Generated at https://next.json-generator.com/N1Tiw308M

reducer.js

const users = require('./users');

const data = users.reduce((U, _u) => {
  
  const u = Object.assign({}, _u);
  u.friends = u.friends.reduce((F, f) => {
    if (users[f] && F.indexOf(f) < 0) {
      F.push(users[f].id);
    }
    return F;
  }, []);
  U[u.id] = u;
  return U;
}, {});

console.log(JSON.stringify(data, null, 2));

users.json

{
  "5a826fdf27256fa9ba444cd4": {
    "id": "5a826fdf27256fa9ba444cd4",
    "email": "nancy.palmer@example.com",
    "fullName": "Nancy Palmer",
    "name": {
      "last": "Palmer",
      "first": "Nancy"
    },
    "picture": "http://placehold.it/32x32",
    "friends": [
      "5a826fdf50f698c2d9c492ca",
      "5a826fdf40b7d805ca2d6e75",
      "5a826fdf066dca8b5f7d127a",
      "5a826fdf27256fa9ba444cd4",
      "5a826fdf6f0122089221a930",
      "5a826fdf50f698c2d9c492ca",
      "5a826fdf6f0122089221a930",
      "5a826fdf40b7d805ca2d6e75"
    ]
  },
  "5a826fdfc3cdf1690c71e00f": {
    "id": "5a826fdfc3cdf1690c71e00f",
    "email": "rebekah.hernandez@example.com",
    "fullName": "Rebekah Hernandez",
    "name": {
      "last": "Hernandez",
      "first": "Rebekah"
    },
    "picture": "http://placehold.it/32x32",
    "friends": [
      "5a826fdf40b7d805ca2d6e75",
      "5a826fdf40b7d805ca2d6e75",
      "5a826fdf27256fa9ba444cd4",
      "5a826fdf6f0122089221a930",
      "5a826fdf2edf272683abe3ca",
      "5a826fdf50f698c2d9c492ca",
      "5a826fdf6f0122089221a930",
      "5a826fdf6f0122089221a930",
      "5a826fdf066dca8b5f7d127a"
    ]
  },
  "5a826fdf9856ad6503d728d5": {
    "id": "5a826fdf9856ad6503d728d5",
    "email": "stanton.hansen@example.com",
    "fullName": "Stanton Hansen",
    "name": {
      "last": "Hansen",
      "first": "Stanton"
    },
    "picture": "http://placehold.it/32x32",
    "friends": [
      "5a826fdf0d054a6fab7bb067",
      "5a826fdf50f698c2d9c492ca",
      "5a826fdf0d054a6fab7bb067"
    ]
  },
  "5a826fdf0d054a6fab7bb067": {
    "id": "5a826fdf0d054a6fab7bb067",
    "email": "barr.wolf@example.com",
    "fullName": "Barr Wolf",
    "name": {
      "last": "Wolf",
      "first": "Barr"
    },
    "picture": "http://placehold.it/32x32",
    "friends": [
      "5a826fdf40b7d805ca2d6e75",
      "5a826fdf40b7d805ca2d6e75"
    ]
  },
  "5a826fdf50f698c2d9c492ca": {
    "id": "5a826fdf50f698c2d9c492ca",
    "email": "hayden.smith@example.com",
    "fullName": "Hayden Smith",
    "name": {
      "last": "Smith",
      "first": "Hayden"
    },
    "picture": "http://placehold.it/32x32",
    "friends": [
      "5a826fdf50f698c2d9c492ca",
      "5a826fdfc3cdf1690c71e00f",
      "5a826fdf2edf272683abe3ca",
      "5a826fdf066dca8b5f7d127a",
      "5a826fdf6f0122089221a930"
    ]
  },
  "5a826fdf066dca8b5f7d127a": {
    "id": "5a826fdf066dca8b5f7d127a",
    "email": "russell.sims@example.com",
    "fullName": "Russell Sims",
    "name": {
      "last": "Sims",
      "first": "Russell"
    },
    "picture": "http://placehold.it/32x32",
    "friends": [
      "5a826fdf9856ad6503d728d5",
      "5a826fdf2edf272683abe3ca",
      "5a826fdf066dca8b5f7d127a",
      "5a826fdf066dca8b5f7d127a",
      "5a826fdfc3cdf1690c71e00f",
      "5a826fdf50f698c2d9c492ca"
    ]
  },
  "5a826fdfa4c1ca0d27881218": {
    "id": "5a826fdfa4c1ca0d27881218",
    "email": "hazel.rodriguez@example.com",
    "fullName": "Hazel Rodriguez",
    "name": {
      "last": "Rodriguez",
      "first": "Hazel"
    },
    "picture": "http://placehold.it/32x32",
    "friends": [
      "5a826fdf27256fa9ba444cd4",
      "5a826fdf066dca8b5f7d127a",
      "5a826fdf9856ad6503d728d5",
      "5a826fdfa4c1ca0d27881218",
      "5a826fdfc3cdf1690c71e00f",
      "5a826fdf27256fa9ba444cd4",
      "5a826fdf0d054a6fab7bb067",
      "5a826fdf50f698c2d9c492ca"
    ]
  },
  "5a826fdf40b7d805ca2d6e75": {
    "id": "5a826fdf40b7d805ca2d6e75",
    "email": "kent.clemons@example.com",
    "fullName": "Kent Clemons",
    "name": {
      "last": "Clemons",
      "first": "Kent"
    },
    "picture": "http://placehold.it/32x32",
    "friends": [
      "5a826fdfc3cdf1690c71e00f",
      "5a826fdf27256fa9ba444cd4",
      "5a826fdf6f0122089221a930",
      "5a826fdf0d054a6fab7bb067",
      "5a826fdf066dca8b5f7d127a"
    ]
  },
  "5a826fdf2edf272683abe3ca": {
    "id": "5a826fdf2edf272683abe3ca",
    "email": "hughes.mcguire@example.com",
    "fullName": "Hughes Mcguire",
    "name": {
      "last": "Mcguire",
      "first": "Hughes"
    },
    "picture": "http://placehold.it/32x32",
    "friends": [
      "5a826fdfc3cdf1690c71e00f"
    ]
  },
  "5a826fdf6f0122089221a930": {
    "id": "5a826fdf6f0122089221a930",
    "email": "maxine.bird@example.com",
    "fullName": "Maxine Bird",
    "name": {
      "last": "Bird",
      "first": "Maxine"
    },
    "picture": "http://placehold.it/32x32",
    "friends": [
      "5a826fdf27256fa9ba444cd4",
      "5a826fdf2edf272683abe3ca"
    ]
  }
}

node reducer.js > _users.json