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[ ] feature request
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Current behavior

The FormArray.at method signature declares the return value to be AbstractControl.

Expected behavior

The FormArray.at method signature declares the return value to be AbstractControl | undefined.

Since:
FormArray.at returns the result of accessing the contained controls array.
undefined is returned when the supplied index is out-of-range, with respect to the target FormArray.

Note: Typescript does not point out an issue, even with strictNullChecks: true.

Minimal reproduction of the problem with instructions

    const formBuilder: FormBuilder = new FormBuilder();
    const formArray: FormArray = formBuilder.array([]);

    const outOfRangeItemPositiveAt: AbstractControl = formArray.at(5);
    const outOfRangeItemNegativeAt: AbstractControl = formArray.at(-1);

    console.log(`outOfRangeItemPositiveAt is defined = [${!!outOfRangeItemPositiveAt}]`);
    console.log(`outOfRangeItemNegativeAt is defined = [${!!outOfRangeItemNegativeAt}]`);

    console.log(`outOfRangeItemPositiveAt = [${outOfRangeItemPositiveAt}]`);
    console.log(`outOfRangeItemNegativeAt = [${outOfRangeItemNegativeAt}]`);

What is the motivation / use case for changing the behavior?

To indicate the possibility of an undefined result.

Please tell us about your environment:

• Angular version: 4.1.3

• Browser: [ Chrome ]

• Language: [ TypeScript 2.3.2 | ES5 ]

• Node (for AoT issues): node --version = v6.9.2