JavaScript & JQuery by Jon Ducket

• Every statement in a script lives in one of three execution contexts:
  • Global context: code that is in the script, but not in a function. There is only one global context per page.
  • Function context: code that is being run within a function. Each function has its own function context.
  • Eval context (not shown): Text is executed like code in an internal function called eval()
• The JavaScript interpreter processes one line of code at a time. When a statement needs data from another function, it stacks (or piles) the new function on top of the current task.
• Each time a script enters a new execution context, there are two phases of activity:
  1. Prepare:
     • The new scope is created
     • Variables, functions, and arguments are created.
     • The preparation phase is often described as taking all of the variables and functions and hoisting them to the top of the execution context.
  2. Execute
     • Now it can assign values to variables
     • Reference functions and run their code
     • Execute statements In the interpreter, each execution context has its own variables object. It holds the variables, functions, and parameters available within it. Each execution context can also access its parent’s variables object.
• Functions in JavaScript are said to have lexical scope. They are linked to the object they are defined within. The scope is the current execution context’s variables object plus the variables object for each parent execution context.
• Debugging is the process of finding errors. It involves a process of deduction.
• The console helps narrow down the area in which the error is located, so you can try to find the exact error. JavaScript has 7 different types of errors. Each creates its own error object, which can tell you its line number and gives a description of the error.
• Handle errors gracefully using try, catch, throw and finally statements.