refactor: replace "for" loops with Array.prototype.* methods as appropriate

Using `for` loops (with the exception of `for..of`) in JS nowadays is discouraged. Generally, anything that can be expressed as a `for` loop can be better expressed declaratively using `Array.prototype` methods ("you need to iterate over an array--but you don't need to tell JS how to do so").

Replace `for` loops with `Array.prototype` methods such as `forEach`, `map`, etc., as appropriate.

The issue names no files or tests. Search the JavaScript source for `for` loops, excluding `for..of`, and determine which Array.prototype method fits each loop while preserving behavior. Run the project's existing tests afterward; done means applicable loops are refactored and the tests still pass.

Written by the indexing model from the issue text.