1: Scope
(function() {
var a = b = 5;
})();
console.log(b);
What will be printed to the console?
Answer
The above code will print 5.
The trick to this problem is that there are two variable declarations, but a is declared using the keyword var. Indicates that it is a local variable of a function. In contrast, b becomes a global variable.
Another trick with this question is that it doesn’t use strict mode (‘use strict’;). If strict mode is enabled, the code will raise a ReferenceError: b is not defined. Remember that strict mode requires explicit specification in order to implement global variable declarations. For example, you should write:
(function() {
'use strict';
var a = window.b = 5;
})();
console.log(b);
2: Create a "native" method
Define a repeatify function for the string object. When passed in an integer n, it returns the result of repeating the string n times. For example:
console.log('hello'.repeatify(3));
should print hellohellohello.
Answer
A possible implementation looks like this:
String.prototype.repeatify = String.prototype.repeatify || function(times) {
var str = '';
for (var i = 0; i str = this;
}
Return str;
};
The current questions test developers’ knowledge about JavaScript inheritance and prototypes. This also verifies that the developer knows how to extend built-in objects (even though this should not be done).
Another important point here is that you need to know how not to override functionality that may already be defined. Test that the function definition did not exist before:
String.prototype.repeatify = String.prototype.repeatify || function(times) {/* code here */};
This technique is especially useful when you are asked to make JavaScript functions compatible.
3: Statement Hoisting
Execute this code and what results will be output.
function test() {
console.log(a);
console.log(foo());
var a = 1;
Function foo() {
Return 2;
}
}
10: test();
Answer
The result of this code is undefined and 2.
The reason is that the declarations of variables and functions are brought forward (moved to the top of the function), but the variables are not assigned any values. So, when printing the variable, it exists in the function (it was declared), but it is still undefined. In other words, the above code is equivalent to the following:
function test() {
var a;
Function foo() {
Return 2;
}
console.log(a);
console.log(foo());
a = 1;
}
test();
4: How this works in JavaScript
What will the following code output? Give your answer.
var fullname = 'John Doe';
var obj = {
Fullname: 'Colin Ihrig',
; prop: {
fullname: 'Aurelio De Rosa',
getFullname: function() {
return this.fullname;
}
}
};
console.log(obj.prop.getFullname());
var test = obj.prop.getFullname;
console.log(test());
Answer
The answer is Aurelio De Rosa and John Doe. The reason is that in a function, the behavior of this depends on how the JavaScript function is called and defined, not just how it is defined.
In the first console.log() call, getFullname() is called as a function of the obj.prop object. Therefore, the context refers to the latter and the function returns the fullname of the object. In contrast, when getFullname() is assigned to the test variable, the context refers to the global object (window). This is because test is implicitly set as a property of the global object. For this reason, the function returns the fullname of the window, which is the value defined on the first line.
5: call() and apply()
Now let's solve the previous problem so that the final console.log() prints Aurelio De Rosa.
Answer
This problem can be changed by forcing the use of call() or apply() to change the function context. Below I'll use call(), but in this case apply() will output the same result:
console.log(test.call(obj.prop));

JavaScript originated in 1995 and was created by Brandon Ike, and realized the language into C. 1.C language provides high performance and system-level programming capabilities for JavaScript. 2. JavaScript's memory management and performance optimization rely on C language. 3. The cross-platform feature of C language helps JavaScript run efficiently on different operating systems.

JavaScript runs in browsers and Node.js environments and relies on the JavaScript engine to parse and execute code. 1) Generate abstract syntax tree (AST) in the parsing stage; 2) convert AST into bytecode or machine code in the compilation stage; 3) execute the compiled code in the execution stage.

The future trends of Python and JavaScript include: 1. Python will consolidate its position in the fields of scientific computing and AI, 2. JavaScript will promote the development of web technology, 3. Cross-platform development will become a hot topic, and 4. Performance optimization will be the focus. Both will continue to expand application scenarios in their respective fields and make more breakthroughs in performance.

Both Python and JavaScript's choices in development environments are important. 1) Python's development environment includes PyCharm, JupyterNotebook and Anaconda, which are suitable for data science and rapid prototyping. 2) The development environment of JavaScript includes Node.js, VSCode and Webpack, which are suitable for front-end and back-end development. Choosing the right tools according to project needs can improve development efficiency and project success rate.

Yes, the engine core of JavaScript is written in C. 1) The C language provides efficient performance and underlying control, which is suitable for the development of JavaScript engine. 2) Taking the V8 engine as an example, its core is written in C, combining the efficiency and object-oriented characteristics of C. 3) The working principle of the JavaScript engine includes parsing, compiling and execution, and the C language plays a key role in these processes.

JavaScript is at the heart of modern websites because it enhances the interactivity and dynamicity of web pages. 1) It allows to change content without refreshing the page, 2) manipulate web pages through DOMAPI, 3) support complex interactive effects such as animation and drag-and-drop, 4) optimize performance and best practices to improve user experience.

C and JavaScript achieve interoperability through WebAssembly. 1) C code is compiled into WebAssembly module and introduced into JavaScript environment to enhance computing power. 2) In game development, C handles physics engines and graphics rendering, and JavaScript is responsible for game logic and user interface.

JavaScript is widely used in websites, mobile applications, desktop applications and server-side programming. 1) In website development, JavaScript operates DOM together with HTML and CSS to achieve dynamic effects and supports frameworks such as jQuery and React. 2) Through ReactNative and Ionic, JavaScript is used to develop cross-platform mobile applications. 3) The Electron framework enables JavaScript to build desktop applications. 4) Node.js allows JavaScript to run on the server side and supports high concurrent requests.


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