![]() ![]() const cities = Ĭonst newCityArr = cities.slice(2,4) How to use the splice() JavaScript array method The new returned array will only include the cities at index 2 and 3 because the end position is not included in the extracted elements. In this example, we have specified a start index of 2 and end index of 4. The end position will not be included in the extracted elements for the new array. If an end position is specified, then the slice() method will extract elements from the start position up to the end position. const newCityArr = cities.slice(5) How to use the slice() JavaScript method using the start and end parameters If the start parameter is greater than the last index of the array, then an empty array will be returned. In this example, we will set the start position at -2 which will select the last two cities in the array and return them in a new array. You can also use negative indexes which will start extracting elements from the end of the array. The original array was not modified as we can see here in this example. In this example, we will set the start position at index 2 which will select the last three cities in the array and return them in a new array. It is important to remember that arrays are zero based indexed. You can use the optional start parameter to set a starting position for selecting elements from the array. How to use the slice() JavaScript method using the start parameter When I console.log the result, then I will see all of the elements from my cities array copied into this new array. I can use the slice() method to create a shallow copy of that array. In this first example, I have created a list of cities from around the world. How to use the slice() JavaScript method without the start and end parameters Let's take a look at some examples to better understand how the slice() method works. Here is the basic syntax: slice(optional start parameter, optional end parameter) It is important to note that the slice() method does not alter the original array but instead creates a shallow copy. The slice() method can be used to create a copy of an array or return a portion of an array. How to use the slice() JavaScript array method In this article, I will walk you through how to use the slice() and splice() array methods using code examples. Given the differences between Chrome's results, and the extremely poor performance in some other browsers, it may be worth sacrificing some performance in Chrome for much better performance in other browsers.When you are first learning JavaScript, it might be difficult to know the difference between the slice() and splice() array methods. Unlike GWT, where you can deploy a certain condition for a certain browser, Closure Compiler just generates one JavaScript file for every browser. ![]() Although, on the whole, indexing performs better than push, that is not the case in Google Chrome. The problem then lies in the fact that the performance varies between browser. In this scenario, the Compiler should be able to replace the push with an array.length index without side effects. ![]() However, we could examine the case where the result of Array.push is not assigned to anything. push ( ' test ' ) // i is 1 var j = b = ' test ' // j is 'test' Converting all Array.push statements would cause semantic problems if the user has assigned that statement to a variable. However, it wouldn't be as simple as converting all pushes to index assignments as there is a subtle difference between the two Array.push returns the length of the array after the element has been added (something you don't get with array). However, with an improved performance in most browsers, you might expect it to. Showing that the Compiler doesn't do anything about push statements in pre-compiled code. We observe the following output: for ( var a =, b = 0 b < 10 b ++ ) a. I put a function on the Array prototype called mypush, which carried out step 3 above. I also defined a fourth function to test. Other than that it's a simple assignment.Īrray = element - Similar to the previous, but this involves a lookup for situations where you don't have access to a pointer. The requirement of this method is that you must have a pointer to the last location. There are 3 distinct ways to do this:Īrray.push(element) - Defined on the native Array object, this is the method challenged by Google.Īrray = element - Assigning an element to a specific index of the array. I had always used push to assign a value to the end of an array, so thought I'd investigate this a little bit further to determine the performance of each of the methods. "Note that since assigning values to an array is faster than using push() you should use assignment where possible." I was recently reading Google's JavaScript Style Guide when I came across the following claim: ![]()
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