Repository: apexcharts/apexcharts.js
File: src/modules/Core.js (JavaScript)

Code before:
      series: [],
      i: [],
    }

    let candlestickSeries = {
      series: [],
      i: [],
    }

    let boxplotSeries = {
      series: [],
      i: [],
    }

    let rangeBarSeries = {
      series: [],
      i: [],
    }

    let rangeAreaSeries = {
      series: [],
      seriesRangeEnd: [],
      i: [],
    }

    let chartType = cnf.chart.type !== 'undefined' ? cnf.chart.type : 'line'
    // Check if the user has specified a type for any series.
    let comboCount = 0
    gl.series.map((serie, st) => {
      // The default type for chart is "line" and the default for series is the
      // chart type, therefore, if the types of all series match the chart type,
      // this should not be considered a combo chart.
      // Combo charts are explicitly excluded from stacking with the exception
      // that series of type "bar" can be stacked if the user sets "stackOnlyBar"
      // true.
      if (typeof ser[st].type !== 'undefined') {
        if (ser[st].type === 'column' || ser[st].type === 'bar') {
          if (gl.series.length > 1 && cnf.plotOptions.bar.horizontal) {
            // horizontal bars not supported in mixed charts, hence show a warning
            console.warn(
              'Horizontal bars are not supported in a mixed/combo chart. Please turn off `plotOptions.bar.horizontal`'
            )
          }
          columnSeries.series.push(serie)
          columnSeries.i.push(st)
          if (chartType !== 'bar') {
            comboCount++
          }
          w.globals.columnSeries = columnSeries.series
        } else if (ser[st].type === 'area') {
          areaSeries.series.push(serie)

Review comment (suggestion):
undefined as a string should be checked as `typeof cnf.chart.type !== 'undefined'`

Code after:
      series: [],
      i: [],
    }

    let candlestickSeries = {
      series: [],
      i: [],
    }

    let boxplotSeries = {
      series: [],
      i: [],
    }

    let rangeBarSeries = {
      series: [],
      i: [],
    }

    let rangeAreaSeries = {
      series: [],
      seriesRangeEnd: [],
      i: [],
    }

    let chartType = cnf.chart.type !== undefined ? cnf.chart.type : 'line'
    // Check if the user has specified a type for any series.
    let comboCount = 0
    gl.series.map((serie, st) => {
      // The default type for chart is "line" and the default for series is the
      // chart type, therefore, if the types of all series match the chart type,
      // this should not be considered a combo chart.
      // Combo charts are explicitly excluded from stacking with the exception
      // that series of type "bar" can be stacked if the user sets "stackOnlyBar"
      // true.
      if (typeof ser[st].type !== 'undefined') {
        if (ser[st].type === 'column' || ser[st].type === 'bar') {
          if (gl.series.length > 1 && cnf.plotOptions.bar.horizontal) {
            // horizontal bars not supported in mixed charts, hence show a warning
            console.warn(
              'Horizontal bars are not supported in a mixed/combo chart. Please turn off `plotOptions.bar.horizontal`'
            )
          }
          columnSeries.series.push(serie)
          columnSeries.i.push(st)
          if (chartType !== 'bar') {
            comboCount++
          }
          w.globals.columnSeries = columnSeries.series
        } else if (ser[st].type === 'area') {
          areaSeries.series.push(serie)