Angular-Starter | Angular 11 Webpack | Frontend Framework library

I'd love an explanation as to why the result of this else if statement doesn't match my expectation. my if statement works fine, but else if statement pushes the same value to my array. what I am trying to do is that, if by end of the period person is greater than 18 i want childEndDate to be dates of person date when he/she will become 18 years old.

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if by end of the period person is greater than 18 i want childEndDate to be the date of person's date when he/she will become 18 years old. in my else if statement i am using date-fns library to add 18 years to dates i have in my this.childBirthDate array. but returned output is wrong: ["1988-01-01T00:00:02.012Z", "1988-01-01T00:00:02.010Z", "2031-11-08T13:24:43.704Z"] output i want returned is: ['2030-02-16T20:00:00.000Z', '2028-05-19T20:00:00.000Z', 2031-11-08T13:24:43.704Z]

here is my stackblitz

I want to store my dates that i generated inside my map function, now it stores as array inside my endDate, i tried using forEach but it returns undefiened

here is my stackblitz