I had deployed a piece of code that was being tested on IE11. When I looked at the console, I saw that it was complaining about the syntax of arrow functions. It turns out arrow functions are not supported in IE. I hadn’t written arrow functions myself, so I had to look it up and port the snippet back to vanilla JavaScript.

Before

var groupedByYear = _(sample_data)
  .groupBy('year')
  .map((objs, key) => ({
    year: key,
    count: _.sumBy(objs, 'count')
  }))
  .value()

After

var groupedByYear = _(sample_data)
  .groupBy('year')
  .map(function(objs, key) {
    return {
      year: key,
      count: _.sumBy(objs, 'count')
    }
  })
  .value()

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