Section 2: Execution Contexts and Lexical Environments

Question 1

Name/Value Pair

Answer 1

A name which maps to a unique value.

The name may be defined more than once, but only can have one value in any given context.

That value may be more name/value pairs.

Question 2

Object

Answer 2

A collection of name value pairs. the simplest definition when talking about Javascript.

This is in Javascript, Object might be different or more complicated in other programming languages.

Question 3

When a javascript file is run, an execution context is created. What two things are created for you when that happens?

Answer 3

1. A Global object

2. a special variable called ‘this’

Question 4

Global Object

Answer 4

Base execution context. There will be a different global object if you’re running on a server but there will always be a global object when running Javascript.

Global Object is available to ALL code running inside the javascript file, the lexical environment.

Global object inside browsers is the ‘window’ object and ‘this’ then refers to the global object, ‘window’.

Global object means “not inside a function”

Question 5

What happens to variables and functions when lexically they’re not sitting inside a function?

Answer 5

They become attached to the window (when running javascript in a browser) or global object.

So, ‘this’ is the window object.

Question 6

Hoisting

Answer 6

tbd…

Question 7

The execution context: creation phase

Answer 7

Phase 1 of execution context

Global object setup, ‘this’, Outer Environment

Also sets up memory space for variables and functions aka “hoisting”

Question 8

Javascript and ‘undefined’

Answer 8

‘undefined’ is a special value in javascript that means the value of the variable has not been set.

DIFFERENT than a reference error… if a variable is set up with the memory space during execution context creation phase, the variable will have a value called ‘undefined’

Question 9

When should you declare a value of ‘undefined’?

Answer 9

NEVER

Question 10

the execution context: execution phase

Answer 10

Phase 2 of execution context

Runs your code line by line, interpreting it, converting it, compiling it, executing it on the computer into something the computer can understand.

Question 11

single threaded (execution)

Answer 11

One command is being executed at a time

Question 12

synchronous execution

Answer 12

One [line of code] at a time AND in order

Question 13

[function] invocation

Answer 13

running or “calling” a function

In Javascript, by using parenthesis ()

Every invocation creates a new execution context which goes through both phases and gets put on the (top of the) execution stack.

Question 14

Variable environment

Answer 14

Where the variables live.

And how they relate to each other in memory.

Question 15

Scope

Answer 15

Where a variable is available in your code

and if it’s truly the same variable, or a new copy

Question 16

Block scoping

Answer 16

In ES6 the variable c that’s inside of the block will not be accessible outside of that block. This is a more familiar concept in other programming languages. You can also use let and var in ES6.

if (a > b) { let c = true; }

Question 17

Asynchronous

Answer 17

More than one at a time.