Js interpreter interpreted

The JavaScript
Interpreter, Interpreted
@marthakelly

Software’s Primary Technical Imperative is
_________________

Software’s Primary Technical Imperative is
managing complexity

JavaScript is (usually) interpreted
Source Code >> Parser >> AST >> Bytecode >>
Machine Code

JavaScript is...really wonderful
● (nearly) everything is an object
● functions are first class objects
○ variables
○ object methods
○ arrays
○ passed as arguments
○ returned as values

To understand JavaScript you must understand
how the JavaScript creates function objects.

Function Objects have two stages
1. Creation
a. << magic happens!>>
b. determines scope and context
2. Execution
a. function is run

It’s all about (Execution) Context
All JavaScript code is executed through an
execution context
● Global
● Function
● Eval

Execution Context Stacks
var kitty = function() {
console.log(“kitty”);
}
var hello = function() {
console.log(“hello”);
kitty();
}
hello();
Stacks:
[[ Function Kitty Context ]]
[[ Function Hello Context ]]
[[ Global Execution Context ]]

Execution Context is an “object”
var executionContext = {
variableObject: {},
scopeChain: {},
this: {}
}

var executionContext = {
variableObject: {
arguments: {
parameterName: argumentValue;
},
functionName: pointer;*
variableName: undefined;*
}
...

var kitten = function() {
console.log(mew);
var mew = 'Mew!';
}
// is interpreted as
var kitten = function() {
var mew = undefined;
console.log(mew);
mew = 'Mew!';
}
kitten() // undefined

The properties created on the Variable object
for local variable declarations initially
assigned as undefined.

Despite JavaScript’s C-like syntax, JavaScript
does not have block level scoping.
JavaScript has function scope (lexical scoping)
Scope

Scope chain
functions have the scope chain of the execution context in
which they are created assigned to their internal [[scope]]
property.
+------------------------------+
| global variable obj |
+------------------------------+
^
|
+-----------------------------+
| variable obj for outer call |
+-----------------------------+
^
|
+-----------------------------+
| variable obj for inner call |
+-----------------------------+

Resolving Identifiers
JavaScript traverses up the scope chain, moving
locally to globally.
Note:
Identifiers are resolved against the scope chain. ECMA
262 categorizes this as a keyword rather than an
identifier.

“this”
this holds a reference to the object that the
function is being applied to.
This doesn't necessarily means that this will equal
the object where the function is stored.

GO AND FIND BEE AND PUPPYCAT ON YOUTUBE NOW

var bee = {name: 'Bee'};
var puppycat = {name: 'PuppyCat'};
bee.sayHello = function(){
return "Hi, I'm " + this.name;
}
puppycat.sayHello = bee.sayHello;

var bee = {name: 'Bee'};
var puppycat = {name: 'PuppyCat'};
bee.sayHello = function(){
return "Hi, I'm " + this.name;
}
puppycat.sayHello = bee.sayHello;
// Hi, I’m PuppyCat

“this”
● top level function
○ global object**
● method
○ the object it’s applied to
● constructor
○ the created object

Omg, Closures
A closure is created when a function
remembers the variables in its scope when it
was created.

Hopefully now you know more about
how JS works
Thanks, everyone!
@marthakelly

Js interpreter interpreted

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