Cartesian product using reduce, map and spread is grouping the result which need to be flattened

Question

class MathSet extends Set{

  constructor(arr){

     super(arr);

     }

  union(set){

     return new MathSet([...this, ...set])

  }

  intersection(set){

     return new MathSet([...this].filter(x => set.has(x)));

  }

  difference(set){

     return new MathSet([...this].filter(x => !set.has(x)));

  }

  cartesian(set){

     return new MathSet( [...this].reduce((acc, i)=> [...acc, [...set].map(j=>[i,j])], []) )

  }

}



let x = new MathSet([1,2,3]);

let y = new MathSet([1,2,3,4,5]);



console.log(JSON.stringify([...x.cartesian(y)]));

//[

//  [[1,1],[1,2],[1,3],[1,4],[1,5]],

//  [[2,1],[2,2],[2,3],[2,4],[2,5]],

//  [[3,1],[3,2],[3,3],[3,4],[3,5]]

// ]

With

cartesian

function expected result is a flattened version of the above array (

[[1,1],[1,2],[1,3],[1,4],[1,5],[2,1],[2,2],[2,3],[2,4],[2,5],[3,1],[3,2],[3,3],[3,4],[3,5]]

), but as you can see somehow its getting grouped into three arrays. The reduce is keeping on concatenating earlier result with spreaded version of new results. Any guess on what I am doing wrong?

Answer 1

To flatten the arrays you need just one more spread:

return new MathSet( [...this].reduce((acc, i)=> [...acc, ...[...set].map(j=>[i,j])], []) )
//                                                       ^^^

(or acc.concat(Array.from(set, j=>[i,j])))

Cartesian product using reduce, map and spread is grouping the result which need to be flattened

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