Using Pipes Within ngModel on Input Elements in Angular

Got an error: Cannot have a pipe in an action expression.

The issue

My model has a property “startDate”, which stores Unix timestamp. When I display, I’d like to show only the full year, like 2018. In Angular, we could achieve this easily using pipe. I did it this way:

<input [(ngModel)]="model.startYear | date: 'yyyy'" >

However, I got an error: Cannot have a pipe in an action expression.

The Reasons

  1. In Angular, [()] is two-way binding.
  2. [] is one-way attribute binding, which binds data from component to the view. The real-time date transform, which how pipe works, happens here.
  3. () is one-way event binding, which passes data from view to component. Pipe function doesn’t work here.
  4. Therefore, I have to split [()] into [] and (), and only use pipe in [].

A Stackoverflow solution:

<input [ngModel]="item.value | somePipeMethod" (ngModelChange)="item.value=$event" />

This solution works well. But it has some drawbacks.

  1. I don’t like (ngModelChange). Imagine a user type in “1234” in the input, four times the model got changed.
  2. Like I said at the beginning, my model’s property “startDate” is Unix timestamp, I need to transfer user input, which is a string, to Unix timestamp. The logic here is simple, however, I fell into a big trouble. Because of the (ngModelChange), user’s input converts to Unix timestamp into real time. If a user types in “201”, model’s startDate is a negative number due to the nature of Unix timestamp. In the input, it shows something like 021, which is a disaster.

My workaround

<input value="{\\{node.startDate | date: 'yyyy' }\\}" 
      (blur)="saveProjectYear(node, startYear.value)" >
  1. In order to show current format, we only use value attribute so that we could pipe it with the right format
  2. Onblur, we save this object using a method. It’s almost the same as the one

References

Using Pipes within ngModel on INPUT Elements in Angular

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