Duplicate numbers in array | Javascript

Question

For the life of me I can't figure out how to duplicate the numbers array.

Expected result: [1, 2, 3, 4, 5, 1, 2, 3, 4, 5]

Here is my code so far:

const numbers = [1, 2, 3, 4, 5];
var result = numbers.map((number) => {
    return number
});

console.log(result);

I can't figure out how you can take the numbers array and then duplicate the array?

I was starting to do if statements - "If number is equal to 1 then return 1" but that would print the numbers like this [1, 1, 2, 2, 3, 3, 4, 4, 5, 5]

https://jsfiddle.net/e6jf74n7/1/

Iterate through each number and return them again at the end of the array — Filth
– Filth, Commented Sep 13, 2016 at 15:24
map is (more or less) for (i = 0; i < a.length; i++) r[i] = f(a[i]); What you describe would be for (i = 0, l = a.length; i < l; i++) a.push(f(a[i]));, which is not what map does. — deceze
– deceze ♦, Commented Sep 13, 2016 at 15:26
Duplicate question [stackoverflow.com/questions/1960473/unique-values-in-an-array] — Shankar
– Shankar, Commented Sep 13, 2016 at 15:28

deceze · Accepted Answer · 2016-09-13 15:32:09Z

3

Map will map all values one-to-one, that's why it's called "map"; it gives you one value, you return a value that should replace it.

To duplicate a list, concat the list to itself:

const numbers = [1, 2, 3, 4, 5];
var result = numbers.concat(numbers);

console.log(result);

edited Sep 13, 2016 at 15:32

answered Sep 13, 2016 at 15:22

deceze♦

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psychodramatic · Accepted Answer · 2016-09-13 15:29:03Z

0

fastest way is to use slice() then concat() to old array.

var arr = [ 1, 2, 3, 4, 5 ];
var clone = arr.slice(0);
var duplicate = arr.concat(clone);

answered Sep 13, 2016 at 15:29

psychodramatic

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1 Comment

deceze Over a year ago

It's even faster if you omit the slice…

KrishCdbry · Accepted Answer · 2016-09-13 15:35:12Z

0

Map won't work in this case just use concat

numbers.concat(numbers);

If you want to concat multiple times then

var concatArr = numbers;
 for (var i=0; i < 9 ; i++ ) {  
        numbers = numbers.concat(concatArr); 
   }
  console.log(numbers);

Concat docs https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Array/concat

edited Sep 13, 2016 at 15:35

answered Sep 13, 2016 at 15:26

KrishCdbry

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