Javascript将值赋给基元 [英] Javascript assigning value to primitive

问题描述

如果JavaScript很容易在基元和对象之间强制转换，为什么向它添加属性会导致未定义？

If JavaScript will readily coerce between primitives and objects why adding properties to it results to undefined??

var a = "abc";
var b = a.length
console.log(b)//outputs 3

强制允许我为原语分配值吗？如果没有原因？

Does coercion allow me to assign values to primitives?If not why ?

推荐答案

强制允许我为原语分配值？

Does coercion allow me to assign values to primitives?

是的。原语包装在一个对象中，并在其上创建一个属性。不会抛出任何异常。

Yes. The primitive is wrapped in an object, and a property is created on that. No exception will be thrown.

为什么向它添加属性会导致未定义？

why adding properties to it results to undefined?

添加本身确实会产生价值。

The adding itself does result in the value.

var str = "abc";
console.log(str.someProperty = 5); // 5

然而，你要求的是从原语中获取属性。这将返回 undefined ，因为基元包含在新包装器对象中 - 而不是您为其分配属性的对象：

Yet, what you're asking for is getting a property from a primitive. This will return in undefined since the primitive is wrapped in a new wrapper object - not the one which you assigned the property on:

console.log(str.someProperty); // undefined

它仅适用于 .length 使用该对象创建的，或者是继承的 slice 或 charAt 方法（参见这些文档。

It only works for special properties like .length that are created with the object, or inherited ones like slice or charAt methods (see the docs for those).

如果你想要这样的东西，你需要明确地创建包装器对象并将其存储在某个地方：

If you wanted such a thing, you'd need to create the wrapper object explicitly and store it somewhere:

var strObj = new String("abc");
strObj.someProperty = 5;
console.log(strObj.someProperty); // 5
// coercion and the `toString` method will even make the object act like a string
console.log(strObj + "def"); // "abcdef"

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