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I'm trying to duplicate each element in an array, but using functional style.

I have this currently:

["a", "b", "c"]

And I'm getting this:

["a","a","b","b","c","c"]

So far I have tried the following, mapping each element to an array, then using flat() to get a 1d array. Is there a cleaner way because it feels like I'm abusing map and flat.

["a", "b", "c"].map(item => [item, item]).flat();

Is there a better way to do this?

I was trying to provide a example as simple as possible but left some details out. The real input is not sorted because elements are not comparable.
It's something like:

[
 
 a:"a"
 b:"b"
 ,
 
 c: 1
 d: 2
 ,
 
 apple: ,
 sellers: ["me", "her"]
 
]

The duplicated result should be something like this, where duplicated elements are next to each other:

[
 
 a:"a"
 b:"b"
 ,
 
 a:"a"
 b:"b"
 ,
 
 c: 1
 d: 2
 ,
 
 c: 1
 d: 2
 ,
 
 apple: ,
 sellers: ["me", "her"]
 ,
 
 apple: ,
 sellers: ["me", "her"]
 
]

Array.reduce is semantically the appropriate method here: take an object (in this case an array) and return an object of a different type, or with a different length or shape (note: edited to use Array.push for faster performance per @slider suggestion):

EDIT: I've edited my answer to reflect OP's updated input data. Note also, that this solution is cross-browser and NodeJS compatible without requiring transpilation.

let data = [
 
 a:"a",
 b:"b",
 ,
 
 c: 1,
 d: 2
 ,
 
 apple: ,
 sellers: ["me", "her"]
 
];

let result = data
 .reduce((acc, el) => 
 acc.push(el, el);
 return acc;
 , );
 
console.log(JSON.stringify(result, null, 2));

Otherwise you could map each element, duplicating it, then combine them:

let data = [
 
 a:"a",
 b:"b",
 ,
 
 c: 1,
 d: 2
 ,
 
 apple: ,
 sellers: ["me", "her"]
 
];

let result = data.map(item => [item, item]).reduce((acc, arr) => acc.concat(arr));

console.log(JSON.stringify(result, null, 2));

As mentioned in other answers here, either of these approaches have the advantage of not requiring the original array to have been sorted.

I would recommend Array.prototype.flatMap

const twice = x =>
 [ x, x ]
 
console.log
 ( [ 'a', 'b', 'c' ] .flatMap (twice) // [ 'a', 'a', 'b', 'b', 'c', 'c' ]
 , [ 1, 2, 3, 4, 5 ] .flatMap (twice) // [ 1, 1, 2, 2, 3, 3, 4, 4, 5, 5 ]
 )

flatMap is useful for all kinds of things

const tree =
 [ 0, [ 1 ], [ 2, [ 3 ], [ 4, [ 5 ] ] ] ]
 
const all = ([ value, ...children ]) =>
 [ value ] .concat (children .flatMap (all))
 
console.log (all (tree))
// [ 0, 1, 2, 3, 4, 5 ]

really cool things

const ranks =
 [ 'J', 'Q', 'K', 'A' ]
 
const suits =
 [ '♡', '♢', '♤', '♧' ]

console.log
 ( ranks .flatMap (r =>
 suits .flatMap (s =>
 [ [ r, s ] ]
 )
 )
 )

// [ ['J','♡'], ['J','♢'], ['J','♤'], ['J','♧']
// , ['Q','♡'], ['Q','♢'], ['Q','♤'], ['Q','♧']
// , ['K','♡'], ['K','♢'], ['K','♤'], ['K','♧']
// , ['A','♡'], ['A','♢'], ['A','♤'], ['A','♧']
// ]

You can use the function reduce and concatenate the same object on each iteration.

let array = ["a", "b", "c"],
 result = array.reduce((a, c) => a.concat(c, c), );
 
console.log(result);

.as-console-wrapper max-height: 100% !important; top: 0; 

You could just do this:

var arr = ["a", "b", "c"];
arr = arr.concat(arr).sort();

This is one of the simplest methods to do what you are asking to do.