为什么TypeScript的方法需要一个变量作为返回类型？

Question

I'm new to typescript, forgive me if the question is stupid.

For the following code:

import { Component, ViewChild } from '@angular/core';
//....
export class MyApp {

  @ViewChild(Nav) nav: Nav;
//....

I understand that Nav is the return type, but what does "nav: Nav" means? why does a return type need a "nav" as a variable? I tried to look at the official document but didn't find a clue(perhaps I barked up to the wrong trees) does this equivalent to:

import { Component, ViewChild } from '@angular/core';
//....
export class MyApp {

  @ViewChild(nav: Nav) Nav;
//....

Answer 1

This actually looks like a property (not a method) of the class called nav , of type Nav , which is decorated by ViewChild . Putting in on one line makes it a little confusing I think. It's easier to see what its doing this way:

import { Component, ViewChild } from '@angular/core';
//....
export class MyApp {

  @ViewChild(Nav)
  nav: Nav;
//....

You can read about decorators here . You can decorate classes, properties, or methods, and decorators use the @decoratorName syntax, much like they do in other languages.

EDIT:

To be a little clearer, in this case ViewChild is a decorator factory , bascially a function that returns a decorator. So you give this function Nav as an argument (which means that Nav is a class constructor function, I presume), and it returns the actual decorator.