fix(js): use Symbol in Function.prototype.myCall

Author: h2jorm

JS/JS-br.md

@@ -752,15 +752,16 @@ Consideramos implementar eles a partir das seguintes regras:
 ```js
 Function.prototype.myCall = function (context) {
   var context = context || window
+  var fn = Symbol('fn')
   // Adiciona uma propriedade ao `context`
   // getValue.call(a, 'yck', '24') => a.fn = getValue
-  context.fn = this
+  context[fn] = this
   // pega os parâmentros do `context`
   var args = [...arguments].slice(1)
   // getValue.call(a, 'yck', '24') => a.fn('yck', '24')
-  var result = context.fn(...args)
+  var result = context[fn](...args)
   // deleta fn
-  delete context.fn
+  delete context[fn]
   return result
 }
 ```

JS/JS-ch.md

@@ -1018,15 +1018,16 @@ getValue.apply(a, ['yck', '24'])
 ```js
 Function.prototype.myCall = function (context) {
   var context = context || window
+  var fn = Symbol('fn')
   // 给 context 添加一个属性
   // getValue.call(a, 'yck', '24') => a.fn = getValue
-  context.fn = this
+  context[fn] = this
   // 将 context 后面的参数取出来
   var args = [...arguments].slice(1)
   // getValue.call(a, 'yck', '24') => a.fn('yck', '24')
-  var result = context.fn(...args)
+  var result = context[fn](...args)
   // 删除 fn
-  delete context.fn
+  delete context[fn]
   return result
 }
 ```

JS/JS-en.md

@@ -751,15 +751,16 @@ We can consider how to implement them from the following rules:
 ```js
 Function.prototype.myCall = function (context) {
   var context = context || window
+  var fn = Symbol('fn')
   // Add an property to the `context`
   // getValue.call(a, 'yck', '24') => a.fn = getValue
-  context.fn = this
+  context[fn] = this
   // take out the rest parameters of `context`
   var args = [...arguments].slice(1)
   // getValue.call(a, 'yck', '24') => a.fn('yck', '24')
-  var result = context.fn(...args)
+  var result = context[fn](...args)
   // delete fn
-  delete context.fn
+  delete context[fn]
   return result
 }
 ```