Add "back" easings, fix error in elastic easings

packages/velocity/src/Velocity/easing/_all.d.ts

@@ -1,4 +1,5 @@
 ///<reference path="easings.ts" />
+///<reference path="back.ts" />
 ///<reference path="bezier.ts" />
 ///<reference path="bounce.ts" />
 ///<reference path="elastic.ts" />

packages/velocity/src/Velocity/easing/back.ts

@@ -0,0 +1,56 @@
+///<reference path="easings.ts" />
+/*
+ * VelocityJS.org (C) 2014-2017 Julian Shapiro.
+ *
+ * Licensed under the MIT license. See LICENSE file in the project root for details.
+ *
+ * Back easings, based on code from https://github.com/yuichiroharai/easeplus-velocity
+ */
+
+namespace VelocityStatic.Easing {
+	export function registerBackIn(name: string, amount: number) {
+		registerEasing([name, function(percentComplete: number, startValue: number, endValue: number): number {
+			if (percentComplete === 0) {
+				return startValue;
+			}
+			if (percentComplete === 1) {
+				return endValue;
+			}
+			return Math.pow(percentComplete, 2) * ((amount + 1) * percentComplete - amount) * (endValue - startValue);
+		}]);
+	}
+
+	export function registerBackOut(name: string, amount: number) {
+		registerEasing([name, function(percentComplete: number, startValue: number, endValue: number): number {
+			if (percentComplete === 0) {
+				return startValue;
+			}
+			if (percentComplete === 1) {
+				return endValue;
+			}
+			return (Math.pow(--percentComplete, 2) * ((amount + 1) * percentComplete + amount) + 1) * (endValue - startValue);
+		}]);
+	}
+
+	export function registerBackInOut(name: string, amount: number) {
+		amount *= 1.525;
+		registerEasing([name, function(percentComplete: number, startValue: number, endValue: number): number {
+			if (percentComplete === 0) {
+				return startValue;
+			}
+			if (percentComplete === 1) {
+				return endValue;
+			}
+			return ((percentComplete *= 2) < 1
+				? (Math.pow(percentComplete, 2) * ((amount + 1) * percentComplete - amount))
+				: (Math.pow(percentComplete -= 2, 2) * ((amount + 1) * percentComplete + amount) + 2)
+			) * 0.5 * (endValue - startValue);
+		}]);
+	}
+
+	registerBackIn("easeInBack", 1.7);
+	registerBackOut("easeOutBack", 1.7);
+	registerBackInOut("easeInOutBack", 1.7);
+
+	// TODO: Expose these as actions to register custom easings?
+};

packages/velocity/src/Velocity/easing/bounce.ts

@@ -52,6 +52,9 @@ namespace VelocityStatic.Easing {
 		if (percentComplete === 1) {
 			return endValue;
 		}
-		return (percentComplete < 0.5 ? easeInBounce(percentComplete * 2) * .5 : easeOutBounce(percentComplete * 2 - 1) * 0.5 + 0.5) * (endValue - startValue);
+		return (percentComplete < 0.5
+			? easeInBounce(percentComplete * 2) * .5
+			: easeOutBounce(percentComplete * 2 - 1) * 0.5 + 0.5
+		) * (endValue - startValue);
 	}]);
 };

packages/velocity/src/Velocity/easing/elastic.ts

@@ -45,10 +45,10 @@ namespace VelocityStatic.Easing {
 			const s = period / pi2 * Math.asin(1 / amplitude);
 
 			percentComplete = percentComplete * 2 - 1;
-			if (percentComplete < 0) {
-				return -0.5 * (amplitude * Math.pow(2, 10 * percentComplete) * Math.sin((percentComplete - s) * pi2 / period));
-			}
-			return amplitude * Math.pow(2, -10 * percentComplete) * Math.sin((percentComplete - s) * pi2 / period) * 0.5 + 1;
+			return (percentComplete < 0
+				? -0.5 * (amplitude * Math.pow(2, 10 * percentComplete) * Math.sin((percentComplete - s) * pi2 / period))
+				: amplitude * Math.pow(2, -10 * percentComplete) * Math.sin((percentComplete - s) * pi2 / period) * 0.5 + 1
+			) * (endValue - startValue);
 		}]);
 	}
 

packages/velocity/velocity.js

@@ -1528,6 +1528,63 @@ var VelocityStatic;
     })(Easing = VelocityStatic.Easing || (VelocityStatic.Easing = {}));
 })(VelocityStatic || (VelocityStatic = {}));
 
+///<reference path="easings.ts" />
+/*
+ * VelocityJS.org (C) 2014-2017 Julian Shapiro.
+ *
+ * Licensed under the MIT license. See LICENSE file in the project root for details.
+ *
+ * Back easings, based on code from https://github.com/yuichiroharai/easeplus-velocity
+ */
+var VelocityStatic;
+
+(function(VelocityStatic) {
+    var Easing;
+    (function(Easing) {
+        function registerBackIn(name, amount) {
+            Easing.registerEasing([ name, function(percentComplete, startValue, endValue) {
+                if (percentComplete === 0) {
+                    return startValue;
+                }
+                if (percentComplete === 1) {
+                    return endValue;
+                }
+                return Math.pow(percentComplete, 2) * ((amount + 1) * percentComplete - amount) * (endValue - startValue);
+            } ]);
+        }
+        Easing.registerBackIn = registerBackIn;
+        function registerBackOut(name, amount) {
+            Easing.registerEasing([ name, function(percentComplete, startValue, endValue) {
+                if (percentComplete === 0) {
+                    return startValue;
+                }
+                if (percentComplete === 1) {
+                    return endValue;
+                }
+                return (Math.pow(--percentComplete, 2) * ((amount + 1) * percentComplete + amount) + 1) * (endValue - startValue);
+            } ]);
+        }
+        Easing.registerBackOut = registerBackOut;
+        function registerBackInOut(name, amount) {
+            amount *= 1.525;
+            Easing.registerEasing([ name, function(percentComplete, startValue, endValue) {
+                if (percentComplete === 0) {
+                    return startValue;
+                }
+                if (percentComplete === 1) {
+                    return endValue;
+                }
+                return ((percentComplete *= 2) < 1 ? Math.pow(percentComplete, 2) * ((amount + 1) * percentComplete - amount) : Math.pow(percentComplete -= 2, 2) * ((amount + 1) * percentComplete + amount) + 2) * .5 * (endValue - startValue);
+            } ]);
+        }
+        Easing.registerBackInOut = registerBackInOut;
+        registerBackIn("easeInBack", 1.7);
+        registerBackOut("easeOutBack", 1.7);
+        registerBackInOut("easeInOutBack", 1.7);
+        // TODO: Expose these as actions to register custom easings?
+        })(Easing = VelocityStatic.Easing || (VelocityStatic.Easing = {}));
+})(VelocityStatic || (VelocityStatic = {}));
+
 ///<reference path="easings.ts" />
 /*
  * VelocityJS.org (C) 2014-2017 Julian Shapiro.
@@ -1795,10 +1852,7 @@ var VelocityStatic;
                 }
                 var s = period / pi2 * Math.asin(1 / amplitude);
                 percentComplete = percentComplete * 2 - 1;
-                if (percentComplete < 0) {
-                    return -.5 * (amplitude * Math.pow(2, 10 * percentComplete) * Math.sin((percentComplete - s) * pi2 / period));
-                }
-                return amplitude * Math.pow(2, -10 * percentComplete) * Math.sin((percentComplete - s) * pi2 / period) * .5 + 1;
+                return (percentComplete < 0 ? -.5 * (amplitude * Math.pow(2, 10 * percentComplete) * Math.sin((percentComplete - s) * pi2 / period)) : amplitude * Math.pow(2, -10 * percentComplete) * Math.sin((percentComplete - s) * pi2 / period) * .5 + 1) * (endValue - startValue);
             } ]);
         }
         Easing.registerElasticInOut = registerElasticInOut;