问我不明白三元运算符是如何工作的，以及我们是如何调用此数组map方法中的函数的

EN

typeof fn === 'function' ? fn : val => val[fn]

const countBy = (arr, fn) =>
  arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val, i) => {
    acc[val] = (acc[val] || 0) + 1;
    return acc;
  }, {});
console.log(countBy([6, 10, 100, 10], Math.sqrt));
console.log(countBy([{b:6}, {b:10}, {b:100}, {b:10}], 'b'));

const countBy = (arr, fn) => {
  let mapped
  if (typeof fn === 'function') {
    mapped = arr.map(fn)
  } else {
    mapped = arr.map(val => val[fn])
  }
  return mapped.reduce((acc, val, i) => {
    acc[val] = (acc[val] || 0) + 1;
    return acc;
  }, {});
}

const countBy = (arr, options) =>
  arr.map(options.processFunction ? options.processFunction : val => val[options.objectProperty]).reduce((acc, val, i) => {
    acc[val] = (acc[val] || 0) + 1;
    return acc;
  }, {});

console.log(countBy([6, 10, 100, 10], {
  processFunction: Math.sqrt
}));


const receipt = [{
  total: 6
}, {
  total: 10
}, {
  total: 100
}, {
  total: 10
}]

console.log(countBy(receipt, {
  objectProperty: 'total'
}));