Section 2: Execution Context and Lexical Environments

Question 1

What is a Syntax Parser

Answer 1

• A program - Read the code - Determines what it does Check if grammar is valid

Question 2

What is a Lexical Environment

Answer 2

Where something sits physically in the code Were code is written is important

Question 3

In this code, where is sitting “var a” lexically

function hello() {

var a = “hello world”;

}

Answer 3

inside the function

Question 4

What is an Execution Context

Answer 4

A wrapper to manage the code that is running. The section of code that is running. There are lots of lexical environments. Which one is currently running is managed via execution contexts. It can contain things beyond what you’ve written in your code

Question 5

What is a Name/Value Pair?

Answer 5

Is a name which maps to a unique value.

Question 6

In a name/value pair, can we define more than one name/value pair?

Answer 6

Yes

Question 7

In a name/value pair, a name, can hold more than one value in any given context?

Answer 7

No, just one value

Question 8

in any particular execution context, a name can only exist and be defined with one value?

Answer 8

Yes

Question 9

in any particular execution context, a name/value pair, can hold more name/value pairs?

Answer 9

Yes

Question 10

what is an An Object in JS?

Answer 10

Is a collection of name/value pairs(and that value can be a collection of more name/value pairs).

Question 11

Whenever code is run in JS, where do we say it´s running?

Answer 11

Inside an execution context

Question 12

What is the base execution context?

Answer 12

the global execution context It´s accesible everywhere to everything in the code

Question 13

A global execution context creates what things?

Answer 13

A global object(window) A special variable called this

Question 14

at the global level, what object and keyword are equal?

Answer 14

the window object, and “this”

Question 15

When we mean Global in JS we mean, what?

Answer 15

Not inside a function

Question 16

The execution context is created in how many phases?

Answer 16

2 phases

Question 17

Name the two phases of an execution context?

Answer 17

1st: Creation phase 2nd: Execution phase

Question 18

What happens in the Creation phase(execution context) in the global environment

Answer 18

• We set up the global object within memory
• creation of “this” keyword
• An outer environment set to null
• variables are set, with a value of undefined
• functions definitions are set with their entire code

Question 19

In the creation phase(execution context) the parser recognize what? and sets up what?

Answer 19

were you´ve created variables and functions and sets up memory space for variables and functions

Question 20

In the creationg phase(execution context) a function how is stored in memory?

Answer 20

its entirety placed into memory, its name and code

Question 21

What is undefined in JS?

Answer 21

It’s actually a special value that JS has within it internally that means that the variable hasn’t been set

Question 22

What’s happening here: console.log(a) // it’s undefined var a = 5; console.log(a) // 5

Answer 22

When we declare “a”, it’s placed into memory during the creation phase. So the execution context saw “var a” and set up “a” in memory, and so even though i haven’t set it to a value, the JS engine, already set it to this special value called undefined.

Question 23

What is happening here:

console.log(a) // a is not declared -> “Uncaught ReferenceError: a is not defined”

Answer 23

That’s because that initial execution context creation, that creation phase, when it went through it didn’t find a “var a” so it never set up the memory space. So when it went to execute this code it said “hey, I don’t have a in memory at all, so it gave you an Uncaught Reference, a is not defined. I don’t have it in memory”

Question 24

What is the name of the second phase in the execution context?

Answer 24

Execution phase

Question 25

what happens in the execution phase?

Answer 25

In the execution phase we already have all those things set up that we had before, and now it runs your code, the code you’ve written, line by line, interpreting it, converting it, compiling it, executing it on the computer into something the computer can understand.

Question 26

What means single threaded

Answer 26

One command is executed at a time JS isn’t the only thing happening in the browser, so when we say JS is single threaded, it may not be under the hood of the browser, but from our perspective as programmers, JS behaves in a single threaded manner.

Question 27

What it means Synchronous execution

Answer 27

In the purposes of programming, one at a time. One line of code being executed for synchronous execution at a time and, in the order that it appears.

Question 28

what it means single threaded synchronous execution

Answer 28

In JS, only one thing is happening at a time.

Question 29

What is invocation in JS

Answer 29

Running or calling a function, in JS by using the parenthesis()

Question 30

what is the execution stack?

Answer 30

one execution context on top of the other on top of the other, And whichever one is on top is the one that’s currently running. So any time you execute or invoke a function in JS, a new execution context is created and put on the execution stack

Question 31

How function invocation happens in JS

Answer 31

Every function creates a new execution context which runs through the create phase, then the execution phase were it executes the code line by line within the function

Question 32

What are Variable Environments?

Answer 32

Were the variables lives, and how they relate to each other in memory.

Question 33

What happens with a variable that it´s created inside a function?

Answer 33

Every variable created and settled inside a function, it will live inside that execution context(the function execution context) and it will be different from others functions and the global execution context.

Every variable is attached to the variable environment of their own execution context. It’s the same to say that every execution context has their own variable environment.

Question 34

Every execution context has a reference to its outer environment?

Answer 34

Yes

Question 35

What is the outer environment?

And explain what happens when a variables is called in a function, and how this relate to the outer environment

Answer 35

The outer environment it’s related to the Lexical environment, it’s, where the function sits lexically (phisically in the code)

When a function searches for a variable, it will look first for the variable in their own variable environment on their own execution context, if it’s not found, it will take a look at the reference to the outer environment , and look at the variable at that point.

Question 36

When the outer reference is created?

Answer 36

When a function invocation takes place.

Question 37

How it’s call to the act of searching for a variable in the chain of references to outer environments

Answer 37

The scope chain

Question 38

In terms of Scope Chain

What scope means

and

What Chain means

Answer 38

Scope means “where a variable is available in your code, and if’s truly the same variable, or a new copy”

Chapin means “those links of outer environments references”

Question 39

Name anoteher way of thinking about “the reference to the outer environment”

Answer 39

Who created me?

the outer reference is to the execution context in which the function was created

“la referencia externa es hacia el contexto de ejecución en donde la función fue creada”

Question 40

In ES6 exist new way of declaring a variable, what is it? and what’s the purpose, and when can we access that variable?

Answer 40

its “let”

Purpose: allows the JS engine to use what’s called block scoping.

We can access a variable declared with “let” only after we declared the variable,if we called before, we get an error

Question 41

A variable declared with “let”, if we called before its declared, we can´t access it, but it’s already in memory?

Answer 41

Yes, its in memory, but the engine just won´t allow to access it.

Question 42

where lives a “let” variable, and what happens if we are using it inside a loop?

Answer 42

it lives inside a block, a block is defined by curly braces { }

If we use it inside a loop, we are going to get a different variable in memory each time the loop is running