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Applying functions to parts of data in a promise chain using JSONPath

published Aug 09, 2018 03:50   by admin ( last modified Aug 13, 2018 03:17 )

When working with promise chains, you sometimes pipe through a bit more complex data than just a value, and would like to apply a pipeline stage to just subparts of that complex data. I used to have a function that did just that in python, but now it is time for javascript!

Here is one way of doing that in javascript that I just came up with. First what it looks like in practice, here we're are calling an uppercase transformation on just part of the data structure being passed through:

.then(forPaths('$', uppity)).then(

The above uppercases the value of any property named "author". With JSON path syntax we could have selected other parts too.

We are using a library called JSONPath to specify what parts of the data structure we would like to apply the function too. Here is a complete working example, using an example data structure from JSONPath's documentation:

const jp = require('jsonpath')

// Here is the general function for applying anything to any
// part of a data structure going through a promise chain.
// It is curried, so the data comes first when the pipeline is executed:

const forPaths = (pathExpr, fun) => data => {
  jp.apply(data, pathExpr, fun)
  return data

// some test data, taken from the docs for JSONPath:
var data = {
  "store": {
    "book": [
        "category": "reference",
        "author": "Nigel Rees",
        "title": "Sayings of the Century",
        "price": 8.95
      }, {
        "category": "fiction",
        "author": "Evelyn Waugh",
        "title": "Sword of Honour",
        "price": 12.99
      }, {
        "category": "fiction",
        "author": "Herman Melville",
        "title": "Moby Dick",
        "isbn": "0-553-21311-3",
        "price": 8.99
      }, {
         "category": "fiction",
        "author": "J. R. R. Tolkien",
        "title": "The Lord of the Rings",
        "isbn": "0-395-19395-8",
        "price": 22.99
    "bicycle": {
      "color": "red",
      "price": 19.95

// An example function for doing some kind of transformation, in this case convert to uppercase
const uppity = text=>text.toUpperCase()

// Finally, an example promise chain:
Promise.resolve(data).then(forPaths('$', uppity)).then(JSON.stringify).then(console.log)