diff --git a/chp00_introduction/NOC_I_01_RandomWalkTraditional/sketch.js b/chp00_introduction/NOC_I_01_RandomWalkTraditional/sketch.js
index 591d18a..0ade478 100644
--- a/chp00_introduction/NOC_I_01_RandomWalkTraditional/sketch.js
+++ b/chp00_introduction/NOC_I_01_RandomWalkTraditional/sketch.js
@@ -27,7 +27,7 @@ class Walker {
   }
 
   step() {
-    var choice = floor(random(4));
+    let choice = floor(random(4));
     if (choice === 0) {
       this.x++;
     } else if (choice == 1) {
diff --git a/chp01_vectors/NOC_1_10_motion101_acceleration/mover.js b/chp01_vectors/NOC_1_10_motion101_acceleration/mover.js
index 4012070..e0dde1c 100644
--- a/chp01_vectors/NOC_1_10_motion101_acceleration/mover.js
+++ b/chp01_vectors/NOC_1_10_motion101_acceleration/mover.js
@@ -14,7 +14,7 @@ class Mover{
 
   update() {
     // Compute a vector that points from position to mouse
-    var mouse = createVector(mouseX,mouseY);
+    let mouse = createVector(mouseX,mouseY);
     this.acceleration = p5.Vector.sub(mouse,this.position);
     // Set magnitude of acceleration
     this.acceleration.setMag(0.2);
diff --git a/chp01_vectors/NOC_1_11_motion101_acceleration_array/mover.js b/chp01_vectors/NOC_1_11_motion101_acceleration_array/mover.js
index d6c105f..ab0a1e6 100644
--- a/chp01_vectors/NOC_1_11_motion101_acceleration_array/mover.js
+++ b/chp01_vectors/NOC_1_11_motion101_acceleration_array/mover.js
@@ -14,7 +14,7 @@ class Mover{
 
   update() {
     // Compute a vector that points from position to mouse
-    var mouse = createVector(mouseX,mouseY);
+    let mouse = createVector(mouseX,mouseY);
     this.acceleration = p5.Vector.sub(mouse,this.position);
     // Set magnitude of acceleration
     this.acceleration.setMag(0.2);
diff --git a/chp01_vectors/NOC_1_11_motion101_acceleration_array/sketch.js b/chp01_vectors/NOC_1_11_motion101_acceleration_array/sketch.js
index 9699fc4..8cd9792 100644
--- a/chp01_vectors/NOC_1_11_motion101_acceleration_array/sketch.js
+++ b/chp01_vectors/NOC_1_11_motion101_acceleration_array/sketch.js
@@ -10,7 +10,7 @@ let movers = [];
 
 function setup() {
   createCanvas(640,360);
-  for (var i = 0; i < 20; i++) {
+  for (let i = 0; i < 20; i++) {
      movers[i] = new Mover();
   }
 }
diff --git a/chp02_forces/NOC_2_01_forces/mover.js b/chp02_forces/NOC_2_01_forces/mover.js
index 885ea8a..789890f 100644
--- a/chp02_forces/NOC_2_01_forces/mover.js
+++ b/chp02_forces/NOC_2_01_forces/mover.js
@@ -11,7 +11,7 @@ class Mover {
   }
 
   applyForce(force) {
-    var f = p5.Vector.div(force, this.mass);
+    let f = p5.Vector.div(force, this.mass);
     this.acceleration.add(f);
   }
 
diff --git a/chp04_systems/NOC_4_02_ArrayParticles/sketch.js b/chp04_systems/NOC_4_02_ArrayParticles/sketch.js
index 0cb2c12..11cf66b 100644
--- a/chp04_systems/NOC_4_02_ArrayParticles/sketch.js
+++ b/chp04_systems/NOC_4_02_ArrayParticles/sketch.js
@@ -13,8 +13,8 @@ function draw() {
   particles.push(new Particle(createVector(width / 2, 50)));
 
   // Looping through backwards to delete
-  for (var i = particles.length - 1; i >= 0; i--) {
-    var p = particles[i];
+  for (let i = particles.length - 1; i >= 0; i--) {
+    let p = particles[i];
     p.run();
     if (p.isDead()) {
       //remove the particle
diff --git a/chp04_systems/NOC_4_05_ParticleSystemInheritancePolymorphism/confetti.js b/chp04_systems/NOC_4_05_ParticleSystemInheritancePolymorphism/confetti.js
index a558fc2..f133380 100644
--- a/chp04_systems/NOC_4_05_ParticleSystemInheritancePolymorphism/confetti.js
+++ b/chp04_systems/NOC_4_05_ParticleSystemInheritancePolymorphism/confetti.js
@@ -13,7 +13,7 @@ class Confetti extends Particle {
     strokeWeight(2);
     push();
     translate(this.position.x, this.position.y);
-    var theta = map(this.position.x, 0, width, 0, TWO_PI * 2);
+    let theta = map(this.position.x, 0, width, 0, TWO_PI * 2);
     rotate(theta);
     rect(0, 0, 12, 12);
     pop();
diff --git a/chp06_agents/NOC_6_01_Seek/vehicle.js b/chp06_agents/NOC_6_01_Seek/vehicle.js
index 430a055..b5e4991 100644
--- a/chp06_agents/NOC_6_01_Seek/vehicle.js
+++ b/chp06_agents/NOC_6_01_Seek/vehicle.js
@@ -34,13 +34,13 @@ class Vehicle {
   // STEER = DESIRED MINUS VELOCITY
   seek(target) {
 
-    var desired = p5.Vector.sub(target, this.position); // A vector pointing from the location to the target
+    let desired = p5.Vector.sub(target, this.position); // A vector pointing from the location to the target
 
     // Scale to maximum speed
     desired.setMag(this.maxspeed);
 
     // Steering = Desired minus velocity
-    var steer = p5.Vector.sub(desired, this.velocity);
+    let steer = p5.Vector.sub(desired, this.velocity);
     steer.limit(this.maxforce); // Limit to maximum steering force
 
     this.applyForce(steer);
diff --git a/chp06_agents/NOC_6_02_Arrive/vehicle.js b/chp06_agents/NOC_6_02_Arrive/vehicle.js
index 1176072..1a7409c 100644
--- a/chp06_agents/NOC_6_02_Arrive/vehicle.js
+++ b/chp06_agents/NOC_6_02_Arrive/vehicle.js
@@ -37,7 +37,7 @@ class Vehicle {
     let d = desired.mag();
     // Scale with arbitrary damping within 100 pixels
     if (d < 100) {
-      var m = map(d, 0, 100, 0, this.maxspeed);
+      let m = map(d, 0, 100, 0, this.maxspeed);
       desired.setMag(m);
     } else {
       desired.setMag(this.maxspeed);
diff --git a/chp07_CA/Exercise_7_01_WolframCA_randomizedrules/CA.js b/chp07_CA/Exercise_7_01_WolframCA_randomizedrules/CA.js
index 779bcac..8790b8b 100644
--- a/chp07_CA/Exercise_7_01_WolframCA_randomizedrules/CA.js
+++ b/chp07_CA/Exercise_7_01_WolframCA_randomizedrules/CA.js
@@ -15,14 +15,14 @@ function CA(r) {
 
   // Make a random ruleset
   this.randomize = function() {
-  	for (var i = 0; i < 8; i++) {
+  	for (let i = 0; i < 8; i++) {
   		this.ruleset[i] = Math.floor(random(2));
   	}
   };
 
   // Reset to generation 0
   this.restart = function() {
-  	for (var i = 0; i < this.cells.length; i++) {
+  	for (let i = 0; i < this.cells.length; i++) {
   		this.cells[i] = 0;
   	}
     // We arbitrarily start with just the middle cell having a state of "1"
@@ -34,13 +34,13 @@ function CA(r) {
   // The process of creating the new generation
   this.generate = function() {
     // First we create an empty array for the new values
-    var nextgen = new Array(this.cells.length);
+    let nextgen = new Array(this.cells.length);
     // For every spot, determine new state by examing current state, and neighbor states
     // Ignore edges that only have one neighor
-    for (var i = 1; i < this.cells.length-1; i++) {
-      var left = this.cells[i-1];   // Left neighbor state
-      var me = this.cells[i];       // Current state
-      var right = this.cells[i+1];  // Right neighbor state
+    for (let i = 1; i < this.cells.length-1; i++) {
+      let left = this.cells[i-1];   // Left neighbor state
+      let me = this.cells[i];       // Current state
+      let right = this.cells[i+1];  // Right neighbor state
       nextgen[i] = this.rules(left, me, right); // Compute next generation state based on ruleset
     }
     // The current generation is the new generation
@@ -50,7 +50,7 @@ function CA(r) {
 
   // This is the easy part, just draw the cells, fill 255 for '1', fill 0 for '0'
   this.display = function() {
-  	for (var i = 0; i < this.cells.length; i++) {
+  	for (let i = 0; i < this.cells.length; i++) {
   		if (this.cells[i] == 1) fill(200);
   		else                    fill(51);
   		noStroke();
diff --git a/chp07_CA/Exercise_7_01_WolframCA_randomizedrules/sketch.js b/chp07_CA/Exercise_7_01_WolframCA_randomizedrules/sketch.js
index d58708b..ad34112 100644
--- a/chp07_CA/Exercise_7_01_WolframCA_randomizedrules/sketch.js
+++ b/chp07_CA/Exercise_7_01_WolframCA_randomizedrules/sketch.js
@@ -9,15 +9,15 @@
 // Mouse click restarts as well
 
  // An object to describe a Wolfram elementary Cellular Automata
-var ca;
+let ca;
 
-var delay = 0;
+let delay = 0;
 
 function setup() {
   createCanvas(640, 360);
   background(51);
   // An initial rule system
-  var ruleset = [0, 1, 0, 1, 1, 0, 1, 0];
+  let ruleset = [0, 1, 0, 1, 1, 0, 1, 0];
   ca = new CA(ruleset);
 }
 
diff --git a/chp07_CA/Exercise_7_04_WolframCA_scrolling/CA.js b/chp07_CA/Exercise_7_04_WolframCA_scrolling/CA.js
index 173bcee..8ca494f 100644
--- a/chp07_CA/Exercise_7_04_WolframCA_scrolling/CA.js
+++ b/chp07_CA/Exercise_7_04_WolframCA_scrolling/CA.js
@@ -16,14 +16,14 @@ function CA(r) {
   this.rows = height/this.w;
   // Store a history of generations in 2D array, not just one
   this.matrix = new Array(this.cols);
-  for( var i = 0; i < this.cols; i++) {
+  for(let i = 0; i < this.cols; i++) {
     this.matrix[i] = new Array(this.rows);
   }
 
   // Reset to generation 0
   this.restart = function() {
-    for (var i = 0; i < this.cols; i++) {
-      for (var j = 0; j < this.rows; j++) {
+    for (let i = 0; i < this.cols; i++) {
+      for (let j = 0; j < this.rows; j++) {
         this.matrix[i][j] = 0;
       }
     }
@@ -34,7 +34,7 @@ function CA(r) {
 
     // Make a random ruleset
   this.randomize = function() {
-    for (var i = 0; i < 8; i++) {
+    for (let i = 0; i < 8; i++) {
       this.ruleset[i] = Math.floor(random(2));
     }
   };
@@ -47,10 +47,10 @@ function CA(r) {
 
     // For every spot, determine new state by examing current state, and neighbor states
     // Ignore edges that only have one neighor
-    for (var i = 0; i < this.cols; i++) {
-      var left  = this.matrix[(i+this.cols-1)%this.cols][this.generation%this.rows];   // Left neighbor state
-      var me    = this.matrix[i][this.generation%this.rows];       // Current state
-      var right = this.matrix[(i+1)%this.cols][this.generation%this.rows];  // Right neighbor state
+    for (let i = 0; i < this.cols; i++) {
+      let left  = this.matrix[(i+this.cols-1)%this.cols][this.generation%this.rows];   // Left neighbor state
+      let me    = this.matrix[i][this.generation%this.rows];       // Current state
+      let right = this.matrix[(i+1)%this.cols][this.generation%this.rows];  // Right neighbor state
       this.matrix[i][(this.generation+1)%this.rows] = this.rules(left, me, right); // Compute next generation state based on ruleset
     }
     this.generation++;
@@ -58,11 +58,11 @@ function CA(r) {
 
   // This is the easy part, just draw the cells, fill 255 for '1', fill 0 for '0'
   this.display = function() {
-    var offset = this.generation%this.rows;
+    let offset = this.generation%this.rows;
 
-    for (var i = 0; i < this.cols; i++) {
-      for (var j = 0; j < this.rows; j++) {
-        var y = j - offset;
+    for (let i = 0; i < this.cols; i++) {
+      for (let j = 0; j < this.rows; j++) {
+        let y = j - offset;
         if (y <= 0) y = this.rows + y;
         // Only draw if cell state is 1
         if (this.matrix[i][j] == 1) {
@@ -77,8 +77,8 @@ function CA(r) {
   // Implementing the Wolfram rules
   // This is the concise conversion to binary way
   this.rules = function(a, b, c) {
-    var s = "" + a + b + c;
-    var index = parseInt(s, 2);
+    let s = "" + a + b + c;
+    let index = parseInt(s, 2);
     return this.ruleset[index];
   };
 
diff --git a/chp07_CA/Exercise_7_04_WolframCA_scrolling/sketch.js b/chp07_CA/Exercise_7_04_WolframCA_scrolling/sketch.js
index 04b2dc0..1ca13cb 100644
--- a/chp07_CA/Exercise_7_04_WolframCA_scrolling/sketch.js
+++ b/chp07_CA/Exercise_7_04_WolframCA_scrolling/sketch.js
@@ -9,11 +9,11 @@
 // Also implements wrap around
 
 // An object to describe a Wolfram elementary Cellular Automata
-var ca;
+let ca;
 
 function setup() {
   createCanvas(360, 600);
-  var ruleset = new Array(0,1,0,1,1,0,1,0);  // Rule 90
+  let ruleset = new Array(0,1,0,1,1,0,1,0);  // Rule 90
 
   //int[] ruleset = {0,1,1,1,1,0,1,1};   // Rule 222  
   //int[] ruleset = {0,1,1,1,1,1,0,1};   // Rule 190  
diff --git a/chp07_CA/GameOfLifeWrapAround/GOL.js b/chp07_CA/GameOfLifeWrapAround/GOL.js
index 85c858c..112fd26 100644
--- a/chp07_CA/GameOfLifeWrapAround/GOL.js
+++ b/chp07_CA/GameOfLifeWrapAround/GOL.js
@@ -8,13 +8,13 @@ function GOL() {
   this.columns = width/this.w;
   this.rows = height/this.w;
   this.board = new Array(this.columns);
-  for (var i = 0; i < this.columns; i++) {
+  for (let i = 0; i < this.columns; i++) {
     this.board[i] = new Array(this.rows);
   }
 
   this.init = function() {
-    for (var i =0;i < this.columns;i++) {
-      for (var j =0;j < this.rows;j++) {
+    for (let i =0;i < this.columns;i++) {
+      for (let j =0;j < this.rows;j++) {
         this.board[i][j] = Math.floor(random(2));
       }
     }
@@ -24,19 +24,19 @@ function GOL() {
   // The process of creating the new generation
   this.generate = function() {
 
-    var next = new Array(this.columns);
-    for (var i = 0; i < this.columns; i++) {
+    let next = new Array(this.columns);
+    for (let i = 0; i < this.columns; i++) {
       next[i] = new Array(this.rows);
     }
 
     // Loop through every spot in our 2D array and check spots neighbors
-    for (var x = 0; x < this.columns; x++) {
-      for (var y = 0; y < this.rows; y++) {
+    for (let x = 0; x < this.columns; x++) {
+      for (let y = 0; y < this.rows; y++) {
         // Add up all the states in a 3x3 surrounding grid
-        var neighbors = 0;
+        let neighbors = 0;
 
-        for (var i = -1; i <= 1; i++) {
-          for (var j = -1; j <= 1; j++) {
+        for (let i = -1; i <= 1; i++) {
+          for (let j = -1; j <= 1; j++) {
             neighbors += this.board[(x+i+this.columns)%this.columns][(y+j+this.rows)%this.rows];
           }
         }
@@ -59,8 +59,8 @@ function GOL() {
 
   // This is the easy part, just draw the cells, fill 255 for '1', fill 0 for '0'
   this.display = function() {
-    for ( var i = 0; i < this.columns;i++) {
-      for ( var j = 0; j < this.rows;j++) {
+    for (let i = 0; i < this.columns;i++) {
+      for (let j = 0; j < this.rows;j++) {
         if ((this.board[i][j] == 1)) fill(0);
         else fill(255);
         stroke(0);
diff --git a/chp07_CA/GameOfLifeWrapAround/sketch.js b/chp07_CA/GameOfLifeWrapAround/sketch.js
index b9c0b3a..dc48a4f 100644
--- a/chp07_CA/GameOfLifeWrapAround/sketch.js
+++ b/chp07_CA/GameOfLifeWrapAround/sketch.js
@@ -6,7 +6,7 @@
 
 // Each cell is now an object!
 
-var gol;
+let gol;
 
 function setup() {
   createCanvas(640, 360);
diff --git a/chp07_CA/NOC_7_01_WolframCA_simple/CA.js b/chp07_CA/NOC_7_01_WolframCA_simple/CA.js
index 6c592dd..be97291 100644
--- a/chp07_CA/NOC_7_01_WolframCA_simple/CA.js
+++ b/chp07_CA/NOC_7_01_WolframCA_simple/CA.js
@@ -9,7 +9,7 @@ function CA() {
   this.w = 10;
   // An array of 0s and 1s
   this.cells = new Array(width/this.w);
-  for (var i = 0; i < this.cells.length; i++) {
+  for (let i = 0; i < this.cells.length; i++) {
     this.cells[i] = 0;
   }
    // We arbitrarily start with just the middle cell having a state of "1"
@@ -22,16 +22,16 @@ function CA() {
   // The process of creating the new generation
   this.generate = function() {
     // First we create an empty array filled with 0s for the new values
-    var nextgen = [];
-    for (var i = 0; i < this.cells.length; i++) {
+    let nextgen = [];
+    for (let i = 0; i < this.cells.length; i++) {
       nextgen[i] = 0;
     }
     // For every spot, determine new state by examing current state, and neighbor states
     // Ignore edges that only have one neighor
-    for (var i = 1; i < this.cells.length-1; i++) {
-      var left   = this.cells[i-1];   // Left neighbor state
-      var me     = this.cells[i];     // Current state
-      var right  = this.cells[i+1];   // Right neighbor state
+    for (let i = 1; i < this.cells.length-1; i++) {
+      let left   = this.cells[i-1];   // Left neighbor state
+      let me     = this.cells[i];     // Current state
+      let right  = this.cells[i+1];   // Right neighbor state
       nextgen[i] = this.rules(left, me, right); // Compute next generation state based on ruleset
     }
     // The current generation is the new generation
@@ -41,7 +41,7 @@ function CA() {
 
   // This is the easy part, just draw the cells
   this.display = function() {
-  	for (var i = 0; i < this.cells.length; i++) {
+  	for (let i = 0; i < this.cells.length; i++) {
   		if (this.cells[i] == 1) fill(200);
   		else                    fill(51);
   		noStroke();
diff --git a/chp07_CA/NOC_7_01_WolframCA_simple/sketch.js b/chp07_CA/NOC_7_01_WolframCA_simple/sketch.js
index a3953f5..ab0a719 100644
--- a/chp07_CA/NOC_7_01_WolframCA_simple/sketch.js
+++ b/chp07_CA/NOC_7_01_WolframCA_simple/sketch.js
@@ -5,7 +5,7 @@
 
 // Simple demonstration of a Wolfram 1-dimensional cellular automata
 
-var ca;
+let ca;
 
 function setup() {
   createCanvas(640, 360);
diff --git a/chp07_CA/NOC_7_02_GameOfLifeSimple/GOL.js b/chp07_CA/NOC_7_02_GameOfLifeSimple/GOL.js
index cc84f9d..251da44 100644
--- a/chp07_CA/NOC_7_02_GameOfLifeSimple/GOL.js
+++ b/chp07_CA/NOC_7_02_GameOfLifeSimple/GOL.js
@@ -8,17 +8,17 @@ function GOL() {
   this.columns = width/this.w;
   this.rows = height/this.w;
   this.board = new Array(this.columns);
-  for (var i = 0; i < this.columns; i++) {
+  for (let i = 0; i < this.columns; i++) {
     this.board[i] = new Array(this.rows);
   }
   // Going to use multiple 2D arrays and swap them
   this.next = new Array(this.columns);
-  for (var i = 0; i < this.columns; i++) {
+  for (let i = 0; i < this.columns; i++) {
     this.next[i] = new Array(this.rows);
   }
   this.init = function() {
-    for (var i =0;i < this.columns;i++) {
-      for (var j =0;j < this.rows;j++) {
+    for (let i =0;i < this.columns;i++) {
+      for (let j =0;j < this.rows;j++) {
         // Lining the edges with 0s
         if (i === 0 || j === 0 || i == this.columns-1 || j == this.rows-1) this.board[i][j] = 0;
         // Filling the rest randomly
@@ -35,13 +35,13 @@ function GOL() {
 
 
     // Loop through every spot in our 2D array and check spots neighbors
-    for (var x = 1; x < this.columns-1; x++) {
-      for (var y = 1; y < this.rows-1; y++) {
+    for (let x = 1; x < this.columns-1; x++) {
+      for (let y = 1; y < this.rows-1; y++) {
         // Add up all the states in a 3x3 surrounding grid
-        var neighbors = 0;
+        let neighbors = 0;
 
-        for (var i = -1; i <= 1; i++) {
-          for (var j = -1; j <= 1; j++) {
+        for (let i = -1; i <= 1; i++) {
+          for (let j = -1; j <= 1; j++) {
             neighbors += this.board[x+i][y+j];
           }
         }
@@ -59,15 +59,15 @@ function GOL() {
     }
 
     // Swap!
-    var temp = this.board;
+    let temp = this.board;
     this.board = this.next;
     this.next = temp;
   };
 
   // This is the easy part, just draw the cells, fill 255 for '1', fill 0 for '0'
   this.display = function() {
-    for ( var i = 0; i < this.columns;i++) {
-      for ( var j = 0; j < this.rows;j++) {
+    for (let i = 0; i < this.columns;i++) {
+      for (let j = 0; j < this.rows;j++) {
         if ((this.board[i][j] == 1)) fill(0);
         else fill(255);
         stroke(0);
diff --git a/chp07_CA/NOC_7_02_GameOfLifeSimple/sketch.js b/chp07_CA/NOC_7_02_GameOfLifeSimple/sketch.js
index 9700ec1..083c6a4 100644
--- a/chp07_CA/NOC_7_02_GameOfLifeSimple/sketch.js
+++ b/chp07_CA/NOC_7_02_GameOfLifeSimple/sketch.js
@@ -8,7 +8,7 @@
 // to describe each individual cell and a "cellular automata" class
 // to describe a collection of cells
 
-var gol;
+let gol;
 
 function setup() {
     createCanvas(640, 360);
diff --git a/chp07_CA/NOC_7_03_GameOfLifeOOP/GOL.js b/chp07_CA/NOC_7_03_GameOfLifeOOP/GOL.js
index 8a62454..f2239fa 100644
--- a/chp07_CA/NOC_7_03_GameOfLifeOOP/GOL.js
+++ b/chp07_CA/NOC_7_03_GameOfLifeOOP/GOL.js
@@ -10,13 +10,13 @@ function GOL() {
 
   // Game of life board
   this.board = new Array(this.columns);
-  for (var i = 0; i < this.columns; i++) {
+  for (let i = 0; i < this.columns; i++) {
     this.board[i] = new Array(this.rows);
   }
 
   this.init = function() {
-    for (var i = 0; i < this.columns; i++) {
-      for (var j = 0; j < this.rows; j++) {
+    for (let i = 0; i < this.columns; i++) {
+      for (let j = 0; j < this.rows; j++) {
         this.board[i][j] = new Cell(i*this.w, j*this.w, this.w);
       }
     }
@@ -25,20 +25,20 @@ function GOL() {
 
   // The process of creating the new generation
   this.generate = function() {
-    for ( var i = 0; i < this.columns;i++) {
-      for ( var j = 0; j < this.rows;j++) {
+    for (let i = 0; i < this.columns;i++) {
+      for (let j = 0; j < this.rows;j++) {
         this.board[i][j].savePrevious();
       }
     }
 
     // Loop through every spot in our 2D array and check spots neighbors
-    for (var x = 0; x < this.columns; x++) {
-      for (var y = 0; y < this.rows; y++) {
+    for (let x = 0; x < this.columns; x++) {
+      for (let y = 0; y < this.rows; y++) {
 
         // Add up all the states in a 3x3 surrounding grid
-        var neighbors = 0;
-        for (var i = -1; i <= 1; i++) {
-          for (var j = -1; j <= 1; j++) {
+        let neighbors = 0;
+        for (let i = -1; i <= 1; i++) {
+          for (let j = -1; j <= 1; j++) {
             neighbors += this.board[(x+i+this.columns)%this.columns][(y+j+this.rows)%this.rows].previous;
           }
         }
@@ -58,8 +58,8 @@ function GOL() {
 
   // This is the easy part, just draw the cells, fill 255 for '1', fill 0 for '0'
   this.display = function() {
-    for ( var i = 0; i < this.columns;i++) {
-      for ( var j = 0; j < this.rows;j++) {
+    for (let i = 0; i < this.columns;i++) {
+      for (let j = 0; j < this.rows;j++) {
         this.board[i][j].display();
       }
     }
diff --git a/chp07_CA/NOC_7_03_GameOfLifeOOP/sketch.js b/chp07_CA/NOC_7_03_GameOfLifeOOP/sketch.js
index b9c0b3a..dc48a4f 100644
--- a/chp07_CA/NOC_7_03_GameOfLifeOOP/sketch.js
+++ b/chp07_CA/NOC_7_03_GameOfLifeOOP/sketch.js
@@ -6,7 +6,7 @@
 
 // Each cell is now an object!
 
-var gol;
+let gol;
 
 function setup() {
   createCanvas(640, 360);
diff --git a/chp08_fractals/NOC_8_04_CantorSet/sketch.js b/chp08_fractals/NOC_8_04_CantorSet/sketch.js
index 3aa11cc..733476b 100644
--- a/chp08_fractals/NOC_8_04_CantorSet/sketch.js
+++ b/chp08_fractals/NOC_8_04_CantorSet/sketch.js
@@ -21,7 +21,7 @@ function draw() {
 
 function cantor(x, y, len) {
   
-  var h = 30;
+  let h = 30;
   
   // recursive exit condition
   if (len >= 1) {
diff --git a/chp08_fractals/NOC_8_05_Koch/KochFractal.js b/chp08_fractals/NOC_8_05_Koch/KochFractal.js
index caaf80e..ae8e6b7 100644
--- a/chp08_fractals/NOC_8_05_Koch/KochFractal.js
+++ b/chp08_fractals/NOC_8_05_Koch/KochFractal.js
@@ -31,7 +31,7 @@ function KochFractal() {
 
   // This is easy, just draw all the lines
   this.render = function() {
-    for(var i = 0; i < this.lines.length; i++) {
+    for(let i = 0; i < this.lines.length; i++) {
       this.lines[i].display();
     }
   }
@@ -45,15 +45,15 @@ function KochFractal() {
 
   // As we do this over and over again, each line gets broken into 4 lines, which gets broken into 4 lines, and so on. . . 
   this.iterate = function(before) {
-    var now = [];    // Create emtpy list
-    for(var i = 0; i < before.length; i++) {
-      var l = before[i];
+    let now = [];    // Create emtpy list
+    for(let i = 0; i < before.length; i++) {
+      let l = before[i];
       // Calculate 5 koch p5.Vectors (done for us by the line object)
-      var a = l.kochA();                 
-      var b = l.kochB();
-      var c = l.kochC();
-      var d = l.kochD();
-      var e = l.kochE();
+      let a = l.kochA();                 
+      let b = l.kochB();
+      let c = l.kochC();
+      let d = l.kochD();
+      let e = l.kochE();
       // Make line segments between all the p5.Vectors and add them
       now.push(new KochLine(a,b));
       now.push(new KochLine(b,c));
diff --git a/chp08_fractals/NOC_8_05_Koch/KochLine.js b/chp08_fractals/NOC_8_05_Koch/KochLine.js
index 59cd133..adcfa17 100644
--- a/chp08_fractals/NOC_8_05_Koch/KochLine.js
+++ b/chp08_fractals/NOC_8_05_Koch/KochLine.js
@@ -24,7 +24,7 @@ function KochLine(a,b) {
 
   // This is easy, just 1/3 of the way
   this.kochB = function() {
-    var v = p5.Vector.sub(this.end, this.start);
+    let v = p5.Vector.sub(this.end, this.start);
     v.div(3);
     v.add(this.start);
     return v;
@@ -32,8 +32,8 @@ function KochLine(a,b) {
 
   // More complicated, have to use a little trig to figure out where this p5.Vector is!
   this.kochC = function() {
-    var a = this.start.copy(); // Start at the beginning
-    var v = p5.Vector.sub(this.end, this.start);
+    let a = this.start.copy(); // Start at the beginning
+    let v = p5.Vector.sub(this.end, this.start);
     v.div(3);
     a.add(v);  // Move to point B
     v.rotate(-PI/3); // Rotate 60 degrees
@@ -43,7 +43,7 @@ function KochLine(a,b) {
 
   // Easy, just 2/3 of the way
   this.kochD = function() {
-    var v = p5.Vector.sub(this.end, this.start);
+    let v = p5.Vector.sub(this.end, this.start);
     v.mult(2/3.0);
     v.add(this.start);
     return v;
diff --git a/chp08_fractals/NOC_8_05_Koch/sketch.js b/chp08_fractals/NOC_8_05_Koch/sketch.js
index 749c648..70170e6 100644
--- a/chp08_fractals/NOC_8_05_Koch/sketch.js
+++ b/chp08_fractals/NOC_8_05_Koch/sketch.js
@@ -6,7 +6,7 @@
 // Renders a simple fractal, the Koch snowflake
 // Each recursive level drawn in sequence
 
-var k;
+let k;
 
 function setup() {
   createCanvas(640,360);
diff --git a/chp08_fractals/NOC_8_05_KochSimple/KochLine.js b/chp08_fractals/NOC_8_05_KochSimple/KochLine.js
index 132d6d6..6716a26 100644
--- a/chp08_fractals/NOC_8_05_KochSimple/KochLine.js
+++ b/chp08_fractals/NOC_8_05_KochSimple/KochLine.js
@@ -24,7 +24,7 @@ function KochLine(a,b) {
 
   // This is easy, just 1/3 of the way
   this.kochB = function() {
-    var v = p5.Vector.sub(this.end, this.start);
+    let v = p5.Vector.sub(this.end, this.start);
     v.div(3);
     v.add(this.start);
     return v;
@@ -32,8 +32,8 @@ function KochLine(a,b) {
 
   // More complicated, have to use a little trig to figure out where this p5.Vector is!
   this.kochC = function() {
-    var a = this.start.copy(); // Start at the beginning
-    var v = p5.Vector.sub(this.end, this.start);
+    let a = this.start.copy(); // Start at the beginning
+    let v = p5.Vector.sub(this.end, this.start);
     v.div(3);
     a.add(v);  // Move to point B
     v.rotate(-PI/3); // Rotate 60 degrees
@@ -43,7 +43,7 @@ function KochLine(a,b) {
 
   // Easy, just 2/3 of the way
   this.kochD = function() {
-    var v = p5.Vector.sub(this.end, this.start);
+    let v = p5.Vector.sub(this.end, this.start);
     v.mult(2/3.0);
     v.add(this.start);
     return v;
diff --git a/chp08_fractals/NOC_8_05_KochSimple/sketch.js b/chp08_fractals/NOC_8_05_KochSimple/sketch.js
index 95eea78..bb6268f 100644
--- a/chp08_fractals/NOC_8_05_KochSimple/sketch.js
+++ b/chp08_fractals/NOC_8_05_KochSimple/sketch.js
@@ -6,37 +6,37 @@
 // Renders a simple fractal, the Koch snowflake
 // Each recursive level drawn in sequence
 
-var lines = [];   // A list to keep track of all the lines
+let lines = [];   // A list to keep track of all the lines
 
 function setup() {
   createCanvas(383, 200);
-  var start = createVector(0, 150);
-  var end   = createVector(width, 150);
+  let start = createVector(0, 150);
+  let end   = createVector(width, 150);
   lines.push(new KochLine(start, end));
   
-  for (var i = 0; i < 5; i++) {
+  for (let i = 0; i < 5; i++) {
     generate();
   }
 }
 
 function draw() {
   background(51);
-  for (var i = 0; i < lines.length; i++) {
-  	var l = lines[i];
+  for (let i = 0; i < lines.length; i++) {
+  	let l = lines[i];
     l.display();
   }
 }
 
 function generate() {
-  var next = [];    // Create emtpy list
-  for (var i = 0; i < lines.length; i++) {
-  	var l = lines[i];
+  let next = [];    // Create emtpy list
+  for (let i = 0; i < lines.length; i++) {
+  	let l = lines[i];
     // Calculate 5 koch p5.Vectors (done for us by the line object)
-    var a = l.kochA();                 
-    var b = l.kochB();
-    var c = l.kochC();
-    var d = l.kochD();
-    var e = l.kochE();
+    let a = l.kochA();                 
+    let b = l.kochB();
+    let c = l.kochC();
+    let d = l.kochD();
+    let e = l.kochE();
     // Make line segments between all the p5.Vectors and add them
     next.push(new KochLine(a, b));
     next.push(new KochLine(b, c));
diff --git a/chp08_fractals/NOC_8_06_Tree/sketch.js b/chp08_fractals/NOC_8_06_Tree/sketch.js
index aa30c33..4843c59 100644
--- a/chp08_fractals/NOC_8_06_Tree/sketch.js
+++ b/chp08_fractals/NOC_8_06_Tree/sketch.js
@@ -6,7 +6,7 @@
 // Renders a simple tree-like structure via recursion
 // Branching angle calculated as a function of horizontal mouse position
 
-var theta;   
+let theta;   
 
 function setup() {
   createCanvas(640, 360);
diff --git a/chp08_fractals/NOC_8_07_TreeStochastic/sketch.js b/chp08_fractals/NOC_8_07_TreeStochastic/sketch.js
index 8f20ef4..eae5818 100644
--- a/chp08_fractals/NOC_8_07_TreeStochastic/sketch.js
+++ b/chp08_fractals/NOC_8_07_TreeStochastic/sketch.js
@@ -7,7 +7,7 @@
 // Angles and number of branches are random
 
 function setup() {
-  var test = createP('Click mouse to generate a new tree');
+  let test = createP('Click mouse to generate a new tree');
   test.position(10,372);
 
   createCanvas(640, 360);
@@ -39,7 +39,7 @@ function newTree() {
 
 function branch(h) {
   // thickness of the branch is mapped to its length
-  var sw = map(h, 2, 120, 1, 5);
+  let sw = map(h, 2, 120, 1, 5);
   strokeWeight(sw);
   // Draw the actual branch
   line(0, 0, 0, -h);
@@ -53,10 +53,10 @@ function branch(h) {
   // Here, ours is when the length of the branch is 2 pixels or less
   if (h > 2) {
     // A random number of branches
-    var n = Math.floor(random(1, 4));
-    for (var i = 0; i < n; i++) {
+    let n = Math.floor(random(1, 4));
+    for (let i = 0; i < n; i++) {
       // Picking a random angle
-      var theta = random(-PI/3, PI/3);
+      let theta = random(-PI/3, PI/3);
       push();      // Save the current state of transformation (i.e. where are we now)
       rotate(theta);     // Rotate by theta
       branch(h);         // Ok, now call myself to branch again
diff --git a/chp08_fractals/NOC_8_08_SimpleLSystem/sketch.js b/chp08_fractals/NOC_8_08_SimpleLSystem/sketch.js
index e70eb22..1167391 100644
--- a/chp08_fractals/NOC_8_08_SimpleLSystem/sketch.js
+++ b/chp08_fractals/NOC_8_08_SimpleLSystem/sketch.js
@@ -7,14 +7,14 @@
 // No drawing
 
 // Start with 'A'
-var current = 'A';
+let current = 'A';
 // Number of  generations
-var count = 0;
+let count = 0;
 
-var instructions;
-var show;
+let instructions;
+let show;
 
-var results = '';
+let results = '';
 
 function setup() {
   instructions = createP('<a href=\'#\'>Click the mouse to generate.</a>');
@@ -29,11 +29,11 @@ function setup() {
 
 function generate() {
   // A new StringBuffer for the next generation
-  var next = '';
+  let next = '';
   
   // Look through the current String to replace according to L-System rules
-  for (var i = 0; i < current.length; i++) {
-    var c = current.charAt(i);
+  for (let i = 0; i < current.length; i++) {
+    let c = current.charAt(i);
     if (c === 'A') {
       // If we find A replace with AB
       next += 'AB';
diff --git a/chp08_fractals/NOC_8_09_LSystem/LSystem.js b/chp08_fractals/NOC_8_09_LSystem/LSystem.js
index 7cde195..dcbc639 100644
--- a/chp08_fractals/NOC_8_09_LSystem/LSystem.js
+++ b/chp08_fractals/NOC_8_09_LSystem/LSystem.js
@@ -16,15 +16,15 @@ function LSystem(axiom, r) {
   // Generate the next generation
   this.generate = function() {
     // An empty StringBuffer that we will fill
-    var nextgen = '';
+    let nextgen = '';
     // For every character in the sentence
-    for (var i = 0; i < this.sentence.length; i++) {
+    for (let i = 0; i < this.sentence.length; i++) {
       // What is the character
       // We will replace it with itself unless it matches one of our rules
-      var replace = this.sentence.charAt(i);
+      let replace = this.sentence.charAt(i);
       // Check every rule
-      for (var j = 0; j < this.ruleset.length; j++) {
-        var a = this.ruleset[j].getA();
+      for (let j = 0; j < this.ruleset.length; j++) {
+        let a = this.ruleset[j].getA();
         // if we match the Rule, get the replacement String out of the Rule
         if (a === replace) {
           replace = this.ruleset[j].getB();
diff --git a/chp08_fractals/NOC_8_09_LSystem/Turtle.js b/chp08_fractals/NOC_8_09_LSystem/Turtle.js
index 0c954c4..5e67245 100644
--- a/chp08_fractals/NOC_8_09_LSystem/Turtle.js
+++ b/chp08_fractals/NOC_8_09_LSystem/Turtle.js
@@ -9,8 +9,8 @@ function Turtle(s, l, t) {
 
   this.render = function() {
     stroke(255);
-    for (var i = 0; i < this.todo.length; i++) {
-      var c = this.todo.charAt(i);
+    for (let i = 0; i < this.todo.length; i++) {
+      let c = this.todo.charAt(i);
       if (c === 'F' || c === 'G') {
         line(0,0,this.len,0);
         translate(this.len,0);
diff --git a/chp08_fractals/NOC_8_09_LSystem/sketch.js b/chp08_fractals/NOC_8_09_LSystem/sketch.js
index 02a21dd..ded9b34 100644
--- a/chp08_fractals/NOC_8_09_LSystem/sketch.js
+++ b/chp08_fractals/NOC_8_09_LSystem/sketch.js
@@ -2,8 +2,8 @@
 // Daniel Shiffman
 // http://natureofcode.com
 
-var lsys;
-var turtle;
+let lsys;
+let turtle;
 
 function setup() {
   createCanvas(600, 600);
@@ -25,7 +25,7 @@ function setup() {
    turtle = new Turtle(lsys.getSentence(),width-1,PI/2);
    */
 
-  var ruleset = [];
+  let ruleset = [];
   ruleset[0] = new Rule('F', "FF+[+F-F-F]-[-F+F+F]");
   lsys = new LSystem("F", ruleset);
   turtle = new Turtle(lsys.getSentence(), height/3, radians(25));
@@ -41,7 +41,7 @@ function draw() {
   turtle.render();
 }
 
-var counter = 0;
+let counter = 0;
 
 function mousePressed() {
   if (counter < 5) {
diff --git a/chp09_ga/NOC_9_02_SmartRockets_superbasic/population.js b/chp09_ga/NOC_9_02_SmartRockets_superbasic/population.js
index 8428fd4..6e72ad9 100644
--- a/chp09_ga/NOC_9_02_SmartRockets_superbasic/population.js
+++ b/chp09_ga/NOC_9_02_SmartRockets_superbasic/population.js
@@ -14,22 +14,22 @@ class Population {
     this.matingPool = []; // ArrayList which we will use for our "mating pool"
     this.generations = 0; // Number of generations
     //make a new set of creatures
-    for (var i = 0; i < num; i++) {
-      var location = createVector(width / 2, height + 20);
+    for (let i = 0; i < num; i++) {
+      let location = createVector(width / 2, height + 20);
       this.population[i] = new Rocket(location, new DNA());
     }
   }
 
   live() {
     // Run every rocket
-    for (var i = 0; i < this.population.length; i++) {
+    for (let i = 0; i < this.population.length; i++) {
       this.population[i].run();
     }
   }
 
   // Calculate fitness for each creature
   fitness() {
-    for (var i = 0; i < this.population.length; i++) {
+    for (let i = 0; i < this.population.length; i++) {
       this.population[i].calcFitness();
     }
   }
@@ -40,17 +40,17 @@ class Population {
     this.matingPool = [];
 
     // Calculate total fitness of whole population
-    var maxFitness = this.getMaxFitness();
+    let maxFitness = this.getMaxFitness();
 
     // Calculate fitness for each member of the population (scaled to value between 0 and 1)
     // Based on fitness, each member will get added to the mating pool a certain number of times
     // A higher fitness = more entries to mating pool = more likely to be picked as a parent
     // A lower fitness = fewer entries to mating pool = less likely to be picked as a parent
-    for (var i = 0; i < this.population.length; i++) {
-      var fitnessNormal = map(this.population[i].getFitness(), 0, maxFitness, 0, 1);
-      var n = floor(fitnessNormal * 100); // Arbitrary multiplier
+    for (let i = 0; i < this.population.length; i++) {
+      let fitnessNormal = map(this.population[i].getFitness(), 0, maxFitness, 0, 1);
+      let n = floor(fitnessNormal * 100); // Arbitrary multiplier
 
-      for (var j = 0; j < n; j++) {
+      for (let j = 0; j < n; j++) {
         this.matingPool.push(this.population[i]);
       }
     }
@@ -59,22 +59,22 @@ class Population {
   // Making the next generation
   reproduction() {
     // Refill the population with children from the mating pool
-    for (var i = 0; i < this.population.length; i++) {
+    for (let i = 0; i < this.population.length; i++) {
       // Sping the wheel of fortune to pick two parents
-      var m = floor(random(this.matingPool.length));
-      var d = floor(random(this.matingPool.length));
+      let m = floor(random(this.matingPool.length));
+      let d = floor(random(this.matingPool.length));
       // Pick two parents
-      var mom = this.matingPool[m];
-      var dad = this.matingPool[d];
+      let mom = this.matingPool[m];
+      let dad = this.matingPool[d];
       // Get their genes
-      var momgenes = mom.getDNA();
-      var dadgenes = dad.getDNA();
+      let momgenes = mom.getDNA();
+      let dadgenes = dad.getDNA();
       // Mate their genes
-      var child = momgenes.crossover(dadgenes);
+      let child = momgenes.crossover(dadgenes);
       // Mutate their genes
       child.mutate(this.mutationRate);
       // Fill the new population with the new child
-      var location = createVector(width / 2, height + 20);
+      let location = createVector(width / 2, height + 20);
       this.population[i] = new Rocket(location, child);
     }
     this.generations++;
@@ -86,8 +86,8 @@ class Population {
 
   // Find highest fitness for the population
   getMaxFitness() {
-    var record = 0;
-    for (var i = 0; i < this.population.length; i++) {
+    let record = 0;
+    for (let i = 0; i < this.population.length; i++) {
       if (this.population[i].getFitness() > record) {
         record = this.population[i].getFitness();
       }