@json2csv/transforms
v7.0.6
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json2csv built-in transforms. A transform is a function that receives a data recod and returns a transformed record. Transforms are executed in order before converting the data record into a CSV row.
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@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.
json2csv ecosystem
There are multiple flavours of json2csv where you can use transforms:
- Plainjs: Includes the
Parser
API and a newStreamParser
API which doesn't the conversion in a streaming fashion in pure js. - Node: Includes the
Node Transform
andNode Async Parser
APIs for Node users. - WHATWG: Includes the
WHATWG Transform Stream
andWHATWG 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.
import { unwind, flatten } from '@json2csv/transforms';
Unwind
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[]> 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 tofalse
.
Examples
Simple unwind
Programmatic APIs
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.
$ json2csv -i data.json --unwind "color"
Flatten
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 totrue
.arrays
<Boolean> Flag indicating whether to flatten Arrays or not. Defaults tofalse
.separator
<String> Separator to use between the keys of the nested JSON properties being flattened. Defaults to.
.
// 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.
function doNothing(item) {
// apply tranformations or create new object
return transformedItem;
}
or using ES6
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).
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.
Programmatic APIs
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);
}
CLI
At the moment, only built-in transforms are supported by the CLI interface.
$ json2csv -i input.json \
--unwind "fieldToUnwind","fieldToUnwind.subfieldToUnwind" \
--unwind-blank \
--flatten-objects \
--flatten-arrays \
--flatten-separator "_"
Complete Documentation
See https://juanjodiaz.github.io/json2csv/#/advanced-options/transforms.
License
See LICENSE.md.