Excel → CSV文字列 → JSON。any[][] を経て csv文字列を愚直に作成。csvtojson で先頭行をPropertyにJSON化。

xlsx2csvString2JSON.ts

    const XlsxPopulate = require('xlsx-populate')
    const workbook: any = await XlsxPopulate.fromFileAsync(inputFullPath)

    const headers: string[] = workbook
      .sheet(sheetName)
      .usedRange()
      .value()
      .shift()

      // CSV
    const valuesArray: any[][] = workbook
      .sheet(sheetName)
      .usedRange()
      .value()
    const delimiter = String.fromCharCode(31)
    // const delimiter = ','
    const csvString = valuesArray
      .map((values: any[]) => {
        return values.reduce((box, column) => {
          if (column) {
            return box + delimiter + column
          } else {
            return box + delimiter
          }
        })
      })
      .reduce(
        (box, current) => `${box}
      ${current}`,
      )
    const instances = await csv({ delimiter: delimiter }).fromString(csvString)