Javascript array format

Your current implementation, is essentially updating the same b objects over and over, x number of times, changing their IDs each time through, ending at 3.

You need to "clone" the objects, so they are separate objects, like so:

  let p = []; // Permutations
  
  let a = [{id:1},{id:2}]; // Renditions...
  // Presets...
  let b = [{count:35, name:'test', age:12}, {count:45, name:'test2', age:9}];
  
  // for each Rendition
  a.forEach(function(a) {
     // for each Preset
     b.forEach(function(b){
         // generate Permutation and Clone, object of b with additional id prop
         p.push(Object.assign({id: a.id}, b)); // <-- Cloned here...
     });
  });

   console.log(p)

for the sake of clarity, you might consider changing the id prop to grouping or group or group_id.

You could use flatMap:

let a=[{id:1},{id:2},{id:3}];
let b=[{count:35, name:'test', age:12}, {count:45, name:'test2', age:9}];

let m = b.flatMap(itemB => a.map(itemA => ({...itemB, ...itemA })))

console.log(m);

I got your problem. The thing is that you are mutating the same b object every iteration, that's why in the end you get id = 3 for every element. You can use a deep copy of b in each cycle, and the error should be gone.

Here's one way to deep clone, not the most efficient but it is very clear:

        const bCopy = JSON.parse(JSON.stringify(b[index]));

        for (const [key, value] of Object.entries(a[j])) {
             //if(j==index)
               bCopy[key]=value;
               k[j]=bCopy;   // what's this for?

        }
        console.log(bCopy);
        c.push(bCopy);

Reduce b and for each item b1, reduce a. When reducing a, just assign both b1 and a1 to a new object.

const expectedValue = JSON.stringify(JSON.parse(
  document.getElementById('expected').value))

let a = [
  { id: 1 },
  { id: 2 },
  { id: 3 }
]

let b = [
  { count: 35, name: 'test',  age: 12 },
  { count: 45, name: 'test2', age: 9  }
]

let c = b.reduce((res, b1) =>
  a.reduce((res1, a1) =>
    [...res1, Object.assign({}, b1, a1)], res), [])

console.log(JSON.stringify(c) === expectedValue)

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

#expected { display: none; }

<textarea id="expected">
[
  { "count": 35, "name": "test",  "age": 12, "id": 1 },
  { "count": 35, "name": "test",  "age": 12, "id": 2 },
  { "count": 35, "name": "test",  "age": 12, "id": 3 },
  { "count": 45, "name": "test2", "age": 9,  "id": 1 },
  { "count": 45, "name": "test2", "age": 9,  "id": 2 },
  { "count": 45, "name": "test2", "age": 9,  "id": 3 }
]
</textarea>

Try this changes.

    let a=[{id:1},{id:2},{id:3}];
    let b=[{count:35, name:'test', age:12}, {count:45, name:'test2', age:9}];
    let c=[];
    for ( var index=0; index<b.length; index++ ) {
        for ( var j=0; j<a.length; j++ ) {
         var obj=b[index];
            for (const [key, value] of Object.entries(a[j])) {
                   obj[key]=value;
            }
            c.push(obj);
        }
    }
    console.log(c);