Week 4 — Linking Node to MySQL

Stuff is really starting to ramp up this week!

What is “callback hell” and how can it be avoided? | It is an execution of multiple asynchronous operations one after the other (aka “The Pyramid of Doom”, a name I much prefer). This can be avoided by using promises, or using the asynch and await keywords.

What are “stubs” in Node.js? | A test stub is a function or object that replaces the actual behavior of a module with a fixed response. The stub can only return the fixed response it was programmed to return.

What are “streams” in Node.JS? | A stream is an abstract interface for working with streaming data in Node.js. The stream module provides an API for implementing the stream interface. There are many stream objects provided by Node.js. For instance, a request to an HTTP server and process.stdout are both stream instances. Streams can be readable, writable, or both. All streams are instances of EventEmitter. There are four fundamental stream types within Node.js:

• Writable: streams to which data can be written (for example, fs.createWriteStream()).
• Readable: streams from which data can be read (for example, fs.createReadStream()).
• Duplex: streams that are both Readable and Writable (for example, net.Socket).
• Transform: Duplex streams that can modify or transform the data as it is written and read (for example, zlib.createDeflate()).

Additionally, this module includes the utility functions stream.pipeline(), stream.finished() and stream.Readable.from().

What does “chaining” mean in Node.JS? | For method chaining, every method returns the object, so that we can chain different calls in single line. For promise chaining, after an initial promise is resolved, the result is passed on through a chain of .then handlers —it works because a call to promise.then returns a promise, so that we can call the next .then on it.

Explain “console” in Node.JS? | It is a class with methods that will display an output in the terminal.

Explain exit codes in Node.JS. List out some exit codes. | The process core module provides a handy method that allows you to programmatically exit from a Node.js program: process.exit(). When Node.js runs this line, the process is immediately forced to terminate. Node will normally exit with a 0 status code when no more async operations are pending. The following status codes are used in other cases:

• 1 Uncaught Fatal Exception - There was an uncaught exception, and it was not handled by a domain or an uncaughtException event handler.
• 2 - Unused (reserved by Bash for builtin misuse)
• 3 Internal JavaScript Parse Error - The JavaScript source code internal in Node's bootstrapping process caused a parse error. This is extremely rare, and generally can only happen during development of Node itself.
• 4 Internal JavaScript Evaluation Failure - The JavaScript source code internal in Node's bootstrapping process failed to return a function value when evaluated. This is extremely rare, and generally can only happen during development of Node itself.
• 5 Fatal Error - There was a fatal unrecoverable error in V8. Typically a message will be printed to stderr with the prefix FATAL ERROR.
• 6 Non-function Internal Exception Handler - There was an uncaught exception, but the internal fatal exception handler function was somehow set to a non-function, and could not be called.
• 7 Internal Exception Handler Run-Time Failure - There was an uncaught exception, and the internal fatal exception handler function itself threw an error while attempting to handle it. This can happen, for example, if a process.on('uncaughtException') or domain.on('error') handler throws an error.
• 8 - Unused. In previous versions of Node, exit code 8 sometimes indicated an uncaught exception.
• 9 - Invalid Argument - Either an unknown option was specified, or an option requiring a value was provided without a value.
• 10 Internal JavaScript Run-Time Failure - The JavaScript source code internal in Node's bootstrapping process threw an error when the bootstrapping function was called. This is extremely rare, and generally can only happen during development of Node itself.
• 12 Invalid Debug Argument - The --debug and/or --debug-brk options were set, but an invalid port number was chosen.
• >128 Signal Exits - If Node receives a fatal signal such as SIGKILL or SIGHUP, then its exit code will be 128 plus the value of the signal code. This is a standard Unix practice, since exit codes are defined to be 7-bit integers, and signal exits set the high-order bit, and then contain the value of the signal code.

What is the difference between cluster and non-cluster Index? | A clustered index defines the order in which data is physically stored in a table. There can be only one clustered index per table. In SQL Server, the primary key constraint automatically creates a clustered index on that particular column. A non-clustered index doesn’t sort the physical data inside the table. In fact, a non-clustered index is stored at one place and table data is stored in another place.

What are user defined functions? What are all types of user defined functions? | Like functions in programming languages, SQL Server user-defined functions are routines that accept parameters, perform an action, such as a complex calculation, and return the result of that action as a value. The return value can either be a single scalar value or a result set.

• Scalar Function: User-defined scalar functions return a single data value of the type defined in the RETURNS clause. For an inline scalar function, the returned scalar value is the result of a single statement. For a multistatement scalar function, the function body can contain a series of Transact-SQL statements that return the single value. The return type can be any data type except text, ntext, image, cursor, and timestamp.
• Table-Valued Functions: User-defined table-valued functions return a table data type. For an inline table-valued function, there is no function body; the table is the result set of a single SELECT statement.
• System Functions: SQL Server provides many system functions that you can use to perform a variety of operations. They cannot be modified.