StructuredRequestBody type definition seemingly has incomplete return values

When using @types/k6, one of the types accepts as a body is StructuredRequestBody. This type is currently defined the following way:

export interface StructuredRequestBody {
    [name: string]: string | FileData;

It seems that number and boolean return values are not allowed when using the index signature. However when I run the following code:

const body = {
  test: 1
const loginResponse ='http://localhost', body as any);

I get test=1 as output from the console.log. The current type definition requires me to toString() the number value for seemingly no reason other than semantics.

Is there any reason why the type definition for StructuredRequestBody is defined this way, when it seems that k6 can serialize numbers just fine? Having to toString() all my data is quite cumbersome.

Hey @octagon,
welcome to the community forum :tada:

as you can read from the Response’s doc, Response.request.body is not the same type as the Request.body. Response.request.body is a string and it contains the encoded body of the request so if you pass an object to it will be the www-form-urlencoded version.

As the example in the doc mentions:

// Using an object as body, the headers will automatically include 
// 'Content-Type: application/x-www-form-urlencoded'.
res =, data);
console.log(res.json(); // Bert

However, it isn’t clear to me where you’re using toString. Can you post an example of how you’re using it and what is the unexpected returned value, please?

Hey @codebien

What I’ve written about Response.request.body was just to demonstrate that k6 seems to handle types other than string just fine when posting content as x-www-form-urlencoded. It is not of much significance to my problem.

My current gripe is that the following code is invalid:

const body = {
    foo: 1
}, body); // TypeError, since foo is an invalid type (expected string | FileData)

while the following example works:

const body = {
    foo: '1'
}, body); // No type errors

Both produce the same result when I ignore the TS type error and read the content from Response.request.body, that’s why I was wondering why the StructuredRequestBody type is defined the way it is. Right now I will have to convert foo to a string before I can pass it to