Polymer Expressions are an expressive syntax that can be used in templates in Dart. Polymer Expressions allow you to write complex binding expressions, with property access, function invocation, list/map indexing, and two-way filtering like:

    {{ person.title + " " + person.getFullName() | upppercase }}


Model-Driven Views (MDV)

MDV allows you to define templates directly in HTML that are rendered by the browser into the DOM. Templates are bound to a data model, and changes to the data are automatically reflected in the DOM, and changes in HTML inputs are assigned back into the model. The template and model are bound together via binding expressions that are evaluated against the model. These binding expressions are placed in double-curly-braces, or “mustaches”.


      <p>Hello {{ person.name }}</p>

MDV includes a very basic binding syntax which only allows a series of dot-separate property names.

Custom Binding Syntaxes with BindingDelegate

While MDV's built-in syntax is very basic, it does allow custom syntaxes called “binding delegates” to be installed and used. A binding delegate can interpret the contents of mustaches however it likes. PolymerExpressions is such a binding delegate.


    <template bind>
      <p>Hello {{ person.title + " " + person.getFullName() | uppercase }}</p>


Installing from Pub

Add the following to your pubspec.yaml file:

      polymer_expressions: any

Hint: check https://pub.dartlang.org/packages/polymer_expressions for the latest version number.

Then import polymer_expressions.dart:

import 'package:polymer_expressions/polymer_expressions.dart';

Registering a Binding Delegate

Polymer Expressions are now the default syntax for <polymer-element> custom elements.

You do not need to manually register the bindingDelegate if your bindings are inside a custom element. However, if you want to use polymer_expressions outside a custom element, read on:

Binding delegates must be installed on a template before they can be used. For example, set the bindingDelegate property of your template elements to an instance of PolymerExpressions. The templates will then use the PolymerExpressions instance to interpret binding expressions.

    import 'dart:html';
    import 'package:polymer_expressions/polymer_expressions.dart';

    main() {
      var template = query('#my_template');
      template.bindingDelegate = new PolymerExpressions();

Registering Top-Level Variables

Before a top-level variable can be used, it must be registered. The PolymerExpressions constructor takes a map of named values to use as variables.

    main() {
      var globals = {
        'uppercase': (String v) => v.toUpperCase(),
        'app_id': 'my_app_123',
      var template = query('#my_template');
      template.bindingDelegate = new PolymerExpressions(globals: globals);


The Model and Scope

Polymer Expressions allow binding to more than just the model assigned to a template instance. Top-level variables can be defined so that you can use filters, global variables and constants, functions, etc. These variables and the model are held together in a container called a Scope. Scopes can be nested, which happens when template tags are nested.

Two-way Bindings

Bindings can be used to modify the data model based on events in the DOM. The most common case is to bind an <input> element's value field to a model property and have the property update when the input changes. For this to work, the binding expression must be “assignable”. Only a subset of expressions are assignable. Assignable expressions cannot contain function calls, operators, and any index operator must have a literal argument. Assignable expressions can contain filter operators as long as all the filters are two-way transformers.

Some restrictions may be relaxed further as allowed.

Assignable Expressions:

  • foo
  • foo.bar
  • items[0].description
  • people['john'].name
  • product.cost | convertCurrency('ZWD') where convertCurrency evaluates to a Tranformer object.

Non-Assignable Expressions:

  • a + 1
  • !c
  • foo()
  • person.lastName | uppercase where uppercase is a filter function.


Expressions are generally null-safe. If an intermediate expression yields null the entire expression will return null, rather than throwing an exception. Property access, method invocation and operators are null-safe. Passing null to a function that doesn't handle null will not be null safe.


Polymer Expressions have experimental support for binding to streams, and when new values are passed to the stream, the template updates. The feature is not fully implemented yet.

See the examples in /example/streams for more details.


Property Access

Properties on the model and in the scope are looked up via simple property names, like foo. Property names are looked up first in the top-level variables, next in the model, then recursively in parent scopes. Properties on objects can be access with dot notation like foo.bar.

The keyword this always refers to the model if there is one, otherwise this is null. If you have model properties and top-level variables with the same name, you can use this to refer to the model property.


Polymer Expressions support number, boolean, string, and map literals. Strings can use either single or double quotes.

  • Numbers: 1, 1.0
  • Booleans: true, false
  • Strings: 'abc', "xyz"
  • Maps: { 'a': 1, 'b': 2 }

List literals are planned, see issue 9

Functions and Methods

If a property is a function in the scope, a method on the model, or a method on an object, it can be invoked with standard function syntax. Functions and Methods can take arguments. Named arguments are not supported. Arguments can be literals or variables.


  • Top-level function: myFunction()
  • Top-level function with arguments: myFunction(a, b, 42)
  • Model method: aMethod()
  • Method on nested-property: a.b.anotherMethod()


Polymer Expressions supports the following binary and unary operators:

  • Arithmetic operators: +, -, *, /, %, unary + and -
  • Comparison operators: ==, !=, <=, <, >, >=
  • Boolean operators: &&, ||, unary !

Expressions do not support bitwise operators such as &, |, << and >>, or increment/decrement operators (++ and --)

List and Map Indexing

List and Map like objects can be accessed via the index operator: []


  • items[2]
  • people['john']

Unlike JavaScript, list and map contents are not generally available via property access. That is, the previous examples are not equivalent to items.2 and people.john. This ensures that access to properties and methods on Lists and Maps is preserved.

Filters and Transformers

A filter is a function that transforms a value into another, used via the pipe syntax: value | filter Any function that takes exactly one argument can be used as a filter.


If person.name is “John”, and a top-level function named uppercase has been registered, then person.name | uppercase will have the value “JOHN”.

The pipe syntax is used rather than a regular function call so that we can support two-way bindings through transformers. A transformer is a filter that has an inverse function. Transformers must extend or implement the Transformer class, which has forward() and reverse() methods.

Repeating Templates

A template can be repeated by using the “repeat” attribute with a binding. The binding can either evaluate to an Iterable, in which case the template is instantiated for each item in the iterable and the model of the instance is set to the item, or the binding can be a “in” iterator expression, in which case a new variable is added to each scope.

The following examples produce the same output.

Evaluate to an iterable:

    <template repeat="{{ items }}">
      <div>{{ }}</div>

“in” expression:

    <template repeat="{{ item in items }}">
      <div>{{ item }}</div>


The syntax implemented is experimental and subject to change, in fact, it will change soon. The goal is to be compatible with Polymer's binding syntax. We will announce breaking changes on the web-ui@dartlang.org mailing list.

Please file issues on Dart project page for any bugs you find or for feature requests. Make a note that it applies to “package:polymer_expressions”

You can discuss Polymer Expressions on the web-ui@dartlang.org mailing list.