I'm trying to combine 2 arrays in JavaScript into one.
var lines = new Array("a","b","c");
lines = new Array("d","e","f");
This is a quick example, I want to be able to combine them so that when the second line is read the 4th element in the array would return "d"
How would I do this?
var a = ['a','b','c'];
var b = ['d','e','f'];
var c = a.concat(b); //c is now an an array with: ['a','b','c','d','e','f']
console.log( c[3] ); //c[3] will be 'd'
Array.prototype.concat.apply([], [[1,2],[3,4],[5,6]])
c=a.push(...b)
Using ES6 JavaScript - spread syntax:
const a = ['a', 'b', 'c'];
const b = ['d', 'e', 'f'];
const c = [...a, ...b]; // c = ['a', 'b', 'c', 'd', 'e', 'f']
It is also the fastest way to concatenate arrays in JavaScript today.
However, when dealing with large arrays, it is more efficient to chain them (concatenate logically):
function chainArrays<T>(...arr: T[][]): Iterable<T> {
return {
[Symbol.iterator](): Iterator<T> {
let i = 0, k = -1, a: T[] = [];
return {
next(): IteratorResult<T> {
while (i === a.length) {
if (++k === arr.length) {
return {done: true, value: undefined};
}
a = arr[k];
i = 0;
}
return {value: a[i++], done: false};
}
};
}
}
}
// usage example:
const a = [1, 2];
const b = [3, 4];
const c = [5, 6];
for (const value of chainArrays(a, b, c)) {
console.log(value); //=> 1, 2, 3, 4, 5, 6
}
Above, we have to use while(i === a.length) logic in order to skip all empty arrays, while also for jumping to the next array at the end of the current one.
A generator approach for the same is much simpler, while also slower:
function* chainArrays<T>(...arr: T[][]) {
for (const i of arr)
for (const v of i)
yield v;
}
// test:
for (const value of chainArrays(a, b, c)) {
console.log(value); //=> 1, 2, 3, 4, 5, 6
}
Internally, a generator is translated into a more verbose IterableIterator, which used to perform slower than a manual iterable, but JavaScript engines keep improving, so it's for you to test it out in your environment ;) But I tested it under Node v20, and on average the manual iterable performs 2 times faster than our generator here.
For a complete TypeScript implementation (including reversed logic), see this gist, or this repo.
spread is much faster for small arrays, while concat is faster for large arrays, while also memory-inefficient (it copies much data), that's why I extended my answer into linking/chaining arrays, for better efficiency.Speed test using local nodejs v16.4.
Object Spread is 3x faster.
ObjectCombining.js
export const ObjectCombining1 = (existingArray, arrayToAdd) => {
const newArray = existingArray.concat(arrayToAdd);
return newArray;
};
export const ObjectCombining2 = (existingArray, arrayToAdd) => {
const newArray = [ ...existingArray, ...arrayToAdd ]
return newArray
};
ObjectCombining.SpeedTest.js
import Benchmark from 'benchmark';
import * as methods from './ObjectCombining.js';
let suite = new Benchmark.Suite();
const existingArray = ['a', 'b', 'c'];
const arrayToAdd = ['d', 'e', 'f'];
Object.entries(methods).forEach(([name, method]) => {
suite = suite.add(name, () => method(existingArray, arrayToAdd));
console.log(name, '\n', method(existingArray, arrayToAdd),'\n');
});
suite
.on('cycle', (event) => {
console.log(`🏎 ${event.target}`);
})
.on('complete', function () {
console.log(`\n🏁 ${this.filter('fastest').map('name')} is fastest.\n`);
})
.run({ async: false });
Result
existingArray.push(...arrayToAdd) test, please?