Support nested columns feature

[jeka1985]

Summary

Thanks for a great lib!

Used it for generating styled dynamic reports and found generating dynamic nested headers pretty complicated because of low-level methods for adding rows and manual cell merging.

It would be great to have some declarative way for such tasks. Imho nested JSON tree looks like a solution.

In a nutshell, recursive walk the tree, collect items meta (such as cell size, including aggregated size of all children). Build flat list, use it for generating header rows and columns.

Test plan

Backward compatibility is not affected, added new worksheet method makeColumns for building flat columns from nested json tree

worksheet.makeColumns([
  {
    id: 1, 
    title: 'Some', 
  },
  {id: 2, title: 'Qwe'},
  {id: 3, title: 'Foo'},
  {
    id: 4,
    title: 'Zoo',
    child: [
      { id: 41, title: 'Zoo 1' },
      { id: 42, title: 'Zoo 2' },
      { id: 44, title: 'Zoo 3' },
      { id: 45, title: 'Zoo 4' },
    ]
  }
]);

Also you can pass column props such as width ad style to "leaf" tree items.
This input will be converted into 3 native entity groups

• Columns
• Rows
• Merge rules

makeColumns(input) {
    const flatter = new ColumnFlatter(input);
    const merges = flatter.getMerges();
    this.columns = flatter.getCoumns();

    this.views.push({state: 'frozen', ySplit: flatter.getRows().length});
    
    this.addRows(flatter.getRows().map(row => row.map(item => item?.title || item?.id || null)));

    merges.forEach(item => {
      this.mergeCells(item);
    });
} 

Related to source code (for typings update)

[jeka1985]

@Siemienik sorry to bother you, but is there any updates on this?

[Siemienik]

Thank you for your effort. Currently, I've no time. I'm going to see this in the first free-time slot.

[Siemienik]

Wow, it is really great feature. I see that I have to analyze it deeply and the relations between nested columns and other features like borders, merged cells, styles etc. please don't blame me that It takes me a lot of time.

[jeka1985]

Wow, it is really great feature. I see that I have to analyze it deeply and the relations between nested columns and other features like borders, merged cells, styles etc. please don't blame me that It takes me a lot of time.

Sure, no problem, take your time )

[jeka1985]

@Siemienik Just wondering about estimates, looking forward to switch to nested-columns released version and drop off ad-hoc solution :)

[jeka1985]

@Siemienik ?

[AlFalahTaieb]

This is awesome @jeka1985 Thank you

[jeka1985]

@alubbe @Alanscut Hi guys, looks like Pawel still have no time for this PR, could you please take a look on this?

[vaynevayne]

How is the progress?

[dmk1111]

@Siemienik any updates on this? This fixes 6-year old issue (#365) but still not merged

[Siemienik]

Suggested change

	this.columns = flatter.getCoumns();
	this.columns = flatter.getColumns();

[Siemienik]

Suggested change

	getCoumns() {
	getColumns() {

[Siemienik]

@Siemienik any updates on this? This fixes 6-year old issue (#365) but still not merged

I'm aware of that. Everything I plan to merge must be thoroughly tested to prevent regressions, which is time-consuming. This is more of a volunteer effort for me, and even though I do my best, there are many other tasks to prioritize.

By the way, all of you can help contribute to this project by:

• Being active on our Discord and responding to other users' questions.

• Reviewing PRs and adding a thumbs-up 👍 to those that are most important to you. I prioritize PRs for review based on the number of thumbs-ups they receive.
• Being active in pinned discussions – these are the most important topics for me as a maintainer. Your active participation there helps me make decisions faster, more accurately, and wisely.

If you're interested in the project's progress, you can check out:
#2351
ExcelJS v4.4.0 Release Notes

I would also like to suggest these discussions as candidates for inclusion in the next release:
#2573
#2574