


Common javascript operation codes for arrays Summary of array methods_javascript skills
var a = [1,2,3,4,5];
var b = a.shift(); //a: [2,3,4,5] b: 1
2. unshift: Add parameters to the beginning of the original array and return the length of the array
var a = [1,2,3,4,5];
var b = a.unshift(-2,-1); //a: [-2,-1,1 ,2,3,4,5] b: 7
Note: The test return value under IE6.0 is always undefined, and the test return value under FF2.0 is 7, so the return value of this method is unreliable and needs to be used When returning a value, splice can be used instead of this method.
3. pop: delete the last item of the original array and return the value of the deleted element; if the array is empty, it returns undefined
var a = [1,2,3,4,5 ];
var b = a.pop(); //a: [1,2,3,4] b: 5
4. push: Add parameters to the end of the original array and return The length of the array
var a = [1,2,3,4,5];
var b = a.push(6,7); //a: [1,2,3, 4,5,6,7] b: 7
5. concat: Returns a new array, which is formed by adding parameters to the original array
var a = [1,2, 3,4,5];
var b = a.concat(6,7); //a:[1,2,3,4,5] b:[1,2,3,4,5, 6,7]
6. splice(start,deleteCount,val1,val2,...): Delete the deleteCount item from the start position and insert val1, val2,...
var a = [1,2,3,4,5];
var b = a.splice(2,2,7,8,9); //a: [1,2,7 ,8,9,5] b:[3,4]
var b = a.splice(0,1); //Same as shift
a.splice(0,0,-2,-1) ; var b = a.length; //Same as unshift
var b = a.splice(a.length-1,1); //Same as pop
a.splice(a.length,0,6, 7); var b = a.length; //Same as push
7. reverse: Reverse the order of the array
var a = [1,2,3,4,5];
var b = a.reverse(); //a: [5,4,3,2,1] b: [5,4,3,2,1]
8. sort(orderfunction ): Sort the array according to the specified parameters
var a = [1,2,3,4,5];
var b = a.sort(); //a: [1, 2,3,4,5] b: [1,2,3,4,5]
9. slice(start,end): Returns the specified start index to the end index in the original array. A new array composed of items between
var a = [1,2,3,4,5];
var b = a.slice(2,5); //a: [1, 2,3,4,5] b: [3,4,5]
10. join(separator): Combine the elements of the array into a string, using separator as the separator. If omitted, use By default, commas are used as delimiters
var a = [1,2,3,4,5];
var b = a.join("|"); //a: [1,2 ,3,4,5] b: "1|2|3|4|5"
Array is an internal object provided by JavaScript. It is a standard collection. We can add (push) and delete (shift) We can also traverse the elements inside through a for loop, so besides arrays, can we have other collections in JavaScript?
Due to the language features of JavaScript, we can dynamically add and delete attributes to general objects. So Object can also be regarded as a special collection of JS. Let’s compare the characteristics of Array and Object:
Array:
New: var ary = new Array(); or var ary = [];
Add: ary.push(value);
Delete :delete ary[n];
Traversal: for ( var i=0 ; i Object:
New: var obj = new Object() ; or var obj = {};
Add: obj[key] = value; (key is string)
Delete: delete obj[key];
Traverse: for ( var key in obj ) obj[ key];
From the above comparison, we can see that Object can be used as a collection. In using the Popup window to create an infinite Web page menu (3), I introduced the __MenuCache__ implemented by Eric, which is also a A mocked collection object.
If we want to retrieve a specified value in Array, we need to traverse the entire array:
Code:
var keyword = ;
for ( var i=0 ; i {
if ( ary[i ] == keyword )
{
// todo
}
}
To retrieve an entry with a specified key in Object, we only need to use :
Code:
var key = '';
var value = obj[key];
// todo
This feature of Object can be used to efficiently retrieve Unique string collections. The time complexity of traversing Array is O(n), while the time complexity of traversing Object is O(1). Although the cost of for retrieval for 10,000 collections is only tens of ms, if it is 1,000*1,000 retrievals or more, the advantages of using Object will be immediately apparent. Before that, I did a mapping, mapping 100 Unique characters to 1000 string arrays, which took 25-30 seconds! Later, I changed the for traversal to the member reference of the Object simulated collection, and the same amount of data was mapped , it takes only 1.7-2s!!!
For the traversal efficiency of the collection (from high to low): var value = obj[key]; > for ( ; ; ) > for ( in ). The least efficient one is for(in). If the collection is too large, try not to use for(in) to traverse.

JavaScript's application in the real world includes front-end and back-end development. 1) Display front-end applications by building a TODO list application, involving DOM operations and event processing. 2) Build RESTfulAPI through Node.js and Express to demonstrate back-end applications.

The main uses of JavaScript in web development include client interaction, form verification and asynchronous communication. 1) Dynamic content update and user interaction through DOM operations; 2) Client verification is carried out before the user submits data to improve the user experience; 3) Refreshless communication with the server is achieved through AJAX technology.

Understanding how JavaScript engine works internally is important to developers because it helps write more efficient code and understand performance bottlenecks and optimization strategies. 1) The engine's workflow includes three stages: parsing, compiling and execution; 2) During the execution process, the engine will perform dynamic optimization, such as inline cache and hidden classes; 3) Best practices include avoiding global variables, optimizing loops, using const and lets, and avoiding excessive use of closures.

Python is more suitable for beginners, with a smooth learning curve and concise syntax; JavaScript is suitable for front-end development, with a steep learning curve and flexible syntax. 1. Python syntax is intuitive and suitable for data science and back-end development. 2. JavaScript is flexible and widely used in front-end and server-side programming.

Python and JavaScript have their own advantages and disadvantages in terms of community, libraries and resources. 1) The Python community is friendly and suitable for beginners, but the front-end development resources are not as rich as JavaScript. 2) Python is powerful in data science and machine learning libraries, while JavaScript is better in front-end development libraries and frameworks. 3) Both have rich learning resources, but Python is suitable for starting with official documents, while JavaScript is better with MDNWebDocs. The choice should be based on project needs and personal interests.

The shift from C/C to JavaScript requires adapting to dynamic typing, garbage collection and asynchronous programming. 1) C/C is a statically typed language that requires manual memory management, while JavaScript is dynamically typed and garbage collection is automatically processed. 2) C/C needs to be compiled into machine code, while JavaScript is an interpreted language. 3) JavaScript introduces concepts such as closures, prototype chains and Promise, which enhances flexibility and asynchronous programming capabilities.

Different JavaScript engines have different effects when parsing and executing JavaScript code, because the implementation principles and optimization strategies of each engine differ. 1. Lexical analysis: convert source code into lexical unit. 2. Grammar analysis: Generate an abstract syntax tree. 3. Optimization and compilation: Generate machine code through the JIT compiler. 4. Execute: Run the machine code. V8 engine optimizes through instant compilation and hidden class, SpiderMonkey uses a type inference system, resulting in different performance performance on the same code.

JavaScript's applications in the real world include server-side programming, mobile application development and Internet of Things control: 1. Server-side programming is realized through Node.js, suitable for high concurrent request processing. 2. Mobile application development is carried out through ReactNative and supports cross-platform deployment. 3. Used for IoT device control through Johnny-Five library, suitable for hardware interaction.


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