refactor: IDrop interface => abstract method

src/drop/blank-drop.ts

@@ -1,11 +1,11 @@
 import { isNil, isString } from '../util/underscore'
-import { isDrop } from '../drop/idrop'
+import { Drop } from '../drop/drop'
 import { EmptyDrop } from '../drop/empty-drop'
 
 export class BlankDrop extends EmptyDrop {
   equals (value: any) {
     if (value === false) return true
-    if (isNil(isDrop(value) ? value.valueOf() : value)) return true
+    if (isNil(value instanceof Drop ? value.valueOf() : value)) return true
     if (isString(value)) return /^\s*$/.test(value)
     return super.equals(value)
   }

src/drop/drop.ts

@@ -1,2 +1,6 @@
 export abstract class Drop {
+  abstract valueOf(): any;
+
+  liquid_method_missing (name: string) { // eslint-disable-line
+  }
 }

src/drop/empty-drop.ts

@@ -1,9 +1,8 @@
 import { Drop } from './drop'
 import { IComparable } from './icomparable'
 import { isObject, isString, isArray } from '../util/underscore'
-import { IDrop } from '../drop/idrop'
 
-export class EmptyDrop extends Drop implements IDrop, IComparable {
+export class EmptyDrop extends Drop implements IComparable {
   equals (value: any) {
     if (isString(value) || isArray(value)) return value.length === 0
     if (isObject(value)) return Object.keys(value).length === 0

src/drop/idrop.ts

@@ -1,10 +0,0 @@
-import { Drop } from './drop'
-import { isFunction } from '../util/underscore'
-
-export interface IDrop {
-  valueOf(): any
-}
-
-export function isDrop (value: any): value is IDrop {
-  return value instanceof Drop && isFunction((value as any).valueOf)
-}

src/drop/null-drop.ts

@@ -1,12 +1,11 @@
 import { Drop } from './drop'
 import { IComparable } from './icomparable'
 import { isNil } from '../util/underscore'
-import { IDrop, isDrop } from '../drop/idrop'
 import { BlankDrop } from '../drop/blank-drop'
 
-export class NullDrop extends Drop implements IDrop, IComparable {
+export class NullDrop extends Drop implements IComparable {
   equals (value: any) {
-    return isNil(isDrop(value) ? value.valueOf() : value) || value instanceof BlankDrop
+    return isNil(value instanceof Drop ? value.valueOf() : value) || value instanceof BlankDrop
   }
   gt () {
     return false