Typescript 参数名称中的问号是什么

Question

export class Thread {
  id: string;
  lastMessage: Message;
  name: string;
  avatarSrc: string;

  constructor(id?: string,
              name?: string,
              avatarSrc?: string) {
    this.id = id || uuid();
    this.name = name;
    this.avatarSrc = avatarSrc;
  }
}

id? 中的? 是干什么用的？

Answer 1

parameter?: type 是 parameter: type | undefined 的简写

那么有什么区别？问号表示“可选”。
更准确地说parameter?: type等于parameter: type | undefined = undefined

Answer 2

是将参数标记为可选。

• TypeScript 手册https://www.typescriptlang.org/docs/handbook/2/functions.html#optional-parameters
• TypeScript 深入了解https://basarat.gitbook.io/typescript/type-system/functions#optional-parameters

Answer 3

参数中的?表示可选参数。 Typescript编译器不需要填写此参数，详情请看下面的代码示例：

// baz: number | undefined means: the second argument baz can be a number or undefined

// = undefined, is default parameter syntax, 
// if the parameter is not filled in it will default to undefined

// Although default JS behaviour is to set every non filled in argument to undefined 
// we need this default argument so that the typescript compiler
// doesn't require the second argument to be filled in
function fn1 (bar: string, baz: number | undefined = undefined) {
    // do stuff
}

// All the above code can be simplified using the ? operator after the parameter
// In other words fn1 and fn2 are equivalent in behaviour
function fn2 (bar: string, baz?: number) {
    // do stuff
}



fn2('foo', 3); // works
fn2('foo'); // works

fn2();
// Compile time error: Expected 1-2 arguments, but got 0
// An argument for 'bar' was not provided.


fn1('foo', 3); // works
fn1('foo'); // works

fn1();
// Compile time error: Expected 1-2 arguments, but got 0
// An argument for 'bar' was not provided.

Answer 4

这是为了使 Optional 类型的变量。否则声明的变量如果不使用此变量，则显示“undefined”。

export interface ISearchResult {  
  title: string;  
  listTitle:string;
  entityName?: string,
  lookupName?:string,
  lookupId?:string  
}