Convert Json.Number into int/int64/float64 in golang

Question

I have a variable data, which is an interface. When I print its type I get it as json.Number. How do I type cast to int/int64/float64

If I try data.(float64), it ends up with panic error

panic: interface conversion: interface {} is json.Number, not float64

Solution

Check this documentation to know the available methods on json.Number: https://golang.org/pkg/encoding/json/#Number

f, err := data.(json.Number).Float64()

Comments

[mkopriva]

data.(json.Number).Int64() You type assert to the actual underlying type, not to an arbitrary type you wish it was. That means that if an interface's underlying type is json.Number you type assert it as json.Number. And also v.(T) in Go is not conversion but type assertion.

[mkopriva]

@horsehair because Go doesn't know the underlying type at compile type and type assertion is a test that helps you figure out the underlying type of an interface value at runtime. This is useful if you need to use the "features" of the underlying type as opposed to the "features" of the interface type.

[mkopriva]

@horsehair panic's are runtime errors, not compile time errors.

[mkopriva]

@horsehair type information is stored in Go's runtime system. Also while json.Number's underlying type may be string the two types are not the same and so type-asserting one to the other is bound to fail, however converting one to the other is ok.

[0x574F4F54]

As a newcomer to go, an example like this is exactly what I needed. While the documentation referenced is accurate, it lacks a straight-forward demonstration of how to use the functionality. Thanks!