From what I understood from your answers what you need is to filter the array:

Copyconst filteredArray = array.filter(element => {
    if (yourConditionIsTrue) { // if this element should be in the filteredArray
        return true;
    } else {
        return false
    }
});

Which can be done in one line:

Copyconst filteredArray = array.filter(element => conditionIsTrue);

This way your array remains untouched and you get a new array (filteredArray) only with the elements you need, but you don't mess up with the array you are iterating.

This is because .slice does a shallow copy and does not copy nested objects. Since you are using two dimensional array, this is expected.

Use: const input = JSON.parse(JSON.stringify(linsys))

Shallow cloning does not copy nested objects. So if an object contains an object, your .slice will keep a reference to the original copy of that object rather than creating a copy. Hence, the value changes.

In case of a shallow copy-

• Objects will reflect change in the original place from where they were shallowly copied because they are stored as references (to their address in the Heap).
• Primitive data types will NOT reflect change in the original place because they are directly stored in the callstack (in Execution Contexts).

For example:

var a = [
    [1, 2],    // This still is referencing to the original reference 
    [5,7]
]

The second argument to Array.slice() is the upper bound of the slice.

Think of it as array.slice(lowestIndex, highestIndex).

When you slice from index 3 to index 100, there is one item (in your case) that has index >= 3 and < 100, so you get an array with that one item. When you try to take a slice from index 3 to index 0, there can't be any items that meet the conditions index >= 3 and < 0, so you get an empty array.

--EDIT--

Also, array.slice() should never return undefined. That's one of the advantages of using it. If there are no matching values in the array, you just get back an empty array. Even if you say var a = new Array() and don't add any values to it, calling a.slice(0,1) will just give you an empty array back. Slicing from outside of the array bounds will just return an empty array also. a.slice(250) will return [] whereas a[250] will be undefined.