We Keep Coding

Array that can store only one type of object?

Is it possible to create an array that will only allow objects of a certain to be stored in it? Is there a method that adds an element to the array I can override?

Yes you can, just override the push array of the array (let's say all you want to store are numbers than do the following:
var myArr = [];
myArr.push = function(){
for(var arg of arguments) {
if(arg.constructor == Number) Array.prototype.push.call(this, arg);
}
}
Simply change Number to whatever constructor you want to match. Also I would probably add and else statement or something, to throw an error if that's what you want.
UPDATE:
Using Object.observe (currently only available in chrome):
var myArr = [];
Array.observe(myArr, function(changes) {
for(var change of changes) {
if(change.type == "update") {
if(myArr[change.name].constructor !== Number) myArr.splice(change.name, 1);
} else if(change.type == 'splice') {
if(change.addedCount > 0) {
if(myArr[change.index].constructor !== Number) myArr.splice(change.index, 1);
}
}
}
});
Now in ES6 there are proxies which you should be able to do the following:
var myArr = new Proxy([], {
set(obj, prop, value) {
if(value.constructor !== Number) {
obj.splice(prop, 1);
}
//I belive thats it, there's probably more to it, yet because I don't use firefox or IE Technical preview I can't really tell you.
}
});

Not directly. But you can hide the array in a closure and only provide your custom API to access it:
var myArray = (function() {
var array = [];
return {
set: function(index, value) {
/* Check if value is allowed */
array[index] = value;
},
get: function(index) {
return array[index];
}
};
})();
Use it like
myArray.set(123, 'abc');
myArray.get(123); // 'abc' (assuming it was allowed)

Avoiding duplication in object- Javascript

I have the next object:
var persons= {};
persons["Matt"].push("A");
persons["Matt"].push("B");
persons["Matt"].push("C");
And I want to know if the object contains the element which I try to insert.
E.g:
persons["Matt"].push("A"); /* The element A already exist... And I don't want duplicate elements.*/
Anybody know one way to make it?
EDIT WITH MORE DETAILS:
I have a the next code:
function insertIfNotThere(array, item) {
if (array.indexOf(item) === -1) {
array.push(item);
}
}
function EventManager(target) {
var target = target || window, events = {};
this.observe = function(eventName, cb) {
if (events[eventName]){
insertIfNotThere(events[eventName], cb);
}else{
events[eventName] = []; events[eventName].push(cb);
}
return target;
};
this.fire = function(eventName) {
if (!events[eventName]) return false;
for (var i = 0; i < events[eventName].length; i++) {
events[eventName][i].apply(target, Array.prototype.slice.call(arguments, 1));
}
};
}
I use your method for checking if the element with the content indicated exist. But... It push the element ever... I don't know what's happening...

First things first. When you do
persons= {};
you are creating a global property called persons and assigning an empty object to it. You might want a local variable here. So, change it to
var persons = {};
And then, when you create a new key in the object, by default, the value will be undefined. In your case you need to store an array. So, you have to initialize it like this
persons['Matt'] = [];
and then you can use the Array.prototype.indexOf function to find out if the item being added is already there in the array or not (it returns -1 if the item is not found in the array), like this
if (persons['Matt'].indexOf("A") === -1) {
persons['Matt'].push("A");
}
if (persons['Matt'].indexOf("B") === -1) {
persons['Matt'].push("B");
}
You can create a function to do this
function insertIfNotThere(array, item) {
if (array.indexOf(item) === -1) {
array.push(item);
}
}
var persons = {};
persons['Matt'] = [];
insertIfNotThere(persons['Matt'], 'A');
insertIfNotThere(persons['Matt'], 'B');
// This will be ignored, as `A` is already there in the array
insertIfNotThere(persons['Matt'], 'A');

Use indexOf to check for the existence of A. If it doesn't exist (is -1), add it to the array:
if (persons['Matt'].indexOf('A') === -1) {
persons['Matt'].push('A');
}

Recursive nested property creation in JavaScript

I am trying to recursively build an object with a tree of properties based on a MongoDB-ish selector "top.middle.bottom". There are some underscorejs helpers as well:
function setNestedPropertyValue(obj, fields, val) {
if (fields.indexOf(".") === -1) {
// On last property, set the value
obj[fields] = val;
return obj; // Recurse back up
} else {
var oneLevelLess = _.first(fields.split("."));
var remainingLevels = _.rest(fields.split(".")).join(".");
// There are more property levels remaining, set a sub with a recursive call
obj[oneLevelLess] = setNestedPropertyValue( {}, remainingLevels, val);
}
}
setNestedPropertyValue({}, "grandpaprop.papaprop.babyprop", 1);
Desired:
{
grandpaprop: {
papaprop: {
babyprop: 1
}
}
}
Outcome:
undefined
Helps and hints would be appreciated.

Instead of recursion I would choose for an iterative solution:
function setNestedPropertyValue(obj, fields, val)
{
fields = fields.split('.');
var cur = obj,
last = fields.pop();
fields.forEach(function(field) {
cur[field] = {};
cur = cur[field];
});
cur[last] = val;
return obj;
}
setNestedPropertyValue({}, "grandpaprop.papaprop.babyprop", 1);

EDIT
And here is another version thanks to the suggestions by Scott Sauyet:
function setPath(obj, [first, ...rest], val) {
if (rest.length == 0) {
return {...obj, [first]: val}
}
let nestedObj = obj[first] || {};
return {...obj, [first]: setPath(nestedObj, rest, val)};
}
function setNestedPropertyValue(obj, field, val) {
return setPath(obj, field.split('.'), val);
}
// example
let test_obj = {};
test_obj = setNestedPropertyValue(test_obj, "foo.bar.baz", 1);
test_obj = setNestedPropertyValue(test_obj, "foo.bar.baz1", 1);
// will output {"foo":{"bar":{"baz":1,"baz1":1}}}, while in the original version only "baz1" will be set
console.log(JSON.stringify(test_obj));
It's plain javascript
It only appends properties and will not override a top level object
setNestedPropertyValue() does not mutate the passed object (although keep in mind it only returns a shallow copy of the object, so some properties may be shared references between the original object and the new one)
I know this is old, but I needed exactly that kind of function and wasn't happy with the implementation, so here is my version:
function setNestedPropertyValue(obj, field, val) {
if (field.indexOf(".") === -1) {
obj[field] = val;
} else {
let fields = field.split(".");
let topLevelField = fields.shift();
let remainingFields = fields.join(".");
if (obj[topLevelField] == null) {
obj[topLevelField] = {};
}
setNestedPropertyValue(obj[topLevelField], remainingFields, val);
}
}
// example
let test_obj = {};
setNestedPropertyValue(test_obj, "foo.bar.baz", 1);
setNestedPropertyValue(test_obj, "foo.bar.baz1", 1);
// will output {"foo":{"bar":{"baz":1,"baz1":1}}}, while in the original version only "baz1" will be set
console.log(JSON.stringify(test_obj));
It's plain javascript
It only appends properties and will not override a top level object
setNestedPropertyValue() does not return the object so it is clear that it mutates the passed object

As mentioned by Jack in the question, I was not returning my object in the last line in the else statement. By adding this, it is now working:
obj[oneLevelLess] = setNestedPropertyValue( {}, remainingLevels, val);
return obj; // Add this line
}

nested hashtables in javascript

I want to create the following data-structure with js:
folders dictionary: folder-id(guid) : ==> pages dictionary
pages array: page-position(int) : ==> files dictionary
files dictionary: file-id(guid) : ==> file object
I want to know at each time how many items are in each collection.
How would you suggest me to implemet this in JS ?
Should I use array or object with dynamically added properties?

Do the classes as follows.
function HashTable() {
var content = {};
var count = 0;
this.Add = function(key, value) {
if (content.hasOwnProperty(key)) throw new Error("Key already exist");
content[key] = value;
count++;
};
this.Set = function(key, value) {
if (!content.hasOwnProperty(key)) count++;
content[key] = value;
};
this.Get = function(key) {
if (!content.hasOwnProperty(key)) throw new Error("No such key");
return content[key];
};
this.AllKeys = function() {
var keys = [];
for (a in content) {
keys.push(a);
}
return keys;
};
this.Count = function() {
return count;
};
this.Remove = function(key) {
if (!content.hasOwnProperty(key)) throw new Error("No such key");
delete content[key];
count--;
};
}
// Then you can use it as follows
var folders = new HashTable();
folders.Add(1, 10);
alert(folders.Count());
alert(folders.Get(1));
folders.Remove(1);
alert(folders.Count());
It gives you a more rigid and OOP approach.
Edit
This ensures that your keys are unique, gives you count at any time and accepts integers and
strings as keys.

You can just write it out:
var folders = {
'folder1-guid': [
{'file1-guid': 'file1-content'},
{'file2-guid': 'file1-content'}
]
};
Alternatively, create Object and Array instances and assign the properties to them.

How to break an functional-style iteration?

I'm experimenting with functional-style Javascript and have encountered an interesting situation. I have a foreach function that takes a collection and function object:
var Utils = {};
// Applies a functor to each item in a collection.
Utils.foreach = function(collection, functor)
{
for (var i = 0; i < collection.length; i++)
{
functor(collection[i]);
}
};
This is cool. However now I want to implement another function:
// Checks if a collection contains an item.
Utils.has = function(collection, item)
{
Utils.foreach(collection, function(obj) {
if (item === obj) {
return true; // How to force a return from the foreach function?
}
});
return false;
};
As you can see I can't implement the "has" function because my return statement doesn't break the iteration.
Can someone recommend a solution for this problem?

I guess what you want is not actually forEach, but rather some (other languages call it any). The couterpart is every (or all in other languages). You'll find an example implementation on MDC.

You need a modification to each.
Start by modifying has:
Utils.has = function (collection, item) {
var found = false;
Utils.foreach(collection, function (obj) {
if (item === obj) {
found = true;
return false;
}
});
return found;
};
Then you need to modify forEach to end early when it gets false
Utils.foreach = function (collection, functor) {
var prop;
for (prop in collection) {
if (prop.hasOwnProperty(prop)) {
if (functor(collection[prop]) === false) {
return;
}
}
}
};

Here is something real quick and untested ( it is friday 4:50 time to go home). I'll try to test and update this post later. see if this helps:
Utils = {};
Utils.foreach = function(collection, functor) {
loop: for (var i in collection) {
if (functor(collection[i])) {
alert("breaking the loop!");
break loop;
}
}
};
Utils.has = function(collection, item) {
var bolReturn = false;
Utils.foreach(collection, function(obj) {
if (item === obj) {
bolReturn = true;
return true;
}
return false;
});
return bolReturn;
};
Utils.has({"test":""}, "");

I don't think you need to abandon your structure-
why not throw and catch an error to break out of the loop?
Utils.has= function(collection, item){
try{
ControlCenter.foreach(collection, function(obj){
if(item=== obj){
throw 'found it!'
}
});
}
catch(er){
if(er== 'found it!') return true;
}
return false;
};