Functions pt2

Overview

Functions Review

Storing function evaluations

Functions within functions

Intro to Scope

Review

var add = function (a, b) {

  return a + b;

};

add(1, 2);

declaration / definition

invocation / call time

< Note 'function'        keyword

< Note  invocation operator ()

Review

var add = function (a, b) {


    return a + b;

};

add(1, 2);

parameters

body

parameters

why functions?

DRY: Do not repeat your code!

Expression: More expressive code!

User Input: We can create actions

without knowing the user input.

Storing INvocations

The value of a function is what is placed after its return statement.

var add = function(a, b){
  return a + b;
};

add(4, 5); // => 9

Storing INvocations

Like anything else, we can assign that value to a variable.

var add = function(a, b){
    return a + b;
};

var nine = add(4, 5);

console.log(nine); // => 9

Storing INvocations

Why would we do this?

var add = function(a, b){
    return a + b;
};

var nine = add(4, 5);

console.log(nine); // => 9

Storing INvocations

Why save function evaluations?

var nine = add(4, 5);

var square = function(a){
  return a * a;
};

var eightyOne = square(nine);

console.log(eightyOne); // => 81

Storing INvocations

Allows us to separate functionality.

// `add` will get us the number that we want.
var nine = add(4, 5);

var square = function(a){
  return a * a;
};

// `square` will square what we want
var eightyOne = square(nine);

console.log(eightyOne); // => 81

Function INception

More complex functionality...

var addAndSquare = function(a, b){
    var sum = a + b;
    var square = sum * sum;
    return square;
};

// This technique is called `imperative` 
// programming. We are building out each step bit
// by bit.
var addAndSquare = function(a, b){
    var sum = add(a, b);
    var squareSum = sum * sum;
    return squareSum;
};

// We have replaced our a + b with 
// our add function invoked with our 
// parameters. 
var addAndSquare = function(a, b){
    var sum = add(a, b);
    var squareSum = square(sum);
    return squareSum;
};

// We have replaced sum * sum with 
// our square function invoked with our 
//sum variable.

Two types of scope

Global Scope

Local Scope

VS

Variables in the global scope can be accessed anywhere, and by anything in your function.

Variables in the local scope can only be accessed in that scope, or in 'child'  scopes relative to that function.

Intro To Scope

  1. Separate and modularize data and functionality. 
     
  2. Makes sure we avoid polluting the global namespace.
     
  3. CLOSURES. We'll get to this later in the week. It's pretty cool. 

WHy is scope important?

Intro To Scope

Intro To Scope

Intro to Scope

var globalScope = true;

var addAndSquare = function(a, b){
    var sum = add(a, b);
    var squareSum = square(sum);
    return square;
};

Scope is local to the `addAndSquare function`

Scope is global to the `addAndSquare` function

Scope is the parent scope to the 

`addAndSquare` function

Scope is child scope to the global object.

Function INception

Let's dive deeper into addAndSquare

var addAndSquare = function(a, b){
    var sum = add(a, b)
    var square = square(sum)
    return square;
};

var eightyOne = addAndSquare(4, 5)
// inside of the local scope addAndSquare
   // passing the arguments into `add` child scope
    var sum = add(4, 5)

// inside local scope of `add` function
    
    return 4 + 5



// inside of the local scope addAndSquare
    var sum = add(4, 5)
// passing our sum into the square function
    var square = square(sum)
    return square;
// inside local scope of `square` function
    
    return sum * sum



// back inside local scope of addAndSquare function
    var sum = add(4, 5)
    var square = square(sum)
    // returning the square variable
    return square;

// back in the global scope of the program
var addAndSquare = function(a, b){
    var sum = add(a, b)
    var square = square(sum)
    return square;
};

// saving the returned results to the variable eightyOne
var eightyOne = addAndSquare(4, 5)
var addAndSquare = function(a, b){
    var sum = add(a, b) // currently here
    var square = square(sum)
    return square;
};
var add = function(a, b){
    return a + b // currently here
};
var addAndSquare = function(a, b){
    var sum = add(a, b)
    // currently here
    var square = square(sum)
    return square;
};
var square = function(num) {
    return num * num // currently here
}
var addAndSquare = function(a, b){
    var sum = add(a, b)
    var square = square(sum)
    // currently here
    return square;
};

var eightyOne = addAndSquare(4, 5)
// currently here
var globalScope = true
var addAndSquare = function(a, b){
    var sum = add(a, b)
    var square = square(sum)
    return square;
};

var eightyOne = addAndSquare(4, 5)

Exercise Time!

https://github.com/TelegraphPrep/06-2016-calculator

Prep+ Week 2: Functions + Intro To Scope

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Prep+ Week 2: Functions + Intro To Scope

An introduction to functional programming

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