JSFiddle - React, Tailwind, and code Playground

HTML

<pre id="out"></pre>

JavaScript

const rows = ['a', 'b', 'c']
const cols = [1, 2, 3, 4, 5]

const result = new Object()
rows.forEach(row => {
  result[row] = cols.map(col => row+col)
})

out.textContent = JSON.stringify(result, null, 4)