deep-reduce

Reduce objects deeply, call reducer for every nested node in object tree. Use deepReduce for transforming/getting values from awfully nested objects. deepReduce also traverse arrays.

Install

npm i deep-reduce

Usage

Try example in browser.

const deepReduce = require('deep-reduce')
const deepEqual = require('assert').deepEqual
 
// let nested leaf values be equal to their path, for demonstration purpose
let deepNestedObject = {
  a: 'a',
  b: { c: 'b.c' },
  c: [
    'c.0',
    {
      d: 'c.1.d',
      e: [
        'c.1.e.0'
      ]
    }
  ]
}
 
// store all values which are strings to reduced[path].
let flattenStrings = (reduced, value, path) => {
  if (typeof value === 'string') {
    reduced[path] = value
  }
  return reduced
}
 
// the reduced value is returned
let reduced = deepReduce(deepNestedObject, flattenStrings)
 
// we should now have this object
deepEqual(reduced, {
  'a': 'a',
  'b.c': 'b.c',
  'c.0': 'c.0',
  'c.1.d': 'c.1.d',
  'c.1.e.0': 'c.1.e.0'
})

Root object may be an array also:

deepReduce([{a: 1},{b: 2},{a: 3}], (r,v) => typeof v === 'number' ? r + v : r, 0)
// 6

Here is how you would collect all items of nested arrays at some specific path:

// we want to get contents from all packets
let transport = {
  id: 'A8811',
  packages: [
    {
      id: 'P100',
      contents: [
        {
          id: 'R88',
          name: 'resistor'
        },
        {
          id: 'C99',
          name: 'capacitor'
        }
      ]
    }, {
      id: 'P101',
      contents: [
        {
          id: 'C96',
          name: 'coil'
        }
      ]
    }
  ]
}
 
let contents = deepReduce(transport, (reduced, value, path) => {
  if (path.match(/packages\.\d+\.contents\.\d+$/)) {
    // path is packages.n.contents.m
    // item n in packages array
    // item m in contents array
    reduced.push(value)
  }
  return reduced
}, [])  // start with an empty array
 
// [ { id: 'R88', name: 'resistor' },
//   { id: 'C99', name: 'capacitor' },
//   { id: 'C96', name: 'coil' } ]

API

deepReduce takes 5 arguments. 2 mandatory and 3 optional:

deepReduce (
  obj: object,
  reducer: (reduced: any, value: any, path: string, root: object) => any,
  reduced = {},
  path = '',
  thisArg = {}): any

Arguments

• obj Object to traverse.

• reducer Function to call with every value in obj-tree. See section below for reducer function signature.

• reduced Initial value of reduced passed to reducer. Defaults to empty object {}.

• path Path to root, start traversing here. Nice to omit looping through parts of obj.

  Example:

  deepReduce({ a: [1,2,3], b: { c: [3, 3] } }, (reduced, val) => reduced + val, 1, 'b.c')
  // only traverses b.c, returns 1 + 3 + 3 = 7

• thisArg Bound to reducer as this.

Arguments for reducer function

The reducer function is called with these arguments:

(reduced: any, value: any, path: string, root: object) => any

• reduced Initial or current reduced value.
• value Value of current node.
• path Path to current value.
• root Root object passed to deepReduce as obj.

The reducer should return the reduced value.

Performance

deep-reduce traverses every node of a 149 kb JSON in 10 milliseconds on a macbook air 2013 i5 1.3GHz, see test.js.

You can omit traversal of parts of the object tree with defining a start path:

deepReduce(object, reducer, initialValue, 'start.at.this.path')

...which may give some performance gains.

Development

git clone https://github.com/arve0/deep-reduce
cd deep-reduce
npm start  # watches index.ts and builds on any change 
npm test  # runs node test.js 

License

MIT © 2017 Arve Seljebu