my solution for code golf "chapter headings"

chapter.js

// my solution for code golf "chapter headings":
// https://codegolf.stackexchange.com/questions/79696/convert-header-levels-to-numbers
// created in https://ttdbin.com

function f(a){
  var result = [];
  
  for( var i in a ){
    let r = [];
    if(0 == i){
      r[i] = 1;
    } else {
      for ( var k=0; k<a[i]; k++ )  {
        r[k] = result[i-1][k];
      }
      if( a[i] > a[i-1] ) {
        r[ a[i]-1 ] = 1;
      } else {
         r[ a[i]-1 ]++;
      }
    }
    result.push(r);
  }
  return result;
}



describe('code golf ', function() {
  
  it('creates 1 entry', function() {
    assert.deepEqual(f([1]), [[1]]);
  });  

  it('coupts up if on same level', function() {
    assert.deepEqual(f([1,1]), [[1],[2]]);
  });
  
  it('coupts up to 3 if on same level', function() {
    assert.deepEqual(f([1,1,1]), [[1],[2],[3]]);
  }); 
  
  it('indent if on next level', function() {
    assert.deepEqual(f([1,2]),   [[1],[1,1]]);
  }); 
  
  it('works for the big example', function() {
    assert.deepEqual(f([1,2,2,1,1,2,3,2,3,3,2,1,1]),
    [[1],[1,1],[1,2],[2],[3],[3,1],[3,1,1],[3,2],[3,2,1],[3,2,2],[3,3],[4],[5]]);
  });

});