spread operator vs array.concat()

Question

What is the difference between spread operator and array.concat()

let parts = ['four', 'five'];
let numbers = ['one', 'two', 'three'];
console.log([...numbers, ...parts]);

Array.concat() function

let parts = ['four', 'five'];
let numbers = ['one', 'two', 'three'];
console.log(numbers.concat(parts));

Both results are same. So, what kind of scenarios we want to use them? And which one is best for performance?

Answer 1

concat and spreads are very different when the argument is not an array.

When the argument is not an array, concat adds it as a whole, while ... tries to iterate it and fails if it can't. Consider:

a = [1, 2, 3]
x = 'hello';

console.log(a.concat(x));  // [ 1, 2, 3, 'hello' ]
console.log([...a, ...x]); // [ 1, 2, 3, 'h', 'e', 'l', 'l', 'o' ]

Here, concat treats the string atomically, while ... uses its default iterator, char-by-char.

Another example:

x = 99;

console.log(a.concat(x));   // [1, 2, 3, 99]
console.log([...a, ...x]);  // TypeError: x is not iterable

Again, for concat the number is an atom, ... tries to iterate it and fails.

Finally:

function* gen() { yield *'abc' }

console.log(a.concat(gen()));   // [ 1, 2, 3, Object [Generator] {} ]
console.log([...a, ...gen()]);  // [ 1, 2, 3, 'a', 'b', 'c' ]

concat makes no attempt to iterate the generator and appends it as a whole, while ... nicely fetches all values from it.

To sum it up, when your arguments are possibly non-arrays, the choice between concat and ... depends on whether you want them to be iterated.

The above describes the default behaviour of concat, however, ES6 provides a way to override it with Symbol.isConcatSpreadable. By default, this symbol is true for arrays, and false for everything else. Setting it to true tells concat to iterate the argument, just like ... does:

str = 'hello'
console.log([1,2,3].concat(str)) // [1,2,3, 'hello']

str = new String('hello');
str[Symbol.isConcatSpreadable] = true;
console.log([1,2,3].concat(str)) // [ 1, 2, 3, 'h', 'e', 'l', 'l', 'o' ]

Performance-wise concat is faster, probably because it can benefit from array-specific optimizations, while ... has to conform to the common iteration protocol. Timings:

let big = (new Array(1e5)).fill(99);
let i, x;

console.time('concat-big');
for(i = 0; i < 1e2; i++) x = [].concat(big)
console.timeEnd('concat-big');

console.time('spread-big');
for(i = 0; i < 1e2; i++) x = [...big]
console.timeEnd('spread-big');


let a = (new Array(1e3)).fill(99);
let b = (new Array(1e3)).fill(99);
let c = (new Array(1e3)).fill(99);
let d = (new Array(1e3)).fill(99);

console.time('concat-many');
for(i = 0; i < 1e2; i++) x = [1,2,3].concat(a, b, c, d)
console.timeEnd('concat-many');

console.time('spread-many');
for(i = 0; i < 1e2; i++) x = [1,2,3, ...a, ...b, ...c, ...d]
console.timeEnd('spread-many');

Answer 2

Well console.log(['one', 'two', 'three', 'four', 'five']) has the same result as well, so why use either here? :P

In general you would use concat when you have two (or more) arrays from arbitrary sources, and you would use the spread syntax in the array literal if the additional elements that are always part of the array are known before. So if you would have an array literal with concat in your code, just go for spread syntax, and just use concat otherwise:

[...a, ...b] // bad :-(
a.concat(b) // good :-)

[x, y].concat(a) // bad :-(
[x, y, ...a]    // good :-)

Also the two alternatives behave quite differently when dealing with non-array values.

Answer 3

.concat() is faster, otherwise they work the same assuming both arguments are lists. Here's the time it takes to merge two arrays with 10 million elements each (lower is better):

Browser	[...a, ...b]	a.concat(b)
Chrome 113	350ms	30ms
Firefox 113	400ms	63ms
Safari 16.4	92ms	71ms

I ran this code on an M1 MacBook Air with 8GB of RAM:

const arraySize = 10000000;
const trials = 50;

const array1 = [];
const array2 = [];
for (let i = 0; i < arraySize; ++i) {
  array1.push(i);
  array2.push(i);
}

let spreadTime = 0;
for (let i = 0; i < trials; ++i) {
  const start = performance.now();
  const array3 = [...array1, ...array2];
  const end = performance.now();

  spreadTime += end - start;
}

let concatTime = 0;
for (let i = 0; i < trials; ++i) {
  const start = performance.now();
  const array3 = array1.concat(array2);
  const end = performance.now();

  concatTime += end - start;
}

// performance.now() returns milliseconds with a
// 5 microsecond resolution in isolated contexts and a
// 100 microsecond resolution in non-isolated contexts.
spreadTime = Math.round(spreadTime / trials * 1000) / 1000;
concatTime = Math.round(concatTime / trials * 1000) / 1000;
console.log(`${arraySize} items - spread: ${spreadTime}ms concat: ${concatTime}ms`);

Answer 4

The one difference I think is valid is that using spread operator for large array size will give you error of Maximum call stack size exceeded which you can avoid using the concat operator.

var  someArray = new Array(600000);
var newArray = [];
var tempArray = [];


someArray.fill("foo");

try {
  newArray.push(...someArray);
} catch (e) {
  console.log("Using spread operator:", e.message)
}

tempArray = newArray.concat(someArray);
console.log("Using concat function:", tempArray.length)

Answer 5

Update:

Concat is now always faster than spread. The following benchmark shows both small and large-size arrays being joined: https://jsbench.me/nyla6xchf4/1

// preparation
const a = Array.from({length: 1000}).map((_, i)=>`${i}`);
const b = Array.from({length: 2000}).map((_, i)=>`${i}`);
const aSmall = ['a', 'b', 'c', 'd'];
const bSmall = ['e', 'f', 'g', 'h', 'i'];

const c = [...a, ...b];
// vs
const c = a.concat(b);

const c = [...aSmall, ...bSmall];
// vs
const c = aSmall.concat(bSmall)

Previous:

Although some of the replies are correct when it comes to performance on big arrays, the performance is quite different when you are dealing with small arrays.

You can check the results for yourself at https://jsperf.com/spread-vs-concat-size-agnostic.

As you can see, spread is 50% faster for smaller arrays, while concat is multiple times faster on large arrays.

Answer 6

There is one very important difference between concat and push in that the former does not mutate the underlying array, requiring you to assign the result to the same or different array:

let things = ['a', 'b', 'c'];
let moreThings = ['d', 'e'];
things.concat(moreThings);
console.log(things); // [ 'a', 'b', 'c' ]
things.push(...moreThings);
console.log(things); // [ 'a', 'b', 'c', 'd', 'e' ]

I've seen bugs caused by the assumption that concat changes the array (talking for a friend ;).

Answer 7

The answer by @georg was helpful to see the comparison. I was also curious about how .flat() would compare in the running and it was by far the worst. Don't use .flat() if speed is a priority. (Something I wasn't aware of until now)

  let big = new Array(1e5).fill(99);
  let i, x;

  console.time("concat-big");
  for (i = 0; i < 1e2; i++) x = [].concat(big);
  console.timeEnd("concat-big");

  console.time("spread-big");
  for (i = 0; i < 1e2; i++) x = [...big];
  console.timeEnd("spread-big");

  console.time("flat-big");
  for (i = 0; i < 1e2; i++) x = [[], big].flat();
  console.timeEnd("flat-big");

  let a = new Array(1e3).fill(99);
  let b = new Array(1e3).fill(99);
  let c = new Array(1e3).fill(99);
  let d = new Array(1e3).fill(99);

  console.time("concat-many");
  for (i = 0; i < 1e2; i++) x = [1, 2, 3].concat(a, b, c, d);
  console.timeEnd("concat-many");

  console.time("spread-many");
  for (i = 0; i < 1e2; i++) x = [1, 2, 3, ...a, ...b, ...c, ...d];
  console.timeEnd("spread-many");

  console.time("flat-many");
  for (i = 0; i < 1e2; i++) x = [1, 2, 3, a, b, c, d].flat();
  console.timeEnd("flat-many");