角反应控制值未正确更新

Question

我正在从此函数返回日期，为什么该函数还会更新StartDate控件的值，例如：如果StartDate为15Dec2019，并且我想返回17Dec2019，并且此方法返回正确的值，但它还会使用该函数的返回日期更新StartDate控件的值。

此值不应在此函数内更新，在这里我得到的是控制值而不是更新。this.createRecord.controls ['StartDate']。value

addDays(date: Date, days: number): Date {
date.setDate(date.getDate() + days);
return date;}



GetDueDate(): Date {
#region Recurring End Type -> No Of Occurences
debugger;
const dtStart: Date = this.createRecord.controls['StartDate'].value;
let returnDate: Date;
let dtEnd: Date = dtStart;
let FrequencyId: number = +this.createRecord.controls['FrequencyId'].value;
for (let i = 1; i < +this.createRecord.controls['Occurances'].value; i++)
{
  //#region Next Date Calculation

  if (FrequencyId == 1) {
    dtEnd = this.addDays(dtStart, 1);//dtStart.(1);
  }
  else if (FrequencyId == 2) {
    dtEnd = this.addDays(dtStart, 2);
  }
  else if (FrequencyId == 3) {
    dtEnd = this.addDays(dtStart, 15);
  }
  else if (FrequencyId == 4) {
    dtEnd = this.addMonths(dtStart,4);
  }
  else if (FrequencyId == 5) {
    dtEnd = this.addMonths(dtStart, 3);
  }
  else if (FrequencyId == 6) {
    dtEnd = this.addMonths(dtStart, 6);
  }
  else if (FrequencyId == 7) {
    dtEnd = this.addYears(dtStart, 1);
  }
 //#endregion

  returnDate = dtEnd;
}

//#endregion
return returnDate;

Answer 1

日期是可变的。您应该创建新的日期。

const addDays = (date: Date, days: number): Date => {
    const newDate = new Date(date.getTime())
    newDate.setDate(newDate.getDate() + days);
    return newDate;
}
const date = new Date();
const after2Days = addDays(date, 2);
console.log('date:',date);
console.log('after2Days:',after2Days);