fix: for tag not respecting Drop#valueOf(), fixes #515

Author: harttle

src/drop/drop.ts

@@ -1,8 +1,4 @@
 export abstract class Drop {
-  public valueOf (): any {
-    return undefined
-  }
-
   public liquidMethodMissing (key: string | number): Promise<string | undefined> | string | undefined {
     return undefined
   }

src/util/collection.ts

@@ -1,6 +1,7 @@
-import { isNil, isString, isObject, isArray, isIterable } from './underscore'
+import { isNil, isString, isObject, isArray, isIterable, toValue } from './underscore'
 
 export function toEnumerable (val: any) {
+  val = toValue(val)
   if (isArray(val)) return val
   if (isString(val) && val.length > 0) return [val]
   if (isIterable(val)) return Array.from(val)

src/util/underscore.ts

@@ -39,7 +39,7 @@ export function stringify (value: any): string {
 }
 
 export function toValue (value: any): any {
-  return value instanceof Drop ? value.valueOf() : value
+  return (value instanceof Drop && isFunction(value.valueOf)) ? value.valueOf() : value
 }
 
 export function isNumber (value: any): value is number {

test/integration/builtin/tags/for.ts

@@ -324,8 +324,7 @@ describe('tags/for', function () {
         yield 'b'
         yield 'c'
       }
-
-      public valueOf (): string {
+      toString () {
         return 'MockIterableDrop'
       }
     }

test/integration/drop/drop.ts

@@ -61,4 +61,15 @@ describe('drop/drop', function () {
     const html = await liquid.parseAndRender(`{{obj.foo}}`, { obj: new PromiseDrop() })
     expect(html).to.equal('FOO')
   })
+  it('should respect valueOf', async () => {
+    class CustomDrop extends Drop {
+      prop = 'not enumerable'
+      valueOf () {
+        return ['foo', 'bar']
+      }
+    }
+    const tpl = '{{drop}}: {% for field in drop %}{{ field }};{% endfor %}'
+    const html = await liquid.parseAndRender(tpl, { drop: new CustomDrop() })
+    expect(html).to.equal('foobar: foo;bar;')
+  })
 })