Member Avatar for costapico

for example if I have two arrys
arr1[10,20,30]
arr2[40,50,60]
can I get help on how to merge them to get arr1[10,20,30,40,50,60]

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  • Latest Post Latest Post by scrappedcola

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A quick, modern summary and a few useful options that build on the replies already in this thread from , , and .

The most concise, idiomatic approach in ES2015+ is the spread operator. It creates a shallow copy and is easy to read:

const merged = [...arr1, ...arr2];

Spread is well-supported and V8 has had targeted optimizations for it, so it is both convenient and performant in modern engines. (developer.mozilla.org)

If the goal is a union (no duplicates) use a Set to filter duplicates while preserving insertion order:

const unique = [...new Set([...arr1, ...arr2])];

Set is the clean, declarative way to dedupe compared with manual loops or repeated indexOf checks; indexOf itself is part of ES5 (older browsers may need a polyfill). (developer.mozilla.org)

When you want to mutate an existing array in-place (avoid allocating a new array), push with apply is a classic choice:

Array.prototype.push.apply(arr1, arr2);

This appends all items without creating a new top-level array. For very large arrays be mindful of engine argument-length limits and memory — sometimes an incremental push or chunked copy is safer. See the push docs for details and caveats. (developer.mozilla.org)

Recommendation: for simple concatenation use spread (clean and fast). For dedupe use Set. For in-place merging prefer push.apply or splice-based insertion when avoiding allocations matters.

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Member Avatar for scrappedcola

Like many things in coding, there are a couple approaches. The simplest is to just use the JS built in function concat.

var newArray = arr1.concat(arr2);

Another approach (and less efficient) would be to loop through the second array and push each element onto the first using

arr1.push(arr2[i])

.
There is yet a third option that would apply if you needed to keep distinct values where you would loop through the both arrays and compare values and then either push the value onto a new array or one of the originals.

Member Avatar for Jayavardhan

The above syntax is absolutely correct that what scrappedcola provided. If you working with multiple arrays you can also use this below syntax

var a=new Array('a','b','c');
var b=new Array('d','e','f');
var d=new Array('x','y','z');
var c=a.concat(b,d);
Member Avatar for Taywin

I hope that your 'merge' definition means 'concatenate'. If your meaning of merge is similar to set union, concatenate won't do it. Just want to make sure.

There is yet a third option that would apply if you needed to keep distinct values where you would loop through the both arrays and compare values and then either push the value onto a new array or one of the originals.

That's the idea for set union, but you don't need to loop through both arrays. You need to loop through only 1 array, and use indexOf() function in another array to check whether the element exists in the array. Example code snippet is below...

var a = [1,2,3,4,5,6]
var b = [4,5,6,7,8]

for (var i=0; i<b.length; i++) {
  if (a.indexOf(b[i])<0) {  // element doesn't exist inside 'a' array
    a.push(b[i])
  }
}
// result is a = [1,2,3,4,5,6,7,8]
//           b = [4,5,6,7,8]
Edited by Taywin because: n/a
Member Avatar for scrappedcola

You need to loop through only 1 array, and use indexOf() function in another array to check whether the element exists in the array.

indexOf may work fine in FF but IE does not currently support indexOf on Arrays and since the OP didn't state which browser I was suggesting cross browser.

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