Javascript - insert an array inside another array

Question

What is the more efficient way to insert an array inside another array.

a1 = [1,2,3,4,5];
a2 = [21,22];

newArray - a1.insertAt(2,a2) -> [1,2, 21,22, 3,4         


        

Answer 1

If you want to insert another array into an array without creating a new one, the easiest way is to use either push or unshift with apply

Eg:

a1 = [1,2,3,4,5];
a2 = [21,22];

// Insert a1 at beginning of a2
a2.unshift.apply(a2,a1);
// Insert a1 at end of a2
a2.push.apply(a2,a1);

This works because both push and unshift take a variable number of arguments. A bonus, you can easily choose which end to attach the array from!

Answer 2

You can now do this if using ES2015 or later:

var a1 = [1,2,3,4,5];
var a2 = [21,22];
a1.splice(2, 0, ...a2);
console.log(a1) // => [1,2,21,22,3,4,5]

Refer to this for documenation on the spread (...) operator https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_operator

Answer 3

The spread operator allows an expression to be expanded in places where multiple arguments (for function calls) or multiple elements (for array literals) are expected.

a2 = [21,22];
a1 = [1,2,...a2,3,4,5];//...a2 is use of spread operator
console.log(a1);

Answer 4

var a1 = [1,2,3,4,5];
var a2 = [21,22];

function injectAt(d, a1, a2) {
    for(var i=a1.length-1; i>=d; i--) {
        a1[i + a2.length] = a1[i];
    }
    for(var i=0; i<a2.length; i++) {
        a1[i+d] = a2[i];
    }
}

injectAt(2, a1, a2);

alert(a1);