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Form: Formatting and Parsing

For demo purposes, below we use <lion-input> which is a basic extension of <lion-field>. Almost all fields share the same functionality as <lion-input>.

Different values

The FormatMixin which is used in all lion fields automatically keeps track of:

  • modelValue, which is a result of parsing
  • formattedValue, which is a result of formatting
  • serializedValue, which is a result of serializing

Our fields can automatically format/parse/serialize user input or an input set imperatively by an Application Developer.

Below are some concrete examples of implementations of formatters and parsers mimicking a (basic) amount input.

For an actual amount input, check out lion-input-amount. This comes with its own formatter, parser, serializer.

Parsers & modelValue

A parser should return a modelValue.

The modelValue is the result of the parser function. It should be considered as the internal value used for validation and reasoning/logic. The modelValue is 'ready for consumption' by the outside world (think of a Date object or a float). The modelValue can (and is recommended to) be used as both input value and output value of the <lion-input>.

In essence, the modelValue acts as a Single Source of Truth in our form fields and therefore a single way of programmatical interaction. Formatted values, serialized values and reflected-back view values are derived from it.

You can listen to model-value-changed event on all fields. Internally this is also used as the main trigger for re-evaluating validation, visibility and interaction states.


  • For a date input: a String '20/01/1999' will be converted to new Date('1999/01/20')
  • For an amount input: a formatted String '1.234,56' will be converted to a Number: 1234.56

You can set a parser function on the <lion-input> to set parsing behavior. In this example, we parse the input and try to convert it to a Number.


If a parser tries to parse and it returns undefined, the modelValue will be an instance of Unparseable.

This object contains a type 'unparseable', and a viewValue which contains the String value of what the user tried to input.

The formatted result of this that is reflected to the user will be the viewValue of the Unparseable instance, so basically nothing happens for the user.


A formatter should return a formattedValue. It accepts the current modelValue and an options object.

Below is a very naive and limited parser that ignores non-digits. The formatter then uses Intl.NumberFormat to format it with thousand separators.

Formatted value is reflected back to the user on-blur of the field, but only if the field has no errors (validation).

The options object holds a fallback value that shows what should be presented on screen when the user input resulted in an invalid modelValue.

Serializers and deserializers

A serializer converts the modelValue to a serializedValue. In this example, we decide we want to store the user input to our hypothetical database, but by parsing it with radix 8 first.

A deserializer converts a value, for example one received from an API, to a modelValue. This can be useful for prefilling forms with data from APIs.

There is no .deserializedValue property that stays in sync by default. You need to call el.deserializer(el.modelValue) manually yourself.


A preprocessor converts the user input immediately on input. This makes it useful for preventing invalid input or doing other processing tasks before the viewValue hits the parser.

In the example below, we do not allow you to write digits.

Flow Diagrams

Below we show three flow diagrams to show the flow of formatting, serializing and parsing user input, with the example of a date input:

Standard flow

Where a user changes the input with their keyboard:

Standard flow

Unparseable flow

Where a user sets the input to something that is not parseable by the parser:

Unparseable flow

Imperative / programmatic flow

Where the developer sets the modelValue of the input programmatically:

Imperative flow