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

src/render/syntax.ts

@@ -6,7 +6,7 @@ import { isComparable } from '../drop/icomparable'
 import { NullDrop } from '../drop/null-drop'
 import { EmptyDrop } from '../drop/empty-drop'
 import { BlankDrop } from '../drop/blank-drop'
-import { isDrop } from '../drop/idrop'
+import { Drop } from '../drop/drop'
 
 const binaryOperators: {[key: string]: (lhs: any, rhs: any) => boolean} = {
   '==': (l: any, r: any) => {
@@ -74,7 +74,7 @@ export function parseExp (exp: string, scope: Scope): any {
 
 export function evalExp (str: string, scope: Scope): any {
   const value = parseExp(str, scope)
-  return isDrop(value) ? value.valueOf() : value
+  return value instanceof Drop ? value.valueOf() : value
 }
 
 function parseValue (str: string, scope: Scope): any {
@@ -93,7 +93,7 @@ function parseValue (str: string, scope: Scope): any {
 
 export function evalValue (str: string, scope: Scope): any {
   const value = parseValue(str, scope)
-  return isDrop(value) ? value.valueOf() : value
+  return value instanceof Drop ? value.valueOf() : value
 }
 
 export function isTruthy (val: any): boolean {