Array.sort() compare function returns undefined

Question

I am trying to sort an array of string dates from oldest to newest. I set up a couple of compare functions, but the console is saying a is undefined. What is going wrong?

//Sort an array of dates in this format
const dates = [
'10',
'23 Apr 2018',
'01 Jun 1943',
'05 Aug 2055',
'22 Sep 1902'
'18 Aug 1970',
'01 Jan 1940',
'08 Mar 2018',
'11 Feb 1982',
'17 Mar 1927',  
];

//remove the data that is not in the correct format
const cleanedDates = dates.filter(date => date.length === 11);

//isolate the day, convert to number
const getDay = (str) => {
  return parseInt(str.slice(0,2));
};

//create a dictionary of months
const monthDict = {
  Jan: 1,
  Feb: 2,
  Mar: 3,
  Apr: 4,
  May: 5,
  Jun: 6,
  Jul: 7,
  Aug: 8,
  Sep: 9,
  Oct: 10,
  Nov: 11,
  Dec: 12
};

//get the month value via dictionary
const getMonth = (str) => {
  const month = str.slice(3,6);
  return monthDict[month];
};

//get the year, convert to number
const getYear = (str) => {
  return parseInt(str.slice(7));
}

//comparison helper functions

//compare day
const compareDay = (a,b) => {
  if (getDay(a) < getDay(b)) {
    return -1;
  } else if (getDay(a) === getDay(b)) {
    return 0;
  }
  } else if (getDay(a) > getDay(b)) {
    return 1;
  }
};

//compare month
const compareMonth = (a,b) => {
  if (getMonth(a) < getMonth(b)) {
    return -1
  } else if (getMonth(a) === getMonth(b)) {
    compareDay(a,b);
  } else if (getMonth(a) > getMonth(b)) {
    return 1;
  }
};

//compare year
const compareYear = (a,b) => {
  if (getYear(a) < getYear(b)) {
    return -1;
  } else if (getYear(a) === getYear(b)) {
    compareMonth(a,b);
  }
  } else if (getYear(a) > getYear(b)) {
    return 1
  }
};

//sort array
const sortedArray = cleanedDates.sort((a,b) => compareYear(a,b));

console.log(sortedArray);

You have additional } at lines 59 and 82 which causes a javascript error. On line 7 you forgot a , (thanks to @Zigzagoon). Upon fixing those the console.log shows what you want, see : jsfiddle.net/hw4and5q — tcj
– tcj, Commented May 16, 2019 at 21:05
You have a lot of a variables, try to name them with more meaningful names, like currentDay and nextDay in compareDay so it is easier to see where the error is — ramirozap
– ramirozap, Commented May 16, 2019 at 21:10

Kai · Accepted Answer · 2019-05-16 21:10:24Z

your syntax is incorrect. The rest is working for me :). you are missing a , when you do const date on the value 22 Sep 1902. And there are extra } on two locations when you do an else if.

Fixing that will get it working:

//Sort an array of dates in this format
const dates = [
    '10',
    '23 Apr 2018',
    '01 Jun 1943',
    '05 Aug 2055',
    '22 Sep 1902',
    '18 Aug 1970',
    '01 Jan 1940',
    '08 Mar 2018',
    '11 Feb 1982',
    '17 Mar 1927'
];

//remove the data that is not in the correct format
const cleanedDates = dates.filter(date => date.length === 11);

//isolate the day, convert to number
const getDay = (str) => {
    return parseInt(str.slice(0, 2));
};

//create a dictionary of months
const monthDict = {
    Jan: 1,
    Feb: 2,
    Mar: 3,
    Apr: 4,
    May: 5,
    Jun: 6,
    Jul: 7,
    Aug: 8,
    Sep: 9,
    Oct: 10,
    Nov: 11,
    Dec: 12
};

//get the month value via dictionary
const getMonth = (str) => {
    const month = str.slice(3, 6);
    return monthDict[month];
};

//get the year, convert to number
const getYear = (str) => {
    return parseInt(str.slice(7));
}

//comparison helper functions

//compare day
const compareDay = (a, b) => {
    if (getDay(a) < getDay(b)) {
        return -1;
    } else if (getDay(a) === getDay(b)) {
        return 0;
    } else if (getDay(a) > getDay(b)) {
        return 1;
    }
};

//compare month
const compareMonth = (a, b) => {
    if (getMonth(a) < getMonth(b)) {
        return -1
    } else if (getMonth(a) === getMonth(b)) {
        compareDay(a, b);
    } else if (getMonth(a) > getMonth(b)) {
        return 1;
    }
};

//compare year
const compareYear = (a, b) => {
    if (getYear(a) < getYear(b)) {
        return -1;
    } else if (getYear(a) === getYear(b)) {
        compareMonth(a, b);
    } else if (getYear(a) > getYear(b)) {
        return 1
    }
};

//sort array
const sortedArray = cleanedDates.sort((a, b) => compareYear(a, b));

console.log(sortedArray);

tiborK · Accepted Answer · 2019-05-16 21:17:57Z

0

After cleaning the dates array and there is no syntax error, try this:

// convert to date
dates.map( el => new Date(el));
// sort it
dates.sort( (a,b) => a>b);

answered May 16, 2019 at 21:17

tiborK

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