TypeScript instanceof不起作用

问题描述

我在使用instanceof运算符时遇到问题，它似乎不起作用。这是我的代码的一部分：

I'm having issues using the instanceof operator and it doesn't seem to work. Here is a part of my code:

        const results = _.map(items, function(item: Goal|Note|Task, index: number) { 
            let result = {};
            if (item instanceof Goal) {
                result = { id: index, title: item.name };
            } else if (item instanceof Note) {
                result = { id: index, title: item.content.text };
            } else if (item instanceof Task) {
                result = { id: index, title: item.name };
            }

            console.log(item);
            console.log(item instanceof Goal);
            console.log(item instanceof Note);
            console.log(item instanceof Task);

            return result; 
        });

我的所有日​​志都为false，这是控制台的样子：

All of my logs say false, here is what the console looks like:

尽管明确指出只有3种类型，但都不匹配。您还可能看到对象本身带有目标类型名，所以我不明白为什么它与目标实例不匹配。

None of them match, despite being explicit that only the 3 types would be possible. You could also see the object itself with a typename of Goal, so I don't get why it doesn't match with instanceof Goal.

有什么想法吗？

推荐答案

instanceof 仅在与之匹配的函数或类匹配时返回true建造了。 项目是简单的对象。

instanceof will return true only if it matches the function or class from which it was constructed. The item here is a plain Object.

const a = { a: 1 } // plain object
console.log(a);

// {a:1}                 <-- the constructor type is empty
//   a: 1
//   __proto__: Object   <-- inherited from

a instanceof A         // false because it is a plain object
a instanceof Object    // true because all object are inherited from Object

如果使用构造函数或类构造，则instanceof将按预期工作：

If it is constructed using a constructor function or a class, then instanceof will work as expected:

function A(a) {
    this.a = a;
}

const a = new A(1);    // create new "instance of" A
console.log(a);

// A {a:1}               <-- the constructor type is `A`

a instanceof A         // true because it is constructed from A
a instanceof Object    // true

如果 Goal 为接口只会检查对象的结构而不是类型。如果 Goal 是构造函数，则对于 instanceof 支票应返回true。

If Goal is an Interface it will only check the structure of the object not its type. If Goal is a constructor then it should return true for instanceof checks.

尝试类似以下操作：

// interface Goal {...}
class Goal {...}        // you will have to change the way it works.

items = [
   new Goal()
];

这篇关于TypeScript instanceof不起作用的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！