How to implement dictionary in java script?

Question

This is my source code but I have problem. I can't implementation showAll() function for Dictionary Data Structure.

Can you help me ?

function Dictionary() {
   this.add = add;
   this.datastore = new Array();
   this.find = find;
   this.remove = remove;
   this.showAll = showAll;
   this.count = count;
   this.clear = clear;
}
function add(key, value) {
   this.datastore[key] = value;
}
function find(key) {
   return this.datastore[key];
}
function remove(key) {
   delete this.datastore[key];
}
function showAll() {
    Object.keys(this.datastore).forEach( function(key, index) {
       console.log(key + "->" + this.datastore[key]);
    });
}
function count() {
    return Object.keys(this.datastore).length;
}
/*function count() {
   // var n = 0;
   // Object.keys(this.datastore).forEach( function(key, index) {
   // for each (var key in Object.keys(this.datastore)) {
   //      ++n;
   //};
   //return n;
}*/
function clear() {
   Object.keys(this.datastore).forEach( function(key, index) {
    // for each (var key in Object.keys(this.datastore)) {
      delete this.datastore[key];
  });
}

var pbook = new Dictionary();

pbook.add("Michael", "012345");
pbook.add("Sam", "54654654");
pbook.add("Shen", "5465464");

console.log("Sam's extension: " + pbook.find("Sam"));
pbook.remove("Sam");
pbook.showAll();

I get this error:

undefined:31
       console.log(key + "->" + this.datastore[key]);
                                              ^
TypeError: Cannot read property 'Michael' of undefined
    at eval (eval at <anonymous> (C:\DATA\NetBeansProjects\StudyForDTwithJS\public_html\js\CH07-Dictionaries\02-TestDictionaryDefinitionClass.js:8:56), <anonymous>:31:47)
    at Array.forEach (native)
    at Dictionary.showAll (eval at <anonymous> (C:\DATA\NetBeansProjects\StudyForDTwithJS\public_html\js\CH07-Dictionaries\02-TestDictionaryDefinitionClass.js:8:56), <anonymous>:30:33)
    at Object.<anonymous> (C:\DATA\NetBeansProjects\StudyForDTwithJS\public_html\js\CH07-Dictionaries\02-TestDictionaryDefinitionClass.js:18:7)
    at Module._compile (module.js:460:26)
    at Object.Module._extensions..js (module.js:478:10)
    at Module.load (module.js:355:32)
    at Function.Module._load (module.js:310:12)
    at Function.Module.runMain (module.js:501:10)
    at startup (node.js:129:16)

Javascript has a function scope for variables, it means that there in no datastore in forEach loop defined. — Mariusz Beltowski
– Mariusz Beltowski, Commented Jun 17, 2015 at 0:25
Prototype: developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… — Adam Jenkins
– Adam Jenkins, Commented Jun 17, 2015 at 0:37
developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… — Bergi
– Bergi, Commented Jun 17, 2015 at 0:48

AmmarCSE · Accepted Answer · 2015-06-17 00:43:12Z

this inside the forEach callback is not the same this before calling because the execution context(fancy for a function has been invoked and a new scope created) has been changed. Preserve it in a variable and use it within the callback.

function showAll() {
    var ds = this.datastore;
    Object.keys(this.datastore).forEach( function(key, index) {
       console.log(key + "->" + ds[key]);
    });
}

function Dictionary() {
  this.add = add;
  this.datastore = new Array();
  this.find = find;
  this.remove = remove;
  this.showAll = showAll;
  this.count = count;
  this.clear = clear;
}

function add(key, value) {
  this.datastore[key] = value;
}

function find(key) {
  return this.datastore[key];
}

function remove(key) {
  delete this.datastore[key];
}

function showAll() {
  var ds = this.datastore;
  Object.keys(this.datastore).forEach(function(key, index) {
    console.log(key + "->" + ds[key]);
  });
}

function count() {
  return Object.keys(this.datastore).length;
}

/*
function count() {
   // var n = 0;
   // Object.keys(this.datastore).forEach( function(key, index) {
   // for each (var key in Object.keys(this.datastore)) {
   //      ++n;
   //};
   //return n;
}
*/
function clear() {
  Object.keys(this.datastore).forEach(function(key, index) {
    // for each (var key in Object.keys(this.datastore)) {
    delete this.datastore[key];
  });
}

var pbook = new Dictionary();

pbook.add("Michael", "012345");
pbook.add("Sam", "54654654");
pbook.add("Shen", "5465464");

console.log("Sam's extension: " + pbook.find("Sam"));
pbook.remove("Sam");
pbook.showAll();

Or, another option is to pass this into forEach as the documentation states

arr.forEach(callback[, thisArg])

function showAll() {
  var ds = this.datastore;
  Object.keys(this.datastore).forEach(function(key, index) {
    console.log(key + "->" + this[key]);
  }, this);
}

function Dictionary() {
  this.add = add;
  this.datastore = new Array();
  this.find = find;
  this.remove = remove;
  this.showAll = showAll;
  this.count = count;
  this.clear = clear;
}

function add(key, value) {
  this.datastore[key] = value;
}

function find(key) {
  return this.datastore[key];
}

function remove(key) {
  delete this.datastore[key];
}


function showAll() {
  var ds = this.datastore;
  Object.keys(this.datastore).forEach(function(key, index) {
    console.log(key + "->" + this[key]);
  }, this);
}

function count() {
  return Object.keys(this.datastore).length;
}

/*
function count() {
   // var n = 0;
   // Object.keys(this.datastore).forEach( function(key, index) {
   // for each (var key in Object.keys(this.datastore)) {
   //      ++n;
   //};
   //return n;
}
*/
function clear() {
  Object.keys(this.datastore).forEach(function(key, index) {
    // for each (var key in Object.keys(this.datastore)) {
    delete this.datastore[key];
  });
}

var pbook = new Dictionary();

pbook.add("Michael", "012345");
pbook.add("Sam", "54654654");
pbook.add("Shen", "5465464");

console.log("Sam's extension: " + pbook.find("Sam"));
pbook.remove("Sam");
pbook.showAll();

For further reading, see What is “this” Context

This works, alternatively the forEach method takes an optional second argument to assign context: .forEach(callback,thisArg)
@Adam, I did not know that! Thanks for mentioning! I have included it in the answer!
@James, if you find an answer solved your problem, mark it as accepted by clicking the checkmark next to the answer.

Ely · Accepted Answer · 2015-06-19 16:13:36Z

0

I'd simply use a JavaScript object to store key/value pairs.

You can simply :

add property
delete property (remove)
read property (find)
show all properties (use for each key in object and hasOwnProperty to skip prototype chain)
count
clear (all properties)

Just a suggestion. Maybe it's more efficient I think.

answered Jun 19, 2015 at 16:13

Ely

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