如何检查ES6的“变量"是否正确?是恒定的吗? [英] How to check that ES6 "variable" is constant?

问题描述

有人知道一些技巧吗?我尝试使用try-catch:

Does anyone know some tricks how to do it? I tried to use try-catch:

"use strict";

const a = 20;

var isConst = false;
try {
   var temp = a; a = a+1; a = temp;
} catch (e) {
   isConst = true;
}

但不幸的是，它仅在严格"模式下有效.如果没有严格使用"，它会静默执行所有语句，而无需修改a.同样，我无法将此代码包装到一些方便的函数(例如isConstant(someConst))中，因为我将传递给该函数的任何参数将是一个新变量.有人知道如何创建isConstant()函数吗?

But unfortunately it works only in "strict" mode. Without "use strict" it perform all statements silently, without modification of a. Also I cannot wrap this code into some handy function (isConstant(someConst) for example) as any argument I'll pass to that function will be a new variable. So anyone know how to create isConstant() function?

推荐答案

我不认为有，但是我也不认为这是个大问题.我认为了解变量是否为const可能很有用，并且它在其他一些语言中也存在，但是实际上，由于您(或团队中的某人)将定义这些变量，因此您会知道变量的范围和类型.换句话说，不，你不能，但这也不是问题.

I don't think there is, but I also don't think this is a big issue. I think it might be useful to have the ability to know if a variable is const, and this exists in some other languages, but in reality since you (or someone on a team) will be defining these variables, you'd know the scope and the type of the variables. In other words, no you can't, but it's also not an issue.

唯一有用的情况是，您可以在运行时更改mutable属性，并且更改此属性具有实际的性能优势. let，const和var与编译器大致相同，唯一的区别是编译器会跟踪const，并会在编译之前检查分配.

The only case where it might be useful is if you could change the mutable property during runtime, and if changing this property had actual performance benefits; let, const, and var are treated roughly equally to the compiler, the only difference is that the compiler keeps track of const and will check assignments before it even compiles.

要注意的另一件事是，与let一样，const的作用域是当前作用域，因此，如果您有类似这样的内容:

Another thing to note is that just like let, const is scoped to the current scope, so if you have something like this:

'use strict';

const a = 12;

// another scope
{
  const a = 13;
}

有效.请注意，如果您未在新作用域中明确声明const a = 13，它将在更高的作用域中查找，并且会出现Read Only或Assignment错误:

it's valid. Just be careful that it will look up in higher scopes if you don't explicitly state const a = 13 in that new scope, and it will give a Read Only or Assignment error:

'use strict';

const a = 12;

{
  a = 13; // will result in error
}

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