https://www.greatfrontend.com/questions/javascript/array-concat?practice=practice&tab=coding
The Array.prototype.concat method on JavaScript arrays is used to merge two or more arrays. This method does not change the existing arrays, but instead returns a new array.
Implement Array.prototype.concat. To avoid overwriting the actual Array.prototype.concat which is being used by the autograder, we shall instead implement it as Array.prototype.myConcat.
[1, 2, 3].myConcat([4, 5, 6]);// [1, 2, 3, 4, 5, 6]
[1, 2, 3].myConcat(4, 5, 6);// [1, 2, 3, 4, 5, 6]
[1, 2, 3].myConcat(4, [5, 6]);// [1, 2, 3, 4, 5, 6]
As seen from the examples, Array.prototype.concat accepts a variadic number of arguments and depending on whether the argument is a primitive or an array/array-like object, they are handled differently. You are not required to handle arguments with the property Symbol.isConcatSpreadable set, but you are recommended to read the specification for Array.prototype.concat on MDN Docs before attempting.