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Add toString to the dictionary

importance: 5

There’s an object dictionary, created as Object.create(null), to store any key/value pairs.

Add method dictionary.toString() into it, that should return a comma-delimited list of keys. Your toString should not show up in over the object.

Here’s how it should work:

let dictionary = Object.create(null);

// your code to add dictionary.toString method

// add some data = "Apple";
dictionary.__proto__ = "test"; // __proto__ is a regular property key here

// only apple and __proto__ are in the loop
for(let key in dictionary) {
  alert(key); // "apple", then "__proto__"

// your toString in action
alert(dictionary); // "apple,__proto__"

The method can take all enumerable keys using Object.keys and output their list.

To make toString non-enumerable, let’s define it using a property descriptor. The syntax of Object.create allows us to provide an object with property descriptors as the second argument.

let dictionary = Object.create(null, {
  toString: { // define toString property
    value() { // the value is a function
      return Object.keys(this).join();
}); = "Apple";
dictionary.__proto__ = "test";

// apple and __proto__ is in the loop
for(let key in dictionary) {
  alert(key); // "apple", then "__proto__"

// comma-separated list of properties by toString
alert(dictionary); // "apple,__proto__"

When we create a property using a descriptor, its flags are false by default. So in the code above, dictionary.toString is non-enumerable.

See the the chapter Property flags and descriptors for review.