JS : insert an array into another using spread operator

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.

Example:

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

 Spread syntax (...)

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_syntax