javascript convert for loop filter on index to functional programming

Question

I have got the following code that returns an object based on a filter i want to convert this to functional programming using map, filter.

var records_object =  {
    "record": [
      "analog",
      "laser",
      "monochrome",
      "digital"
    ],
    "vcd": [
      12,
      3,
      6,
      0
    ],
    "dvd": [
      1,
      0,
      0,
      16
    ]
  }
  
  var arr_idx = [];
for (i = 0; i < records_object.record.length; i++) {
  if (records_object.record[i].match(/digital/i) != null||
  records_object.record[i].match(/analog/i) != null) {
    arr_idx.push(i);
  }
}

for (el in records_object) {
  records_object[el] = records_object[el].filter(function (x, i) {
    return arr_idx.indexOf(i) != -1;
  });
}

console.log(records_object);

so far i was able to do this , now i am stuck

const getIndex = (data) => {
  return data.record
.map((e, i) =>
  e.includes("analog") || e.includes("digital") ? i : undefined
)
.filter((x) => x !== undefined);
};

You are doing a lot in single line of code. To find the cause of your problem you should go step by step. First create a normal function and put each part on a separate line using variables. Then you can console.log the output of each line, to find out where your mistake is. — vrijdenker
– vrijdenker, Commented Nov 4, 2020 at 6:22
@varun, still trying to figure out the reduce function, any ideas? — Samtech
– Samtech, Commented Nov 4, 2020 at 6:40

Md Sabbir Alam · Accepted Answer · 2020-11-04 06:48:18Z

1

You can do this,

var records_object =  {
    "record": [
      "analog",
      "laser",
      "monochrome",
      "digital"
    ],
    "vcd": [
      12,
      3,
      6,
      0
    ],
    "dvd": [
      1,
      0,
      0,
      16
    ]
  }
  
  let arrayIndexes = records_object.record.map((item, index) => {
       if(item.match(/digital/i) != null || item.match(/analog/i) !== null) {
          return index;
       }
  }).filter(item => item !== undefined);

  let newObject = Object.keys(records_object).reduce((prev, key) => {
       prev[key] = records_object[key].filter((item, index) => arrayIndexes.includes(index));
       return prev;
  }, {});
console.log(newObject);

edited Nov 4, 2020 at 6:48

answered Nov 4, 2020 at 6:27

Md Sabbir Alam

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3 Comments

Samtech Over a year ago

thank you @sabbir.alam, i am looking for a more functional approach maybe using reduce?

Md Sabbir Alam Over a year ago

I don't know how reduce will make it more functional than it is with map. Anyway I have converted my code to use reduce method.

Samtech Over a year ago

Thank you for @sabbir.alam, actually i was hoping both map function and reduce is clarified in this example. appreciated much

Ketan Ramteke · Accepted Answer · 2020-11-04 06:32:23Z

1

The problem was with filter.

when you are running map it returns array [0, undefined, undefined, 3] and that array is being filtered and as you are using filter(x => x), this will iterate through the returned array and remove all the falsy values and return the resulting array.

In [0, undefined, undefined, 3]'s case, only 3 is the truthy value and that's why you are getting only [3] as 0 too is falsy.

You can modify your code slightly to get this resolved.

var records_object = {
  record: ["analog", "laser", "monochrome", "digital"],
  vcd: [12, 3, 6, 0],
  dvd: [1, 0, 0, 16],
};

const getIndex = (data) => {
  return data.record
.map((e, i) =>
  e.includes("analog") || e.includes("digital") ? i : undefined
)
.filter((x) => x !== undefined);
};

console.log(getIndex(records_object));

edited Nov 4, 2020 at 6:32

answered Nov 4, 2020 at 6:21

Ketan Ramteke

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2 Comments

Samtech Over a year ago

this really helped me understand the filter and map function

Ketan Ramteke Over a year ago

Glad it was helpful, happy coding.

Varun Arya · Accepted Answer · 2020-11-04 07:45:16Z

1

Here is the solution using reduce and filter function. I've saved the result in new object.

var records_object = {
  "record": [
    "analog",
    "laser",
    "monochrome",
    "digital"
  ],
  "vcd": [
    12,
    3,
    6,
    0
  ],
  "dvd": [
    1,
    0,
    0,
    16
  ]
};

const matchByString = ['analog', 'digital'];

const isMatch = (el, stringElements) => stringElements.some((strEl) => el.match(new RegExp(strEl, 'i')) != null);

const filterByIndex = records_object.record.reduce((acc, el, index) => isMatch(el, matchByString) ? [...acc, index] : acc, [])

const result = {};
Object.keys(records_object).forEach(i => result[i] = records_object[i].filter((el, i) => filterByIndex.includes(i)));

console.log(result)

edited Nov 4, 2020 at 7:45

answered Nov 4, 2020 at 6:45

Varun Arya

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2 Comments

Samtech Over a year ago

how can i change the includes to match in your filterByIndex function if i were to look for part of a string that matches the array?

Varun Arya Over a year ago

We have to use regex and iterate the array of string, I'll update that in my answer

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