域内的功能视为FunctionExpression，而Opera和Chrome将其视为FunctionDeclaration.

Evidently Firefox treats the function inside the block scope as a FunctionExpression while Opera and Chrome treat it as a FunctionDeclaration.

我的问题是，为什么Firefox的行为有所不同?哪个实现更合乎逻辑?哪个符合标准?

My question is why does Firefox behave differently? Which implementation is more logical? Which one is standards compliant?

我了解到JavaScript中的声明是悬挂式的，因此，如果在同一作用域中的两个或多个不同块中声明相同的函数，则会发生名称冲突.

I understand that declarations in JavaScript are hoisted and so if the same function is declared in two or more different blocks in the same scope then there'll be a name conflict.

但是，每次声明函数时都重新声明该函数更合乎逻辑，这样您就可以执行以下操作:

However wouldn't it be more logical to redeclare the function every time it's declared so that you can do something like this:

greet(); // Merry Christmas!

function greet() {
    alert("Merry Christmas!");
}

greet(); // Happy New Year!

function greet() {
    alert("Happy New Year!");
}

除了解决我上面描述的块作用域问题外，我认为这将非常有用.

I think this would be very useful, in addition to solving the block scoping problem I described above.

推荐答案

实际上，显然不是 标准化了块作用域内的函数声明，并且行为依赖于实现.不同的实现有不同的响应.如果尝试在if语句中声明一个函数，您会感到很奇怪.

Actually, function declarations inside block scopes is expressly not standardized and the behavior is implementation dependent. Different implementation respond differently. You'd get the same weirdness if you tried to declare a function inside an if statement.

ES5规范建议实现者将在块内的函数声明标记为警告或错误.

The ES5 spec recommends that implementers make function declarations inside blocks be marked as a warning or error.

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