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tform

Purpose

Written in TypeScript and short for record transformer, Tform applies a given set of rules to transform JSON-like structured data into a different shape. Rules take the form of easy-to-read-and-write functional expressions that can be serialized and applied safely at runtime.

Transformation state does not persist between records. Tform works well as a pre-processor for data from external sources, especially for record canonicalization.

Design Philosophy

• Easy-to-read-and-write rules. Rules should be concise and comprehensible, providing an overview of the transformation at glance
• Extensible. Third-party code can customize the transformation behavior easily and flexibly.
• Exception handling. Tform handles errors gracefully, collecting errors to report later.
• Serializable. Rules are simple enough to be serialized and thrown into a database. Tform provides de-serialization functionality.
• Type-checking. Tform allows your rules to be type-checked at compile time.
• Statelessness. State should be minimized, and where needed, made explicit.

Documentation

Overview of API

1. Define an interface for your JSON input

2. Define rules for transformation (or de-serialize rules as described later on)

3. Create a Tform instance

4. Use the instance to transform records

    interface IRawRecord {
      ...
    }
   
    interface IPerson {
      ...
    }
   
    const record: IRawRecord = ...;
   
    const rules: IRules<IRawRecord, IPerson> = { ... };
   
    const tform = new Tform(rules);
    const output = tform.transform(record);
   

Rules Syntax

To transform records, you define some rules as map/dictionary. Each rule is a function that takes in a special object X. You can then access some property foo of your input JSON using the syntax X.foo(). Tform is written to with type-checking in mind: If you defined foo as a string in your interface, X.foo() will type-check as a string!

If the input record does not define property foo, tform logs an error (see exception handling below). Optionally, you can provide a default value as so: X.foo('default_value').

Rules do not have to be flat; they can be nested.

Extending Tform

Ideally, rules are no more than a line (or two) long; this way, you can glance at some rules and grasp the shape of the transformation. If you have a complicated transformation, consider extracting functionality into helper functions.

Tform comes with a core set of utility functions:

• splitList()
• wrapList()

You are encouraged to also create your own library of small, reusable functions specific to your code base to help with rule transformation.

Full Example

interface IPerson {
  job: string;
  name: {
    first: string;
    last: string;
  };
  age: number;
  hobbies: string[];
  address: {
    city: string;
    zip: string;
  };
}

interface IRecord {
  job: string;
  name: string;
  hobbies: string;
  address: {
    city: string;
    zip: string | null;
  };
}

const record: IRecord = {
  job: 'Engineer ',
  name: 'John Doe',
  hobbies: 'Biking; Skating;;',
  address: {
    city: 'Cupertino',
    zip: null,
  },
};

const rules: IRules<IRecord, IPerson> = {
  job: (X) => X.job(''),
  name: {
    first: (X) => X.name('').split(' ')[0],
    last: (X) => X.name('').split(' ')[1],
  },
  age: (X) => X.age(-1),
  hobbies: (X) => splitList(';', X.hobbies('')),
  address: {
    city: (X) => X.address.city(''),
    zip: (X) => X.address.zip(''),
  },
};

With the above interface and rules, the input record is transformed into:

{
  job: 'Engineer',
  name: {
    first: 'John',
    last: 'Doe',
  },
  age: -1,
  hobbies: ['Biking', 'Skating'],
  address: {
    city: 'Cupertino',
    zip: null,
  },
};

Exception Handling

On encountering an error, Tform catches and collects the error. You can later access all the errors collected so far using tform.getErrors(), which returns a list of TformErrors. The errors can be cleared using tform.clearErrors().

By default, Tform reports which record caused the error as well as which field (if any). Optionally, you can have the TformError identify the associated record by an ID property of the record, which you supply as the idKey argument to the Tform constructor.

License

tform is MIT licensed.