对Angular2承诺感到困惑[重复](Confused about Angular2 promise [duplicate])

对Angular2承诺感到困惑[重复](Confused about Angular2 promise [duplicate])

这个问题在这里已有答案：

如何从异步调用返回响应？ 31个答案

我有非常简单的代码，但我无法理解它。

我有一个返回当前用户名的服务

所以我正在做以下来获取登录的用户名并显示在网页上。 请注意当前登录的用户是“demo”

this.userData.getUsername().then(value => { this.username = value ; console.log("Inside -->" + value); }); console.log("Outside -- >" + this.username);

所以现在我运行这段代码。 我没有在this.username中获得用户名

这是console.log的输出 Outside -- >undefined Inside -->demo 在这两种情况下，不要显示“演示”。 （内部和外部）为什么外部未定义？

请帮我理解这个。

This question already has an answer here:

How do I return the response from an asynchronous call? 34 answers

I have very simple code , But I am not able to understand it.

I have a service which returns current Username

so I am doing following to get logged username and display on webpage. Please note current logged in user is "demo"

this.userData.getUsername().then(value => { this.username = value ; console.log("Inside -->" + value); }); console.log("Outside -- >" + this.username);

So now when I run this code. I do NOT get username in this.username

This is the output of console.log Outside -- >undefined Inside -->demo Shoulnd't this display 'demo' in both case. (inside and outside) Why outside is undefined ??

Please help me understand this.

最满意答案

这是预期的行为。

这是稍后返回的异步调用

this.userData.getUsername().then(value => { this.username = value ; console.log("Inside -->" + value); });

在此期间，执行继续进行

console.log("Outside -- >" + this.username);

当异步调用最终完成时， Promise执行该函数

value => { this.username = value ; console.log("Inside -->" + value); }

那是早先通过的。

This is the expected behavior.

This is an async call that returns later

this.userData.getUsername().then(value => { this.username = value ; console.log("Inside -->" + value); });

In the meantime execution continues here

console.log("Outside -- >" + this.username);

When the async call completes eventually the Promise executes the function

value => { this.username = value ; console.log("Inside -->" + value); }

that was passed earlier.