@json2csv/transforms


A transform is a function to preprocess data before it is converted into CSV by json2csv (in any of its flavours).
Each transform receives each data record, performs some processing and returns a transformed record.

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There are multiple flavours of json2csv where you can use transforms:

* Plainjs: Includes the Parser API and a new StreamParser API which doesn't the conversion in a streaming fashion in pure js.
* Node: Includes the Node Transform and Node Async Parser APIs for Node users.
* WHATWG: Includes the WHATWG Transform Stream and WHATWG Async Parser APIs for users of WHATWG streams (browser, Node or Deno).
* CLI: Includes the CLI interface.

Built-in transforms

There is a number of built-in transform provided by this package.

``js
import { unwind, flatten } from '@json2csv/transforms';
`

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The unwind transform deconstructs an array field from the input item to output a row for each element. It's similar to MongoDB's \$unwind aggregation.

The transform needs to be instantiated and takes an options object as arguments containing:

* paths [<String[]>](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array) List of the paths to the fields to be unwound. It's mandatory and should not be empty.
* blankOut <Boolean> Flag indicating whether to unwind using blank values instead of repeating data or not. Defaults to false.


#### Examples

##### Simple unwind

###### Programmatic APIs

`js
import { Parser } from '@json2csv/plainjs';
import { unwind } from '@json2csv/transforms';

const data = [
{ "carModel": "Audi", "price": 0, "colors": ["blue","green","yellow"] },
{ "carModel": "BMW", "price": 15000, "colors": ["red","blue"] },
{ "carModel": "Mercedes", "price": 20000, "colors": "yellow" },
{ "carModel": "Porsche", "price": 30000, "colors": ["green","teal","aqua"] },
{ "carModel": "Tesla", "price": 50000, "colors": []}
];

try {
const opts = {
transforms: [
unwind({ paths: ['colors'] })
]
};
const parser = new Parser(opts);
const csv = parser.parse(data);
console.log(csv);
} catch (err) {
console.error(err);
}
`

###### CLI
At the moment, only built-in transforms are supported by the CLI interface.

`bash
$ json2csv -i data.json --unwind "color"
`

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Flatten nested JavaScript objects into a single level object.

The transform needs to be instantiated and takes an options object as arguments containing:

* objects <Boolean> Flag indicating whether to flatten JSON objects or not. Defaults to true.
* arrays<Boolean> Flag indicating whether to flatten Arrays or not. Defaults to false.
* separator <String> Separator to use between the keys of the nested JSON properties being flattened. Defaults to ..

`js
// Default
flatten();

// Custom separator '__'
flatten({ separator: '_' });

// Flatten only arrays
flatten({ objects: false, arrays: true });
`

Custom transforms

Users can create their own transforms as simple functions.

`js
function doNothing(item) {
// apply tranformations or create new object
return transformedItem;
}
`

or using ES6

`js
const doNothing = (item) => {
// apply tranformations or create new object
return transformedItem;
};
`

For example, let's add a line counter to our CSV, capitalize the car field and change the price to be in Ks (1000s).

`js
function addCounter() {
let counter = 1;
return (item) => ({
counter: counter++,
...item,
car: item.car.toUpperCase(),
price: item.price / 1000,
});
}
`

The reason to wrap the actual transform in a factory function is so the counter always starts from one and you can reuse it. But it's not strictly necessary.

How to use transforms

Transforms are added to the transforms option when creating a parser.
They are applied in the order in which they are declared.

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`js
import { Parser } from '@json2csv/plainjs';
import { unwind, flatten } from '@json2csv/transforms';
import { addCounter } from './custom-transforms';

try {
const opts = {
transforms: [
unwind({ paths: ['fieldToUnwind','fieldToUnwind.subfieldToUnwind'], blankOut: true }),
flatten({ object: true, array: true, separator: '_'}),
addCounter()
]
};
const parser = new Parser(opts);
const csv = parser.parse(myData);
console.log(csv);
} catch (err) {
console.error(err);
}
`

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`bash
$ json2csv -i input.json \
--unwind "fieldToUnwind","fieldToUnwind.subfieldToUnwind" \
--unwind-blank \
--flatten-objects \
--flatten-arrays \
--flatten-separator "_"
``

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See https://juanjodiaz.github.io/json2csv/#/advanced-options/transforms.

License

See LICENSE.md.