在Javascript中有条件地初始化一个常量

2022-01-23 00:00:00 constants javascript ecmascript-6

从 ES6 开始，我们有 const.

ES6 onwards we have const.

这是不允许的:

const x; //declare first
//and then initialize it
if(condition) x = 5;
else x = 10;

这是有道理的，因为它阻止我们在初始化之前使用常量.

This makes sense because it prevents us from using the constant before it's initialized.

如果我这样做了

if(condition)
    const x = 5;

else 
    const x = 10;

x 变为块作用域.

那么如何有条件地创建一个常量呢?

So how to conditionally create a constant?

推荐答案

如您所知，您的问题是 const 必须在声明它的相同表达式中初始化.

Your problem, as you know, is that a const has to be intialised in the same expression that it was declared in.

这并不意味着您分配给常量的值必须是文字值.它实际上可以是任何有效的表达式 - 三元:

This doesn't mean that the value you assign to your constant has to be a literal value. It could be any valid expression really - ternary:

const x = IsSomeValueTrue() ? 1 : 2;

或者也许只是将它分配给一个变量的值?

Or maybe just assign it to the value of a variable?

let y = 1;
if(IsSomeValueTrue()) {
    y = 2;
}

const x = y;

您当然也可以将其分配给函数的返回值:

You could of course assign it to the return value of a function, too:

function getConstantValue() {
    return 3;
}

const x = getConstantValue();

因此，有很多方法可以使值动态化，您只需要确保它只分配在一个地方.

So there's plenty of ways to make the value dynamic, you just have to make sure it's only assigned in one place.

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