Dart2js numeric types: determining if a value is an int or a double

Question

I'm trying to determine if a `dynamic` parameter to a function is really an `int` or a `double` and I'm finding surprising behavior (at least to me). Can anyone explain this output (produced on dartpad)? ``` foo(value) { print("$value is int: ${value is int}"); print("$value is double: ${value is double}"); print("$value runtimetype: ${value.runtimeType}"); } void main() { foo(1); foo(2.0); int x = 10; foo(x); double y = 3.1459; foo(y); double z = 2.0; foo(z); } ``` The output: ``` 1 is int: true 1 is double: true 1 runtimetype: int 2 is int: true 2 is double: true 2 runtimetype: int 10 is int: true 10 is double: true 10 runtimetype: int 3.1459 is int: false 3.1459 is double: true 3.1459 runtimetype: double 2 is int: true 2 is double: true 2 runtimetype: int ```

Accepted Answer

In the browser there is no distinction possible between `int` and `double`. JavaScript doesn't provide any such distinction and introducing a custom type for this purpose would have a strong impact on performance, which is why this wasn't done.

Therefore for web applications it's usually better to stick with `num`.

You can check if a value is integer by using for example:

```
var val = 1.0;
print(val is int);
```

prints `true`

This only indicates that the fraction part is or is not `0`.

In the browser there is no type information attached to the value, so `is int` and `is double` seem to just check to see if there is a fractional component to the number and decide based on that alone.

Dart2js numeric types: determining if a value is an int or a double

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