具有负值索引的array.splice意外行为

Question

I noticed MDN mentioned in the description of start Parameter :

• Index at which to start changing the array (with origin 0)
• If negative , will begin that many elements from the end of the array (with origin -1)

This description working very good for me on removing elements for me :

Removing 1st element :

 let foo = ["a", "b", "c"]; foo.splice(0,1) // "a" removed console.log(foo);//["b", "c"] 

Removing last element :

 let foo = ["a", "b", "c"]; foo.splice(-1,1) // "c" removed console.log(foo);//["a", "b"] 

Now let's Try add elements

add element at 1st :

 let foo = ["a", "b", "c"]; foo.splice(0,0,"x") // "x" added to 1st console.log(foo);//["x", "a", "b", "c"] 

add element to the last :

 let foo = ["a", "b", "c"]; foo.splice(-1,0,"x") // I expected to added "x" as a last element // but instead it shows ["a", "b", "x", "c"] console.log(foo); 

can anyone explain why ?

the last case shows ["a", "b", "x" , "c"]

if you wanna remove the same element from it's position we should remove it from index of -2 not -1 :

 let foo = ["a", "b", "c"]; foo.splice(-1,0,"x") console.log(foo); //["a", "b", "x", "c"] foo.splice(-2,1) console.log(foo); 

Thanks...

Answer 1

Initially lets consider foo as ["a", "b", "c"] and foo.length is 3.

So when you are adding the element using foo.splice(-1,0,"x") , the index is calculated as foo.length - 1 which is 2.

So the new element is inserted at the index 2:

["a", "b", "c"]
  0    1    2
            ^
            |__________"x" added here 

So the new array becomes ["a", "b", "x", "c"] as "x" is inserted at index 2 and "c" is shifted by one index.

When you remove the element using foo.splice(-2,0) , the index at which it is removed foo.length - 2 which is 2:

["a", "b", "x", "c"]
  0    1    2    3
            ^
            |__________ "x" is removed

And the array becomes ["a", "b", "c"] .

Answer 2

Splice function accepts three parameters,

• start : Index from where to change the array.
• deleteCount : Number of elements to remove.
• items(varargs) : Elements to add, beginning from start index.

let foo = ["a", "b", "c"];
foo.splice(-1,0,"x") 
// I expected to added "x" as a last element
// but instead it shows ["a", "b", "x", "c"]
console.log(foo);

In the above code snippet, you are passing start as -1, so it's get added on last index (which is calculated as arr.length - 1 ) and deleteCount as 0 so no elements are deleted instead they are moved to right by one index.