JavaScript - Array mapping with another array

Question

I have two arrays which starts off as this:

let days = [null, null, null, null, null, null, null];

const weekdays = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday']

Depending on what data I'm given, I want to map a new array with the corresponding weekday, but still keep it the same size array.

let numDays = ["0","2","4"]

let days = days.map(?)

days
> ['Sunday', null, 'Tuesday', null, 'Thursday', null, null]

I have attempted a conversion function which can convert numDays to its correspondingweekdays

const convert = function (c) {
  return weekdays[c];
}

numDays.map(convert)
> ['Sunday', 'Tuesday', 'Thursday']

But not sure how to retain the array size for the result I need.

Cheers!

kevguy · Accepted Answer · 2016-12-19 03:28:09Z

2

I don't think you need the days variable to be [null, null, null, null, null, null, null].

const weekdays = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday'];

let numDays = ["0","2","4"];

var days = weekdays.map(function(day, index){
  if (numDays.indexOf(index.toString()) >= 0){
    return day;
  } else {
    return null;
  }
});

console.log(days);

answered Dec 19, 2016 at 3:28

kevguy

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dtkaias · Accepted Answer · 2016-12-19 02:46:07Z

2

This approach might be easier than using map, you can use forEach to loop over the required indexes, look up the day and add them to your days array as required.

const weekdays = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday']
let numDays = ["0","2","4"]
let days = [null, null, null, null, null, null, null];
numDays.forEach(i=>days[i] = weekdays[i]);
console.log(days);

edited Dec 19, 2016 at 2:46

answered Dec 19, 2016 at 2:41

dtkaias

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2 Comments

Aᴍɪʀ Over a year ago

but you need to define an array with 7 null elements.

dtkaias Over a year ago

The question stated that the array of nulls was already defined, if a more general array of n nulls is needed you could use something like let days = Array(7).fill(null)

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