SelectionModel<T> initialized with empty array results in selected.length==1

[simeyla]

Bug, feature request, or proposal:

If you create a SelectionModel with isMulti=false and pass an empty array for initiallySelectedValues?: T[] it results in undefined being being added to the selection therefore you get selection.selected.length == 1 instead of the expected 0.

What is the expected behavior?

var selection = new SelectionModel<CustomerSearchResult>(false, []);

//Expect `selection.selected.length == 0`

What is the current behavior?

var selection = new SelectionModel<CustomerSearchResult>(false, []);
// selection.selected.length == 1

What are the steps to reproduce?

Code above is sufficient.

What is the use-case or motivation for changing an existing behavior?

It is confusing to the user, especially given that the method signature is initiallySelectedValues?: T[]

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Material CDK

Is there anything else we should know?

Suggested fix :

Add && initiallySelectedValues.length to the first if statement for the constructor.
This would also result in the unnecessary forEach being skipped when isMulti==true

Then if [] is provided selected will remain an empty array.

Also it would be nice if I could just pass in the value I want selected instead of it needing to be an array. That's another point I think should be addressed.

constructor(
    private _isMulti = false,
    initiallySelectedValues?: T[],
    private _emitChanges = true) {

    if (initiallySelectedValues && initiallySelectedValues.length) {
      if (_isMulti) {
        initiallySelectedValues.forEach(value => this._markSelected(value));
      } else {
        this._markSelected(initiallySelectedValues[0]);
      }

      // Clear the array in order to avoid firing the change event for preselected values.
      this._selectedToEmit.length = 0;
    }
  }

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