问题
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
回答1:
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.
来源:https://stackoverflow.com/questions/43949093/dart2js-numeric-types-determining-if-a-value-is-an-int-or-a-double