The at() method is equivalent to the bracket notation when index
is non-negative.
For example, array0 and array.at(0) both return the first item.
However, when counting elements from the end of the array, you cannot use
array-1 like you may in Python or R, because all values inside the square
brackets are treated literally as string properties, so you will end up
reading array"-1", which is just a normal string property instead of
an array index.
The concat method creates a new array.
The array will first be populated by the elements in the object on
which it is called. Then, for each argument, its value will be
concatenated into the array — for normal objects or primitives,
the argument itself will become an element of the final array;
for arrays or array-like objects with the property
Symbol.isConcatSpreadable set to a truthy value, each element of
the argument will be independently added to the final array.
The concat method does not recurse into nested array arguments.
The copyWithin() method works like C and C++'s memmove, and is a
high-performance method to shift the data of an Array.
This especially applies to the TypedArray method of the same name.
The sequence is copied and pasted as one operation; the pasted sequence
will have the copied values even when the copy and paste region overlap.
The every() method is an iterative method.
It calls a provided callbackFn function once for each element in an
array, until the callbackFn returns a falsy value. If such an element
is found, every() immediately returns false and stops iterating through
the array. Otherwise, if callbackFn returns a truthy value for all
elements, every() returns true.
The filter() method is an iterative method. It calls a provided callbackFn
function once for each element in an array, and constructs a new array
of all the values for which callbackFn returns a truthy value.
Array elements which do not pass the callbackFn test are not included
in the new array.
The find() method is an iterative method. It calls a provided callbackFn
function once for each element in an array in ascending-index order,
until callbackFn returns a truthy value. find() then returns that
element and stops iterating through the array. If callbackFn never
returns a truthy value, find() returns undefined.
The findIndex() is an iterative method. It calls a provided callbackFn
function once for each element in an array in ascending-index order,
until callbackFn returns a truthy value. findIndex() then returns the
index of that element and stops iterating through the array.
If callbackFn never returns a truthy value, findIndex() returns -1.
