How to retain objects with a specific unique attribute in an array in javascript?

Question

I have an array like so:

var myarray = [
    {"scaleId":"001","currentWeight":0.200}
    {"scaleId":"002","currentWeight":0.300},
    {"scaleId":"001","currentWeight":0.255},
    {"scaleId":"002","currentWeight":0.000},
    {"scaleId":"003","currentWeight":0.340},
    ]

and I want to retain the objects which have unique scaleId. So, for the above example, the output would like ( any random object can be retained if the scaleId is duplicate):

myarray = [
    {"scaleId":"001","currentWeight":0.200}
    {"scaleId":"002","currentWeight":0.300},
    {"scaleId":"003","currentWeight":0.340},
    ]

I'm already using underscore library in my application, so I'm fine with the implementation using underscore. I had come up with a solution but it is not working as expected. So, any help will be great.

brk · Accepted Answer · 2020-05-04 16:13:53Z

1

Use reduce and inside callback check if in accumulator object there is a key with same scaleId. If so then add the value to that key. Finally use Object.values to create an array of values

var myarray = [{
    "scaleId": "001",
    "currentWeight": 0.200
  },
  {
    "scaleId": "002",
    "currentWeight": 0.300
  },
  {
    "scaleId": "001",
    "currentWeight": 0.255
  },
  {
    "scaleId": "002",
    "currentWeight": 0.000
  },
  {
    "scaleId": "003",
    "currentWeight": 0.340
  }
]

let data = myarray.reduce((acc, curr) => {
  if (!acc[curr.scaleId]) {
    acc[curr.scaleId] = curr
  }
  return acc;
}, {});

console.log(Object.values(data))

answered May 4, 2020 at 16:13

brk

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Comments

Scott Sauyet · Accepted Answer · 2020-05-04 17:45:32Z

1

Maps and Sets are often the appropriate structures to maintain uniqueness. Here is a simple function that uses a Map to maintain a single unique value given a property name:

const uniqByProp = (prop) => (xs) =>
  [... new Map (xs .map ((x) => [x [prop], x])) .values ()]

var myarray = [{scaleId: "001", currentWeight: 0.200}, {scaleId: "002", currentWeight: 0.300}, {scaleId: "001", currentWeight: 0.255}, {scaleId: "002", currentWeight: 0.000}, {scaleId: "003", currentWeight: 0.340}]

console .log (
  uniqByProp ('scaleId') (myarray)
)

.as-console-wrapper {min-height: 100% !important; top: 0}

This version keeps the last matching value. If you want the first matching one, it would be only slightly more complex.

edited May 4, 2020 at 17:45

answered May 4, 2020 at 16:25

Scott Sauyet

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4 Comments

Patrick Roberts Over a year ago

Out of interest, is there a resource I can read about the particular code style you use? I don't see this used often, though I'm aware it's common among those that favor FP.

Scott Sauyet Over a year ago

@PatrickRoberts: I'm not sure what about my style interests you. I am an FP fan, so I guess I'm matching some stereotype! I prefer curried functions. I prefer simple single-purpose but ideally generic functions, but that just seems common sense to me. I'm willing to write fairly wide lines, but I like some unusual whitespace (e.g. xs .map (...) rather than xs.map(...), but that's a minor idiosyncracy more than a consistent style. Or is it something else?

Patrick Roberts Over a year ago

It was the whitespace usage I was noting particularly. There's no standard for that then? I've seen that used quite consistently when FP-oriented people write JavaScript.

Scott Sauyet Over a year ago

@PatrickRoberts. I know of no styleguide that suggests it. I borrowed it and some other formatting ideas from another SO user who has since changed whitespace style. Posts from that user contain a wealth of useful techniques. The formatting is clearly derived from LISP-like language formatting.

YGilk1 · Accepted Answer · 2020-05-04 16:28:48Z

0

You can do it this way (if you are using ES6/ES2015 or later) Using Set to filter unique scaleIds first and feeding a new array with the Array.find method

var myarray = [
    {"scaleId":"001","currentWeight":0.200},
    {"scaleId":"002","currentWeight":0.300},
    {"scaleId":"001","currentWeight":0.255},
    {"scaleId":"002","currentWeight":0.000},
    {"scaleId":"003","currentWeight":0.340},
]

let scaleIds = [...new Set(myarray.map(item => item.scaleId))];
let filtered = []
scaleIds.forEach(scaleId => filtered.push(myarray.find(item => item.scaleId === scaleId)))

console.log(filtered)

answered May 4, 2020 at 16:28

YGilk1

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Comments

Patrick Roberts · Accepted Answer · 2020-05-04 16:38:40Z

You can also use Array.prototype.reduceRight() to simplify aggregation logic:

let myarray = [
  { scaleId: '001', currentWeight: 0.2 },
  { scaleId: '002', currentWeight: 0.3 },
  { scaleId: '001', currentWeight: 0.255 },
  { scaleId: '002', currentWeight: 0 },
  { scaleId: '003', currentWeight: 0.34 }
];

myarray = Object.values(
  myarray.reduceRight(
    (acc, cur) => (acc[cur.scaleId] = cur, acc),
    {}
  )
).reverse(); // optional if order of output matters to you

console.log(myarray);

or construct a Map from key-value pairs generated using Array.prototype.map():

let myarray = [
  { scaleId: '001', currentWeight: 0.2 },
  { scaleId: '002', currentWeight: 0.3 },
  { scaleId: '001', currentWeight: 0.255 },
  { scaleId: '002', currentWeight: 0 },
  { scaleId: '003', currentWeight: 0.34 }
];

myarray = Array.from(
  new Map(
    myarray.map(
      val => [val.scaleId, val]
    ).reverse()
  ).values()
).reverse(); // optional if order of output matters to you

console.log(myarray);

We use Array.prototype.reverse() on the array passed to the constructor to ensure that the insertion order allows earlier values to take precedence over later values.

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