Array.In the deep winter snow falls. It packs onto trees and the earth. As the blizzard continues the snow builds up. It is added to.
An arraystores one element after another. It is a linear collection of elements. Like snow it can keep expanding. We initialize with square brackets. We use push() to add.Initialize Array
Simple example.Here we see an array literal with 3 elements. We can access the length of the array with the length property. Here we have a 3-element array.
Push.This function appends an element to the end of an array. It is an "add" or "append" function. The original array is modified, so we would need to copy the original to retain it.
Initial size.It is possible to create an array with an initial size. This can prevent an array from needing to be resized during use. An initial size can improve performance.
Tip: This is like an array capacity, but all the initial elements exist and are undefined.
Pop.This function removes the last element from an array and returns it. With push and pop we use an array like a stack. We can ignore the return value if we do not need it.
Tip: Pop() removes the last element, so it leaves no gap in the array. No elements need to be shifted forward after a pop.
IndexOf.We search arrays with the indexOf method. If an element matches, its index is returned. If nothing is found, the special value -1 is returned.
LastIndexOf.With this function we search an array from its end. Other than the search direction and starting point, it is the same as indexOf.
Here: We have 3 "blue" strings in an array. We find the last one with lastIndexOf—its index is 2, meaning the third element.
For-in loop.With a for-in loop we can access all the indexes of an array without specifying the start or end indexes. This is a simpler way to loop over array elements.
Tip: With the for-in loop on an array, we must still access the element with an index. The iteration variable is an index.
Note: The array indexes can be returned in any order by the for-in loop. So this loop is probably not the best one for arrays.
For-of loop.This loop returns all the elements in the array. No indexes are returned—just elements. This is a way to enumerate all the elements in an array with clear code.
Slice.This method takes part of an array. It accepts 2 arguments: the first is the start index of the slice, and the second is the last index (not an element count).
Splice.This method removes elements and adds new ones in their place. It both removes and inserts in a single method call. It receives multiple arguments.
Argument 1: The index we want to start removing from. So to remove the second element onwards, we pass 1.
Argument 2: The count of elements we want to remove. To remove 2 elements we pass the value 2.
Finally: We pass any number of arguments to be inserted in the region specified. The array grows to accommodate these elements.
Reverse.This method inverts the ordering of an array. Reverse() modifies the existing array and does not create a copy. So we must copy the original to keep it around.
Adjacent elements.Here we loop over two elements at a time, accessing the left and right element. We start at index 1 to avoid accessing an element at index -1.
And: This saves memory. Engines like V8 decide when to use sparse arrays with logical rules (heuristics).
Shift.This method removes and returns the first element of an array. The following elements are shifted forward so that the second element becomes the first element.
Note: It is possible to modify arrays. But often for best performance, leaving arrays alone and using indexes to access them is best.
ToString.This method converts an array into a string. Each element is separated by a comma. We see that an array of 3 elements is converted to a string with a length of 20 characters.
Note: No quotes are in the output string. So if a string contains a comma, it may be impossible to convert this string back into an array.
JSON: For a way to convert an array into a string format that can be reused with no data loss, consider JSON.stringify.JSON
Join.With this method we combine elements from an array into a string. We specify the separator string (which can be zero or more characters).
Tip: Join is the opposite of split. If the delimiter is not found in any of the strings, we can use split() to reverse a join.
ForEach function.Looping over the elements in an array can be done with the forEach function. We must provide a function argument that handles each element.function
Here: We call forEach on an array, and call the handleElement function on each individual element.
Tip: Using forEach instead of a for-loop can reduce errors and lead to more elegant code.
Push, unshift benchmark.Here we test adding 1000 numbers to an array. The first version uses push, and the second version uses unshift. The total time required is recorded for each page.
Note: The data in the "numbers" array is different for the pages, but the same total count of elements is added in both.
Analysis: When adding elements to an array, push() is a faster option than unshift. Push is optimized better in browsers.
Array creation benchmark.Suppose we want to create a 1-element array with a certain value in it. We can use the literal syntax or use an empty array and call push().
Result: Using a single expression with literal syntax is faster in Chrome. Reducing method calls (like push) tends to help performance.
Question: Is it faster to use push() or to assign to the next element? And does the difference matter?
Answer: In Chrome I found that assignment performs lightly better. But even in a long-running simulation this difference is small.
Initial size benchmark.We can improve performance by specifying an initial array size and then assigning the undefined elements. Here we test two programs.
Program 1: This uses an empty array literal and appends 200 elements with the push method.
Program 2: This one uses "new Array(200)" and assigns the 200 undefined elements. The array should never resize.
Result: Program 2 is faster as the array is never resized—it has all 200 elements at creation time and never resizes, unlike program 1.
For, array benchmark.A for-loop from 0 to the length of an array tends to perform well in Google Chrome. More complex loops (while, or ones with hoisted variables) are harder to optimize.for
Split.We can use split() to get a string array from a string that contains delimiter characters. This is a way an array can be represented in a more compact way in a web page.split
Linked list.Arrays can be used to create a linked list—each list item is a 2-element array. One of the array elements is a reference to another unit in the linked list.Linked List
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