TypeScript抱怨string.[]类型数组上不存在array.map [英] TypeScript complaining that array.map is not present on array of type string[]

问题描述

我有一个带有TypeScript的Angular 2项目.我不明白为什么TypeScript实时编译器抱怨 info.map ?

I have an Angular 2 project with TypeScript. I don't understand why TypeScript live compiler is complaining about the info.map?

错误消息:无法解析的函数或方法map()

Error message: unresolved function or method map()

当我在浏览器中运行它时， 工作正常 ，但是我不明白为什么会发生，更重要的是TypeScript如何确定/else我在这里.

When I run it in the browser, is working just fine, but I don't understand why it happens and more importantly how TypeScript determines in which if /else branch I'm in.

代码:

let addInfo = (info: string|string[]) =>  {
        if(_.isString(info)){

            console.log('info is a string');
            infos.push(info);

        }else if(_.isArray(info)){

            console.log('info is an array');
            info.map( x => { infos.push(x) }); // here is the compiling error- map is red.
        }
 }

这是快照:

推荐答案

Typescript编译器无法将lodash的函数调用识别为类型检查.您可以尝试"if(info instanceof String)"，在"else"分支打字稿中可以假定"info"为数组.

Typescript compiler doesn't recognise lodash's function call as a type check. You can try "if(info instanceof String)" and in the "else" branch typescript can assume the "info" is array.

这在打字稿游乐场中对我有用:

let addInfo = (info: string|string[]) =>  {
    if(info instanceof String){
        info;
    } else {
        info.map(s => s);
    }
 }

更新

感谢@Tamas Hegedus，他们分享了有关自定义类型保护的知识.我以前从没听说过.我决定在此处引用打字稿文档中有关用户定义的报价类型防护:

Update

Thanks to @Tamas Hegedus, shared knoweledge about custom type-guards. I haven't heard about them before. I've decided to put here quotation from Typescript documentation about User-Defined Type Guards:

恰恰是TypeScript有一种叫做类型保护的东西.类型防护是一些执行运行时检查的表达式，在一定范围内保证类型.要定义类型保护，我们只需需要定义一个返回类型为类型谓词的函数:

It just so happens that TypeScript has something called a type guard. A type guard is some expression that performs a runtime check that guarantees the type in some scope. To define a type guard, we simply need to define a function whose return type is a type predicate:

function isFish(pet: Fish | Bird): pet is Fish {
    return (<Fish>pet).swim !== undefined;
}

在此示例中，

宠物是鱼是我们的类型谓词.谓词表格 parameterName是类型，其中 parameterName 必须是名称来自当前函数签名的参数.

pet is Fish is our type predicate in this example. A predicate takes the form parameterName is Type, where parameterName must be the name of a parameter from the current function signature.

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