在Typescript参数名称中有什么问号

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;
  }
}

在id? ?为什么？

Answer 1

将参数标记为可选。

• TypeScript手册https://www.typescriptlang.org/docs/handbook/functions.html#optional-and-default-parameters
• TypeScript Deep Dive https://basarat.gitbooks.io/typescript/content/docs/types/functions.html#optional-parameters

Answer 2

parameter?: type是parameter: type | undefined

的简写

Answer 3

这是为了使变量为Optional类型。如果未使用此变量，则声明的变量显示为“ undefined ”。

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

Answer 4

参数中的?表示可选参数。 Typescript编译器不需要填写此参数。有关更多详细信息，请参见下面的代码示例：

// 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.