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List.cast() is not a native Array at browser env. with js-interop (same error between DDC and Dart2JS) #35622

jumperchen opened this Issue Jan 10, 2019 · 3 comments


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jumperchen commented Jan 10, 2019

Dart version: 2.1.0

We use a their-party library to interop with JS and they declare a List<String> as setValue() parameter, so in Dart 2, we need to pass a List<dynamic> to List<String> by using list.cast().

For example,

// this is foo.dart
library foo;

import 'package:js/js.dart';

class Foo {
  external factory Foo();
  external void setValue(List<String> data); // this library declares List<String> here
// this is index.html
<!DOCTYPE html>
<html lang="en">
    <meta charset="UTF-8">
    <script defer src="main.dart.js" ></script>
    <script> = function() {
          this.setValue = function (v) {
              console.log(v); // print "CastList<dynamic, String>" here
              for (var i = 0; i < v.length; i++) {
                  console.log(v[i]);  // <- print "undefined" here


As you can see above, in the JS implementation, it will print "undefined" there.

// this is main.dart
import 'foo.dart';

main() {
  var list = [];
  Foo().setValue(list.cast()); // we need to cast here, otherwise, Dart will throw runtime error or compile time error.

If the list = []; without .cast(), it will be a native Array at browser env, so is the inconsistent behavior expected?


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jumperchen commented Jan 11, 2019

We are trying to avoid this issue from our side, but it seems only to copy whole List to a new List and passes the new one to a js-interop method to solve the JS array issue.
Because we cannot guarantee the list which passed from outside of the method whether has been casted or not.
Any suggestion?


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sigmundch commented Jan 11, 2019

@jumperchen thanks for reaching out.

There is a lot of work happening these days to improve js-interop, @jmesserly can hopefully provide more details.

As you noted, some operations in Dart that help provide appropriate strong types don't play well with JS-interop.

In this case, there are a few options you can use:

  • create the list with the right type initially:
var list = <String>[]; // now you have a List<String> that is represented as a JsArray internally
  • create a copy of the list, instead of using .cast:
setValue(new List<String>.from(list));
  • use a weaker type in your JS-interop interface
void setValue(List<Object> ...);

@jmesserly - do we have plans to support non-native List types (like CastList) or plans to provide hints to guide developers on this?


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jumperchen commented Jan 18, 2019

@sigmundch Thanks for the suggestion, we will use new List<String>.from() for the moment, and wait for @jmesserly's response.

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