02: Tools and Context

Learning Objectives

• Students will understand the difference between context and scope
• Students will be able to control a functions context using call, apply, and bind
• Students will be able to handle thrown errors using try and catch
• Understand the role context plays in constructor functions
• Students will be able to define npm scripts for automating command line tasks

Resources

Package.json

• Read About package.json

NPM Scripts

• Skim NPM scripts as build tools
• Skim NPM scripts docs
• Skim semver

Context

• Read MDN this
• Watch Javascript context tutorial

Prototype

• Read MDN inheritance and the prototype chain
• Skim MDN New
• Skim MDN object prototype

Errors

• Read Node error docs

package.json

• A package.json is a config file used for configuring metadata about a node module.
• It only requires a name and version field, but a typical package.json has much more information.
• Dependencies and Dev-dependencies
  • Dependencies : a list of packages required to run the main program
  • Dev-dependencies : a list of packages required to develop the module
• NPM scripts are used to manage common tasks for working a node project
• Setting up a project checklist
  • Create a new directory named after your project
  • Navigate inside your directory
  • Run npm init and answer the questions
  • Now you should find a package.json inside your current directory :)

Context vs Scope

• Every function invocation has both a scope and a context associated with it.
• Scope is function-based while context is object-based.
• Scope pertains to the variable access of a function when it is invoked and is unique to each invocation.
• Context is always the value of the this keyword which is a reference to the object that “owns” the currently executing code.

Node Errors

Error

• a generic error
• .stack - a String. Describes the point in the code where the Error was instantiated
• .message - a String. Description set by calling the new Error(message)

ReferenceError

• Indicates that an attempt is being made to access a variable that is not defined
• ReferenceError is a subclass of Error

SyntaxError

• indicates a program is not valid javascript
• SyntaxError is a subclass of Error

TypeError

• indicates that a provided argument is not an allowable type
• TypeError is a subclass of Error

SystemError

• .code - A String describing the error code
• .errno - A Number describing the error code
• .syscall - A String describing the syscall that failed
• SystemError is not a subclass of Error
• Common System Errors
• EACCESS - An attempt to access a file without the right permissions
• EADDRINUSE - An attempt to start a server on a PORT that is already in use
• ECONNREFUSED - A connection was deliberately refused by the target machine
• ECONNRESET - A connection was forcibly closed by a peer
• EEXIST - A file exists and the attempted action required that it didn't
• EISDIR - An action expected to act on a file but found a directory
• EMFILE - To many files were open for your operating system to handle
• ENOENT - An action expected a file, but did not find one
• ENOTDIR - An action expected a directory, but found something else
• ENOTEMPTY - An action expected an empty directory, but found one with data in it
• EPERM - An attempt to do something that you currently don't have permissions to do
• EPIPE - An attempt to write data to a connection that had been closed

Throw Try Catch

• If an un handled error is thrown in javascript the program will crash
• try catch blocks allow you to safely throw a an error and handle it

Pass by reference vs Pass by value

• Objects are passed by reference
• Strings, Numbers, Bools, undefined, and null are passed by value

Call, Bind, Apply

• when a function has a this we say that this is the functions context
• unlike scope a functions context can be configured
• If a function is not a property on an object, by default it has no context
• If a function is a property on an object, by default that object is the context for that function
• call, bind, and apply are function prototype methods that allow us to change the context of a function
• call is a methods on a function that invokes a function with a specified context and arguments
• call passes comma separated arguments
• apply is a methods on a function that invokes a function with a specified context and arguments
• apply passes arguments from an array
• don't mess with __proto__ its slow not even to read a property
• if you want to determine an objects prototype use Object.getPrototypeOf(someObject)
• don't nest a lot of prototype's it will have a speed impact on your code
• if you look up a property that is not on any object on the prototype chain it will still look through the whole prototype chain