withSpliced performs more work than necessary.

[acutmore]

Array.prototype.withSpliced step 13 is copying more values into the result than it needs to.

proposal-change-array-by-copy/spec.html

Line 93 in 17669b8

1. Repeat, while _k_ < _newLen_,

Repeat, while _k_ < _newLen_, should be Repeat, while _k_ < _actualStart_,

[acutmore]

Note, the polyfill already implements the correct version:

proposal-change-array-by-copy/polyfill.js

Line 172 in 17669b8

while (k < actualStart) {