What is the difference between for in loop and for loop in js
JavaScript for...in statement
The for...in statement is used to loop through the properties of an array or object.
for ... in Each time the code in the loop is executed, an operation will be performed on the elements of the array or the properties of the object.
Tip: for-in loops should be used to traverse non-array objects. Using for-in to loop is also called "enumeration". ·
Syntax:
for (变量 in 对象)
{ 在此执行代码}"Variable" is used to specify variables. The specified variables can be array elements or attributes of objects.
Example: Use for ... in to loop through the array.
<html><body><script type="text/javascript">var x
var mycars = new Array()
mycars[0] = "Saab"
mycars[1] = "Volvo"
mycars[2] = "BMW"
for (x in mycars)
{
document.write(mycars[x] + "<br />")
}</script></body></html>Note 1: The for in loop will not arrange the output according to the subscript of the attribute.
//code from http://caibaojian.com/js-loop-for-in.html"first":"first", "zoo":"zoo", "2":"2", "34":"34", "1":"1", "second":"second"};for (var i in obj) { console.log(i); };输出：1234firstzoosecondWhen executing, press chrome to execute. First, extract the non-negative integer keys, sort the output, and then output the rest in the order of definition. Due to this weird setting, avalon's ms-with object sorting is not output as expected. You can only force users not to define key names with pure numbers.
Example 1: A new attribute is defined on the prototype object of the array, and there is no problem using the for loop
function getNewArray(){var array=[1,2,3,4,5];Array.prototype.age=13;
var result = [];for(var i=0;i<array.length;i++){
result.push(array[i]);
}alert(result.join(''));
}Example 2: A for in loop is used, but it gives us the same expectation Still got the correct result of 12345
function getArrayTwo(){var array=[1,2,3,4,5 ];
var result=[];
for(var i in array){
result.push(array[i]);
}alert(result.join(''));
}Example 3: After adding attributes to the prototype, it is enumerated by default, and finally outputs 1234513
function getNewArrayTwo(){var array=[1,2,3,4,5 ];
Array.prototype.age=13;var result=[];
for(var i in array){
result.push(array[i]);
}alert(result.join(''));
}So it is recommended not to perform a for in loop on the array, in fact In the book High Performance JavaScript, it is also emphasized that the for in loop is not good, because it always accesses the prototype of the object to see if there are attributes on the prototype, which unintentionally adds extra time to the traversal. pressure.
Solution:
If an object has a property with the given name, then Object.prototype.hasOwnProperty(name) returns true. If the object inherits this property from the prototype chain, or does not have such a property at all, false is returned. Limit the for in loop to traverse the current through hasOwnProperty, without considering its prototype properties.
function finalArray(){var array=[1,2,3,4,5 ];
Array.prototype.age=13;
var result=[];for(var i in array){if(array.hasOwnProperty(i)){
result.push(array[i]);
}
}alert(result.join(''));
}Notes:
1. Some browsers, such as the early Safari browser, do not support this method
2. Objects often Being used as a hash value, there is a risk that hasOwnProperty is blocked by another attribute (but I guess no one is so boring to use this attribute)
The above is the detailed content of What is the difference between for in loop and for loop in js. For more information, please follow other related articles on the PHP Chinese website!
Hot AI Tools
Undresser.AI Undress
AI-powered app for creating realistic nude photos
AI Clothes Remover
Online AI tool for removing clothes from photos.
Undress AI Tool
Undress images for free
Clothoff.io
AI clothes remover
AI Hentai Generator
Generate AI Hentai for free.
Hot Article
Hot Tools
Notepad++7.3.1
Easy-to-use and free code editor
SublimeText3 Chinese version
Chinese version, very easy to use
Zend Studio 13.0.1
Powerful PHP integrated development environment
Dreamweaver CS6
Visual web development tools
SublimeText3 Mac version
God-level code editing software (SublimeText3)
Hot Topics
1385
52
The difference between char and wchar_t in C language
Apr 03, 2025 pm 03:09 PM
In C language, the main difference between char and wchar_t is character encoding: char uses ASCII or extends ASCII, wchar_t uses Unicode; char takes up 1-2 bytes, wchar_t takes up 2-4 bytes; char is suitable for English text, wchar_t is suitable for multilingual text; char is widely supported, wchar_t depends on whether the compiler and operating system support Unicode; char is limited in character range, wchar_t has a larger character range, and special functions are used for arithmetic operations.
The difference between multithreading and asynchronous c#
Apr 03, 2025 pm 02:57 PM
The difference between multithreading and asynchronous is that multithreading executes multiple threads at the same time, while asynchronously performs operations without blocking the current thread. Multithreading is used for compute-intensive tasks, while asynchronously is used for user interaction. The advantage of multi-threading is to improve computing performance, while the advantage of asynchronous is to not block UI threads. Choosing multithreading or asynchronous depends on the nature of the task: Computation-intensive tasks use multithreading, tasks that interact with external resources and need to keep UI responsiveness use asynchronous.
What is the function of C language sum?
Apr 03, 2025 pm 02:21 PM
There is no built-in sum function in C language, so it needs to be written by yourself. Sum can be achieved by traversing the array and accumulating elements: Loop version: Sum is calculated using for loop and array length. Pointer version: Use pointers to point to array elements, and efficient summing is achieved through self-increment pointers. Dynamically allocate array version: Dynamically allocate arrays and manage memory yourself, ensuring that allocated memory is freed to prevent memory leaks.
What is the difference between char and unsigned char
Apr 03, 2025 pm 03:36 PM
char and unsigned char are two data types that store character data. The main difference is the way to deal with negative and positive numbers: value range: char signed (-128 to 127), and unsigned char unsigned (0 to 255). Negative number processing: char can store negative numbers, unsigned char cannot. Bit mode: char The highest bit represents the symbol, unsigned char Unsigned bit. Arithmetic operations: char and unsigned char are signed and unsigned types, and their arithmetic operations are different. Compatibility: char and unsigned char
What are the basic requirements for c language functions
Apr 03, 2025 pm 10:06 PM
C language functions are the basis for code modularization and program building. They consist of declarations (function headers) and definitions (function bodies). C language uses values to pass parameters by default, but external variables can also be modified using address pass. Functions can have or have no return value, and the return value type must be consistent with the declaration. Function naming should be clear and easy to understand, using camel or underscore nomenclature. Follow the single responsibility principle and keep the function simplicity to improve maintainability and readability.
c What are the differences between the three implementation methods of multithreading
Apr 03, 2025 pm 03:03 PM
Multithreading is an important technology in computer programming and is used to improve program execution efficiency. In the C language, there are many ways to implement multithreading, including thread libraries, POSIX threads, and Windows API.
The difference between H5 and mini-programs and APPs
Apr 06, 2025 am 10:42 AM
H5. The main difference between mini programs and APP is: technical architecture: H5 is based on web technology, and mini programs and APP are independent applications. Experience and functions: H5 is light and easy to use, with limited functions; mini programs are lightweight and have good interactiveness; APPs are powerful and have smooth experience. Compatibility: H5 is cross-platform compatible, applets and APPs are restricted by the platform. Development cost: H5 has low development cost, medium mini programs, and highest APP. Applicable scenarios: H5 is suitable for information display, applets are suitable for lightweight applications, and APPs are suitable for complex functions.
Concept of c language function
Apr 03, 2025 pm 10:09 PM
C language functions are reusable code blocks. They receive input, perform operations, and return results, which modularly improves reusability and reduces complexity. The internal mechanism of the function includes parameter passing, function execution, and return values. The entire process involves optimization such as function inline. A good function is written following the principle of single responsibility, small number of parameters, naming specifications, and error handling. Pointers combined with functions can achieve more powerful functions, such as modifying external variable values. Function pointers pass functions as parameters or store addresses, and are used to implement dynamic calls to functions. Understanding function features and techniques is the key to writing efficient, maintainable, and easy to understand C programs.
