ndjson-apply
A CLI tool to transform a stream of newline-delimited JSON by applying a JS function to each JSON object.
Features:
- take the JS function to apply from a file
- the function may return async results
- preview the transformation results with the
--diff
option
Summary
Install
npm i -g ndjson-apply
How To
Basic
cat some_data.ndjson | ndjson-apply some_transform_function.js > some_data_transformed.ndjson
# Which can also be written
ndjson-apply some_transform_function.js < cat some_data.ndjson > some_data_transformed.ndjson
where some_transform_function.js
just needs to export a JS function. This should work both with the ESM export syntax
// some_transform_function.js
export default function (doc) {
doc.total = doc.a + doc.b
if (doc.total % 2 === 0) {
return doc
} else {
// returning null or undefined drops the entry
}
}
or with the CommonJS export syntax
// some_transform_function.js
module.exports = function (doc) {
doc.total = doc.a + doc.b
if (doc.total % 2 === 0) {
return doc
} else {
// returning null or undefined drops the entry
}
}
Async
That function can also be async:
import { getSomeExtraData } from './path/to/get_some_extra_data.js'
// some_async_transform_function.js
export default async function (doc) {
doc.total = doc.a + doc.b
if (doc.total % 2 === 0) {
doc.extraData = await getSomeExtraData(doc)
return doc
} else {
// returning null or undefined drops the entry
}
}
Diff mode
As a way to preview the results of your transformation, you can use the diff mode
cat some_data.ndjson | ndjson-apply some_transform_function.js --diff
which will display a colored diff of each line before and after transformation.
For more readability, each line diff output is indented and on several lines.
Filter mode
Use the js function only to filter lines: lines returning true
will be let through. No transformation will be applied.
cat some_data.ndjson | ndjson-apply some_transform_function.js --filter
Use sub-function
Given a function_collection.js
file like:
// function_collection.js
export function foo (obj) {
obj.timestamp = Date.now()
return obj
}
export function bar (obj) {
obj.count += obj.count
return obj
}
You can use those subfunction by passing their key as an additional argument
cat some_data.ndjson | ndjson-apply ./function_collection.js foo
cat some_data.ndjson | ndjson-apply ./function_collection.js bar
This should also work with the CommonJS syntax:
// function_collection.cjs
module.exports = {
foo: (obj) => {
obj.timestamp = Date.now()
return obj
},
bar: (obj) => {
obj.count += obj.count
return obj
}
}
Pass additional arguments
Any remaining argument will be passed to the function
# Pass '123' as argument to the exported function
cat some_data.ndjson | ndjson-apply ./function.js 123
# Pass '123' as argument to the exported sub-function foo
cat some_data.ndjson | ndjson-apply ./function_collection.js foo 123
See also
- jq is great to work with NDJSON:
cat entries_array.json | jq '.[]' -cr > entries.ndjson
- ndjson-cli#map
- json-apply