Create new custom visuals from the scratch code

Hi,

I want to create custom Power BI visuals for the report. I follow the different links to create custom visuals. But did't get the code for that. I need to know how to write a code in the visuals.ts file. Can you please provide me some basic Idea, How can I write the code? Or Is there any document fot it How to write a code?

I have some inbuilt code but I am getting the following error

Argument of type '{ value: string | number | boolean | Date; identity: string; }' is not assignable to parameter of type 'CategoryViewModel'.
  Types of property 'value' are incompatible.
    Type 'string | number | boolean | Date' is not assignable to type 'string'.
      Type 'number' is not assignable to type 'string'.

Thanks