它将参数标记为可选。

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

这将使变量的类型为Optional。否则声明的变量显示“未定义”，如果这个变量没有被使用。

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

参数?: type是参数:type | undefined的简写

那么有什么不同呢?问号表示“可选”。更精确的参数?参数:type | undefined = undefined

的吗?在参数中表示一个可选参数。Typescript编译器不需要填写此参数。查看下面的代码示例了解更多细节:

// 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.

从TypeScript 4.7开始，编译器是这样处理这些场景的:

interface OptTest {
  test: string
  test2: string
  test3?: string
  test4?: string | null
  test5: string | null
  test6: null
}

const testVar: OptTest = {} 
// Type '{}' is missing the following 
// properties from type 'OptTest': test, test2, test5, test6