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Questions and Answers
What are the three ways to declare a variable in JavaScript?
let, const, and var
What is the difference between global and local variables in JavaScript?
Global variables are declared outside of any function and are accessible from anywhere, while local variables are declared within a function and are only accessible within that function.
What is the purpose of the return statement in a JavaScript function?
To return a value from a function
What is an Immediately Invoked Function Expression (IIFE) in JavaScript?
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What is the difference between a function declaration and a function expression in JavaScript?
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Flask is a high-level framework that provides an architecture, templates, and an ORM system.
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The requests library is used for parsing HTML and XML documents.
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The finally block is used to handle the exception raised in the try block.
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SyntaxError is raised when an error occurs during execution.
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It's a good practice to use bare except clauses in exception handling.
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Study Notes
Variables
-
Declaration: Variables are declared using the
let,const, orvarkeywords. -
Types: JavaScript has dynamic typing, which means the data type of a variable is determined at runtime. The main types are:
- Number: numeric values, e.g.
1,2.5 - String: text values, e.g.
"hello",'hello' - Boolean: true or false values
- Null: represents the absence of any object value
- Undefined: represents an uninitialized variable
- Object: a collection of key-value pairs
- Array: a collection of values of the same type
- Number: numeric values, e.g.
-
Scope: Variables have either global or local scope.
- Global variables are declared outside of any function and are accessible from anywhere.
- Local variables are declared within a function and are only accessible within that function.
Functions
-
Declaration: Functions are declared using the
functionkeyword. -
Types: There are two types of functions:
- Function declaration: a function declared as a separate statement.
- Function expression: a function assigned to a variable or property.
- Arguments: Functions can take arguments, which are values passed to the function when it is called.
-
Return: Functions can return a value using the
returnstatement. - Hoisting: Function declarations are "hoisted" to the top of their scope, which means they can be used before they are declared.
-
Arrow functions: A concise way to declare functions using the
=>operator. - Immediately Invoked Function Expressions (IIFE): A function that is called immediately after it is defined.
Variables
- Variables are declared using
let,const, orvarkeywords. - JavaScript has dynamic typing, which means the data type of a variable is determined at runtime.
- There are several data types in JavaScript:
- Number: e.g.
1,2.5 - String: e.g.
"hello",'hello' - Boolean:
trueorfalsevalues - Null: represents the absence of any object value
- Undefined: represents an uninitialized variable
- Object: a collection of key-value pairs
- Array: a collection of values of the same type
- Number: e.g.
- Variables have either global or local scope.
- Global variables are declared outside of any function and are accessible from anywhere.
- Local variables are declared within a function and are only accessible within that function.
Functions
- Functions are declared using the
functionkeyword. - There are two types of functions:
- Function declaration: a function declared as a separate statement.
- Function expression: a function assigned to a variable or property.
- Functions can take arguments, which are values passed to the function when it is called.
- Functions can return a value using the
returnstatement. - Function declarations are "hoisted" to the top of their scope, which means they can be used before they are declared.
- Arrow functions provide a concise way to declare functions using the
=>operator. - Immediately Invoked Function Expressions (IIFE) are functions that are called immediately after they are defined.
Web Development
- Python is a popular choice for web development due to its extensive range of libraries and frameworks.
- Django framework provides an architecture, templates, and an ORM system, making it suitable for complex and scalable projects.
- Flask framework offers flexibility and a lightweight approach, ideal for small to medium-sized projects.
- requests library is used for making HTTP requests, essential for web development.
- BeautifulSoup library is used for parsing HTML and XML documents, useful for web scraping and crawling.
- Scrapy library is used for building web scrapers, ideal for extracting data from websites.
- Python can be used for building web applications and APIs, creating web services and microservices, and scraping and crawling websites.
Exception Handling
- Exception handling is a mechanism to handle runtime errors in Python, ensuring that the program continues to execute despite errors.
- The try block is used to enclose the code that might raise an exception, allowing for error detection.
- The except block is used to handle the exception raised in the try block, providing a way to respond to errors.
- The finally block is used to execute code regardless of whether an exception was raised, ideal for releasing resources.
- SyntaxError is raised when there is a syntax error in the code, preventing the program from executing.
- RuntimeError is raised when an error occurs during execution, such as division by zero.
- TypeError is raised when there is a type mismatch, such as passing a string to a function that expects an integer.
- ValueError is raised when a function or method receives an argument with the wrong value, such as passing a negative value to a function that expects a positive value.
- Best practices for exception handling include handling specific exceptions, providing informative error messages, using the finally block to release resources, and avoiding bare except clauses.
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Description
Learn about declaring and typing variables in JavaScript. Understand the different types of variables, including numbers, strings, booleans, null, undefined, and objects.
