角度中的 TypeScript 错误：声明类型既不是'void'也不是'any'的函数必须返回一个值

Question

我正试图突出显示日历上的日期。我在dateClass中使用有角度的材质日期选择器。逻辑很好，但我不明白为什么会出现以下错误。有人能告诉我为什么会出现以下错误吗？

A function whose declared type is neither 'void' nor 'any' must return a value.

HTML:

<mat-calendar *ngIf="loadCalendar" [dateClass]="dateClass()" [selected]="selectedDate" (selectedChange)="onSelect($event)"></mat-calendar>

Ts:

import { DatePipe } from "@angular/common";
import { Component, OnInit } from "@angular/core";
import { MatCalendarCellCssClasses } from "@angular/material";
import { DataService } from "../app/services/data.service";
@Component({
selector: "app-root",
templateUrl: "./app.component.html",
styleUrls: ["./app.component.scss"],
providers: [DatePipe],
})
export class AppComponent implements OnInit {
cardData;
dates: string[] = [];
loadCalendar: boolean = false;
constructor(private data: DataService, public datepipe: DatePipe) {}
ngOnInit() {
this.data.getData().subscribe((data) => {
this.cardData = data;
this.cardData.forEach((element) => {
//  console.log(this.datepipe.transform(element.start, 'yyyy/MM/dd'));
this.dates.push(element.start);
this.loadCalendar = true;
});
});
}
dateClass() { //Error is caused by this function
return (date: Date): MatCalendarCellCssClasses => {
this.dates.forEach((sD)=>{
let startDate = this.datepipe.transform(sD, "yyyy/MM/dd");
let calendarDate = this.datepipe.transform( date.getDate(),"yyyy/MM/dd");

if (calendarDate === startDate) {
return "special-date";
} else {
return "";
}
})
};
}
}

Answer 1

return (date: Date): MatCalendarCellCssClasses => {  <-- !!!You say it returns a MatCalendarCellCssClasses
this.dates.forEach((sD)=>{
let startDate = this.datepipe.transform(sD, "yyyy/MM/dd");
let calendarDate = this.datepipe.transform( date.getDate(),"yyyy/MM/dd");

if (calendarDate === startDate) {
return "special-date";
} else {
return""   
}

})
return "" <--- you must also return something here incase forEach does not loop 
}

编辑：问题更新时更新了答案