Generate an array with values that are a range of multiples of 7 using javascript

Question

I would like the most modern way to make an array like this

const nextchunk = [];
nextchunk[0] = [0, 6];
nextchunk[1] = [7, 13];

Each nextchunk must have 7 places in it as shown. These represent a limit query later in in the code. So nextchunk[1] gets out item 7 - 13, nextchunk[2] gets out 14 - 20

I would like the flexibility of being able to call nextchunk[20] at runtime and have it return the correct values.

Start by using a loop to generate them, then go for [`Array.from({length: 21}, (_, i) => …)` or something similar](https://stackoverflow.com/q/3746725/1048572) — Bergi, Apr 08 '18 at 11:42
Amazing. -3 yet 3 answers from very experienced stackers - that would suggest the opposite. — 32423hjh32423, Apr 08 '18 at 11:57
You know that question downvotes mean "*This post does not show any effort*"? You can still [edit] to improve. — Bergi, Apr 08 '18 at 12:07

Jonas Wilms · Accepted Answer · 2018-04-08T11:50:09.893

5

   function chunks(size) {
     return function getChunk(index) {
       return [size * index, size * (index + 1) - 1];
     }
   }

   const nextchunk = chunks(7);
   console.log(
      nextchunk(0), 
      nextchunk(1), 
      nextchunk(20)
   );

You can easily calculate the value. There is no need to generate it before it is actually needed. If you really need the array, its easy to build it up with the upper helper:

  const nextchunk = Array.from({length: 21}, (_, i) => chunks(7)(i));
  console.log(nextchunk[20]);

edited Apr 08 '18 at 11:50

answered Apr 08 '18 at 11:42

Jonas Wilms

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1

…apart from when you need `nextchunk(0) == nextchunk(0)` – Bergi Apr 08 '18 at 11:45
I guess returned function's name (`get`) is a bit excess as it is not being used anywhere. – Mohammad Usman Apr 08 '18 at 11:48
2

@mohammad it improves readability and helps furing debugging. – Jonas Wilms Apr 08 '18 at 11:49

score 4 · Answer 2 · answered Apr 08 '18 at 11:54

I would like the flexibility of being able to call nextchunk[20] at runtime and have it return the correct values.

Though I don't recommend to use this approach in production (mainly because you won't be able to transpile it to work in elder js engines) but you could achive nextchuck[20] syntax using Proxies.

// just for fun
const nextChunk = new Proxy([], {
  get(target, prop) {
    const maybeIndex = parseInt(prop, 10)
    const has = Reflect.has(target, prop)
    const value = Reflect.get(target, prop)
    
    if (has) { 
      return value
    }
    
    if(!Number.isNaN(maybeIndex)) {
      const newValue = [maybeIndex * 7, 6 + maybeIndex * 7]
      Reflect.set(target, prop, newValue );
      
      return newValue;
    }
    
    return value;
  }
});

console.log(nextChunk[0], nextChunk[1], nextChunk[20])
console.log(nextChunk[0] === nextChunk[0])

@JonasW. Used Proxies for some PoC code this week. :) Just can't stop seeing em everywhere. — Yury Tarabanko, Apr 08 '18 at 12:05

score 2 · Answer 3 · answered Apr 08 '18 at 11:47

You could create custom function that takes array and number and adds new element to array based on your calculation.

const arr = [];
const nextchunk = (array, chunk) => {
  const start = chunk * 6 + chunk;
  array.push([start, start + 6])
}

nextchunk(arr, 0);
nextchunk(arr, 1);
nextchunk(arr, 2);
nextchunk(arr, 20);

console.log(arr)

Generate an array with values that are a range of multiples of 7 using javascript

3 Answers3