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

1npm i deep-reduce

Usage

Try example in browser.

1const deepReduce = require('deep-reduce')
2const deepEqual = require('assert').deepEqual
3
4// let nested leaf values be equal to their path, for demonstration purpose
5let deepNestedObject = {
6  a: 'a',
7  b: { c: 'b.c' },
8  c: [
9    'c.0',
10    {
11      d: 'c.1.d',
12      e: [
13        'c.1.e.0'
14      ]
15    }
16  ]
17}
18
19// store all values which are strings to reduced[path].
20let flattenStrings = (reduced, value, path) => {
21  if (typeof value === 'string') {
22    reduced[path] = value
23  }
24  return reduced
25}
26
27// the reduced value is returned
28let reduced = deepReduce(deepNestedObject, flattenStrings)
29
30// we should now have this object
31deepEqual(reduced, {
32  'a': 'a',
33  'b.c': 'b.c',
34  'c.0': 'c.0',
35  'c.1.d': 'c.1.d',
36  'c.1.e.0': 'c.1.e.0'
37})

Root object may be an array also:

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

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

1// we want to get contents from all packets
2let transport = {
3  id: 'A8811',
4  packages: [
5    {
6      id: 'P100',
7      contents: [
8        {
9          id: 'R88',
10          name: 'resistor'
11        },
12        {
13          id: 'C99',
14          name: 'capacitor'
15        }
16      ]
17    }, {
18      id: 'P101',
19      contents: [
20        {
21          id: 'C96',
22          name: 'coil'
23        }
24      ]
25    }
26  ]
27}
28
29let contents = deepReduce(transport, (reduced, value, path) => {
30  if (path.match(/packages\.\d+\.contents\.\d+$/)) {
31    // path is packages.n.contents.m
32    // item n in packages array
33    // item m in contents array
34    reduced.push(value)
35  }
36  return reduced
37}, [])  // start with an empty array
38
39// [ { id: 'R88', name: 'resistor' },
40//   { id: 'C99', name: 'capacitor' },
41//   { id: 'C96', name: 'coil' } ]

API

deepReduce takes 5 arguments. 2 mandatory and 3 optional:

1deepReduce (
2  obj: object,
3  reducer: (reduced: any, value: any, path: string, root: object) => any,
4  reduced = {},
5  path = '',
6  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:

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

1(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:

1deepReduce(object, reducer, initialValue, 'start.at.this.path')

...which may give some performance gains.

Development

1git clone https://github.com/arve0/deep-reduce
2cd deep-reduce
3npm start  # watches index.ts and builds on any change
4npm test  # runs node test.js

License

MIT © 2017 Arve Seljebu