1.0 is int not double in dart web.

[CaiJingLong]

1.0 is int not double in dart web.

~ dart --version
Dart VM version: 2.0.0 (Fri Aug 3 10:53:23 2018 +0200) on "macos_x64"

same code

const str = """{"num1":1.0,"num2":1.1}
    """;

  var map = json.decode(str);
  var n1 = map["num1"];
  if (n1 is int) {
    print("is int");
  } else if (n1 is double) {
    print("is double");
  } else {
    print("is other");
  }
  print(map["num1"].runtimeType);
  print(map["num2"].runtimeType);

run in dart vm or flutter then result is

is double
double
double

run in dart web

is int
int
double

What do I do to make 1 recognises double instead of int?

[eernstg]

Numbers in Dart are represented natively when compiling to JavaScript, that is, every number is represented using the IEEE 754 double-precision format. As an optimization, in ways that programmers cannot observe, a JavaScript virtual machine may choose to use a different representation (such as a 64 bit 2's complement format), but "in principle" a JavaScript number is a double, and it is far too expensive to faithfully emulate a genuine int type on top of that. So you won't get anything but doubles on the web at this point. In the future we may get other choices, but we'll see when that happens. ;-)

So right now Dart web numbers use JavaScript Number objects to emulate the desired semantics as closely as possible.

In particular, when testing whether a number has type int, it is tested whether the given double value is a whole number, not whether it "is an integer", and this means that you will get the following:

main() {
  print(1.0 is double); // 'true'
  print(1.0 is int); // 'true'
}

It would be very, very nice to have a more faithful semantics for Dart numbers on the web, but it is not considered realistic because of the performance implications.

Returning to your question, this means that 1 is recognized as an int and 1.0 is recognized as a double, but you also get true if you swap the types.

For some additional details, check #25493.

[CaiJingLong]

thx,get it.
You mean I just need to change the order, decide whether it's double, and then decide whether it's int, to get the truth type.

[eernstg]

With the JavaScript Number semantics it is in fact so that int is a subtype of double, which means that you could test for is int first; if true then you can use it as an int, otherwise test is double; if true then you can use it as a double; otherwise it is not a number at all. With the opposite order you'd never discover that some numbers are ints, which might break your code.

[CaiJingLong]

Actually I found this problem by writing a json2dart web page for the json_serializable library.
I want to get the number type to generate dart code, but I can't. So in the dart web project,I use num instead of int to achieve my goal.