I'm trying to access a property of an object using a dynamic name. Is this possible?
const something = { bar: "Foobar!" };
const foo = 'bar';
something.foo; // The idea is to access something.bar, getting "Foobar!"
There are two ways to access properties of an object:
Dot notation: something.bar
Bracket notation: something['bar']
The value between the brackets can be any expression. Therefore, if the property name is stored in a variable, you have to use bracket notation:
var something = {
bar: 'foo'
};
var foo = 'bar';
// both x = something[foo] and something[foo] = x work as expected
console.log(something[foo]);
console.log(something.bar)
This is my solution:
function resolve(path, obj) {
return path.split('.').reduce(function(prev, curr) {
return prev ? prev[curr] : null
}, obj || self)
}
Usage examples:
resolve("document.body.style.width")
// or
resolve("style.width", document.body)
// or even use array indexes
// (someObject has been defined in the question)
resolve("part.0.size", someObject)
// returns null when intermediate properties are not defined:
resolve('properties.that.do.not.exist', {hello:'world'})
In javascript we can access with:
dot notation - foo.bar
square brackets - foo[someVar] or foo["string"]
But only second case allows to access properties dynamically:
var foo = { pName1 : 1, pName2 : [1, {foo : bar }, 3] , ...}
var name = "pName"
var num = 1;
foo[name + num]; // 1
// --
var a = 2;
var b = 1;
var c = "foo";
foo[name + a][b][c]; // bar
Following is an ES6 example of how you can access the property of an object using a property name that has been dynamically generated by concatenating two strings.
var suffix = " name";
var person = {
["first" + suffix]: "Nicholas",
["last" + suffix]: "Zakas"
};
console.log(person["first name"]); // "Nicholas"
console.log(person["last name"]); // "Zakas"
This is called computed property names
You can achieve this in quite a few different ways.
let foo = {
bar: 'Hello World'
};
foo.bar;
foo['bar'];
The bracket notation is specially powerful as it let's you access a property based on a variable:
let foo = {
bar: 'Hello World'
};
let prop = 'bar';
foo[prop];
This can be extended to looping over every property of an object. This can be seem redundant due to newer JavaScript constructs such as for ... of ..., but helps illustrate a use case:
let foo = {
bar: 'Hello World',
baz: 'How are you doing?',
last: 'Quite alright'
};
for (let prop in foo.getOwnPropertyNames()) {
console.log(foo[prop]);
}
Both dot and bracket notation also work as expected for nested objects:
let foo = {
bar: {
baz: 'Hello World'
}
};
foo.bar.baz;
foo['bar']['baz'];
foo.bar['baz'];
foo['bar'].baz;
Object destructuring
We could also consider object destructuring as a means to access a property in an object, but as follows:
let foo = {
bar: 'Hello World',
baz: 'How are you doing?',
last: 'Quite alright'
};
let prop = 'last';
let { bar, baz, [prop]: customName } = foo;
// bar = 'Hello World'
// baz = 'How are you doing?'
// customName = 'Quite alright'
You can do it like this using Lodash get
_.get(object, 'a[0].b.c');
UPDATED
Accessing root properties in an object is easily achieved with obj[variable], but getting nested complicates things. Not to write already written code I suggest to use lodash.get.
Example
// Accessing root property
var rootProp = 'rootPropert';
_.get(object, rootProp, defaultValue);
// Accessing nested property
var listOfNestedProperties = [var1, var2];
_.get(object, listOfNestedProperties);
Lodash get can be used in different ways, the documentation lodash.get
To access a property dynamically, simply use square brackets [] as follows:
const something = { bar: "Foobar!" };
const userInput = 'bar';
console.log(something[userInput])
The problem
There's a major gotchya in that solution! (I'm surprised other answers have not brought this up yet). Often you only want to access properties that you've put onto that object yourself, you don't want to grab inherited properties.
Here's an illustration of this issue. Here we have an innocent-looking program, but it has a subtle bug - can you spot it?
const agesOfUsers = { sam: 16, sally: 22 }
const username = prompt('Enter a username:')
if (agesOfUsers[username] !== undefined) {
console.log(`${username} is ${agesOfUsers[username]} years old`)
} else {
console.log(`${username} is not found`)
}
When prompted for a username, if you supply "toString" as a username, it'll give you the following message: "toString is function toString() { [native code] } years old". The issue is that agesOfUsers is an object, and as such, automatically inherits certain properties like .toString() from the base Object class. You can look here for a full list of properties that all objects inherit.
Solutions
Use a Map data structure instead. The stored contents of a map don't suffer from prototype issues, so they provide a clean solution to this problem.
const agesOfUsers = new Map()
agesOfUsers.set('sam', 16)
agesOfUsers.set('sally', 2)
console.log(agesOfUsers.get('sam')) // 16
Use an object with a null prototype, instead of the default prototype. You can use Object.create(null) to create such an object. This sort of object does not suffer from these prototype issues, because you've explicitly created it in a way that it does not inherit anything.
const agesOfUsers = Object.create(null)
agesOfUsers.sam = 16
agesOfUsers.sally = 22;
console.log(agesOfUsers['sam']) // 16
console.log(agesOfUsers['toString']) // undefined - toString was not inherited
You can use Object.hasOwn(yourObj, attrName) to first check if the dynamic key you wish to access is directly on the object and not inherited (learn more here). This is a relatively newer feature, so check the compatibility tables before dropping it into your code. Before Object.hasOwn(yourObj, attrName) came around, you would achieve this same effect via Object.prototype.hasOwnProperty.call(yourObj, attrName). Sometimes, you might see code using yourObj.hasOwnProperty(attrName) too, which sometimes works but it has some pitfalls that you can read about here.
// Try entering the property name "toString",
// you'll see it gets handled correctly.
const user = { name: 'sam', age: 16 }
const propName = prompt('Enter a property name:')
if (Object.hasOwn(user, propName)) {
console.log(`${propName} = ${user[propName]}`)
} else {
console.log(`${propName} is not found`)
}
If you know the key you're trying to use will never be the name of an inherited property (e.g. maybe they're numbers, or they all have the same prefix, etc), you can choose to use the original solution.
I came across a case where I thought I wanted to pass the "address" of an object property as data to another function and populate the object (with AJAX), do lookup from address array, and display in that other function. I couldn't use dot notation without doing string acrobatics so I thought an array might be nice to pass instead. I ended-up doing something different anyway, but seemed related to this post.
Here's a sample of a language file object like the one I wanted data from:
const locs = {
"audioPlayer": {
"controls": {
"start": "start",
"stop": "stop"
},
"heading": "Use controls to start and stop audio."
}
}
I wanted to be able to pass an array such as: ["audioPlayer", "controls", "stop"] to access the language text, "stop" in this case.
I created this little function that looks-up the "least specific" (first) address parameter, and reassigns the returned object to itself. Then it is ready to look-up the next-most-specific address parameter if one exists.
function getText(selectionArray, obj) {
selectionArray.forEach(key => {
obj = obj[key];
});
return obj;
}
usage:
/* returns 'stop' */
console.log(getText(["audioPlayer", "controls", "stop"], locs));
/* returns 'use controls to start and stop audio.' */
console.log(getText(["audioPlayer", "heading"], locs));
ES5 // Check Deeply Nested Variables
This simple piece of code can check for deeply nested variable / value existence without having to check each variable along the way...
var getValue = function( s, context ){
return Function.call( context || null, 'return ' + s )();
}
Ex. - a deeply nested array of objects:
a = [
{
b : [
{
a : 1,
b : [
{
c : 1,
d : 2 // we want to check for this
}
]
}
]
}
]
Instead of :
if(a && a[0] && a[0].b && a[0].b[0] && a[0].b[0].b && a[0].b[0].b[0] && a[0].b[0].b[0].d && a[0].b[0].b[0].d == 2 ) // true
We can now :
if( getValue('a[0].b[0].b[0].d') == 2 ) // true
Cheers!
Others have already mentioned 'dot' and 'square' syntaxes so I want to cover accessing functions and sending parameters in a similar fashion.
Code jsfiddle
var obj = {method:function(p1,p2,p3){console.log("method:",arguments)}}
var str = "method('p1', 'p2', 'p3');"
var match = str.match(/^\s*(\S+)\((.*)\);\s*$/);
var func = match[1]
var parameters = match[2].split(',');
for(var i = 0; i < parameters.length; ++i) {
// clean up param begninning
parameters[i] = parameters[i].replace(/^\s*['"]?/,'');
// clean up param end
parameters[i] = parameters[i].replace(/['"]?\s*$/,'');
}
obj[func](parameters); // sends parameters as array
obj[func].apply(this, parameters); // sends parameters as individual values
I asked a question that kinda duplicated on this topic a while back, and after excessive research, and seeing a lot of information missing that should be here, I feel I have something valuable to add to this older post.
Firstly I want to address that there are several ways to obtain the value of a property and store it in a dynamic Variable. The first most popular, and easiest way IMHO would be:
let properyValue = element.style['enter-a-property'];
however I rarely go this route because it doesn't work on property values assigned via style-sheets. To give you an example, I'll demonstrate with a bit of pseudo code.
let elem = document.getElementById('someDiv');
let cssProp = elem.style['width'];
Using the code example above; if the width property of the div element that was stored in the 'elem' variable was styled in a CSS style-sheet, and not styled inside of its HTML tag, you are without a doubt going to get a return value of undefined stored inside of the cssProp variable. The undefined value occurs because in-order to get the correct value, the code written inside a CSS Style-Sheet needs to be computed in-order to get the value, therefore; you must use a method that will compute the value of the property who's value lies within the style-sheet.
Henceforth the getComputedStyle() method!
function getCssProp(){
let ele = document.getElementById("test");
let cssProp = window.getComputedStyle(ele,null).getPropertyValue("width");
}
W3Schools getComputedValue Doc This gives a good example, and lets you play with it, however, this link Mozilla CSS getComputedValue doc talks about the getComputedValue function in detail, and should be read by any aspiring developer who isn't totally clear on this subject.
As a side note, the getComputedValue method only gets, it does not set. This, obviously is a major downside, however there is a method that gets from CSS style-sheets, as well as sets values, though it is not standard Javascript.
The JQuery method...
$(selector).css(property,value)
...does get, and does set. It is what I use, the only downside is you got to know JQuery, but this is honestly one of the very many good reasons that every Javascript Developer should learn JQuery, it just makes life easy, and offers methods, like this one, which is not available with standard Javascript.
Hope this helps someone!!!
For anyone looking to set the value of a nested variable, here is how to do it:
const _ = require('lodash'); //import lodash module
var object = { 'a': [{ 'b': { 'c': 3 } }] };
_.set(object, 'a[0].b.c', 4);
console.log(object.a[0].b.c);
// => 4
Documentation: https://lodash.com/docs/4.17.15#set
Also, documentation if you want to get a value: https://lodash.com/docs/4.17.15#get
You can do dynamically access the property of an object using the bracket notation. This would look like this obj[yourKey] however JavaScript objects are really not designed to dynamically updated or read. They are intended to be defined on initialisation.
In case you want to dynamically assign and access key value pairs you should use a map instead.
const yourKey = 'yourKey';
// initialise it with the value
const map1 = new Map([
['yourKey', 'yourValue']
]);
// initialise empty then dynamically assign
const map2 = new Map();
map2.set(yourKey, 'yourValue');
console.log(map1.get(yourKey));
console.log(map2.get(yourKey));
demo object example
let obj = {
name: {
first_name: "Bugs",
last_name: "Founder",
role: "Programmer"
}
}
dotted string key for getting the value of
let key = "name.first_name"
Function
const getValueByDottedKeys = (obj, strKey)=>{
let keys = strKey.split(".")
let value = obj[keys[0]];
for(let i=1;i<keys.length;i++){
value = value[keys[i]]
}
return value
}
Calling getValueByDottedKeys function
value = getValueByDottedKeys(obj, key)
console.log(value)
output
Bugs
const getValueByDottedKeys = (obj, strKey)=>{
let keys = strKey.split(".")
let value = obj[keys[0]];
for(let i=1;i<keys.length;i++){
value = value[keys[i]]
}
return value
}
let obj = {
name: {
first_name: "Bugs",
last_name: "Founder",
role: "Programmer"
}
}
let key = "name.first_name"
value = getValueByDottedKeys(obj, key)
console.log(value)
I bumped into the same problem, but the lodash module is limited when handling nested properties. I wrote a more general solution following the idea of a recursive descendent parser. This solution is available in the following Gist:
Recursive descent object dereferencing
Finding Object by reference without, strings,
Note make sure the object you pass in is cloned , i use cloneDeep from lodash for that
if object looks like
const obj = {data: ['an Object',{person: {name: {first:'nick', last:'gray'} }]
path looks like
const objectPath = ['data',1,'person',name','last']
then call below method and it will return the sub object by path given
const child = findObjectByPath(obj, objectPath)
alert( child) // alerts "last"
const findObjectByPath = (objectIn: any, path: any[]) => {
let obj = objectIn
for (let i = 0; i <= path.length - 1; i++) {
const item = path[i]
// keep going up to the next parent
obj = obj[item] // this is by reference
}
return obj
}
You can use getter in Javascript
getter Docs
Check inside the Object whether the property in question exists,
If it does not exist, take it from the window
const something = {
get: (n) => this.n || something.n || window[n]
};
You should use JSON.parse, take a look at https://www.w3schools.com/js/js_json_parse.asp
const obj = JSON.parse('{ "name":"John", "age":30, "city":"New York"}')
console.log(obj.name)
console.log(obj.age)
I'm currently trying to port some Specman code that takes certain actions based on whether a variable is part of an enumeration. In Specman the code looks something like this:
define COMP_STATES do_add, do_sub;
define WAIT_STATES wait_X, wait_Y;
defube RUN_STATES run_X, run_Y;
type my_states : [
do_add = 3'b000;
do_sub = 3'b001;
wait_X = 3'b010;
wait_Y = 3'b011;
run_X = 3'b100;
run_Y = 3'b101;
] (bits:3);
and then later:
if(state in [COMP_STATES, RUN_STATES]){
/* DO STUFF */
} else if(state in [WAIT_STATES]){
/* DO STUFF */
}
I now would like to do this in SystemVerilog but have hit a little bit of a snag. My currently best approach uses arrays:
my_state which_state[$] = {`COMP_STATES, `RUN_STATES};
int indexes[$] = which_state.get_index( index ) where ( index == state );
int num_indexes = indexes.size(); //If state doesn't exist in my_state then the returned array of indexes will be empty.
if(num_indexes > 0) begin /* DO STUFF */ end
But there has to be a more elegant and concise way? find_first_index comes to mind but I couldn't find what it would return in case no match was found.
There are a couple of ways you could do this. If you can depend on the encoding, then you can define your collection with a wildcard and use the wildcard equality operator or inside operator
let COMP_STATES = 3'b00?; // or parameter COMP_STATES = 3'b00?;
let RUN_STATES = 3'b01?; // or parameter RUN_STATES = 3'b01?;
let WAIT_STATES = 3'b10?; // or parameter WAIT_STATES = 3'b10?;
if (my_states inside {COMP_STATES,RUN_STATES})
...
else if (my_state ==? WAIT_STATES)
...
or you could just create an expression
module top;
enum bit [2:0] {
do_add = 3'b000,
do_sub = 3'b001,
wait_X = 3'b010,
wait_Y = 3'b011,
run_X = 3'b100,
run_Y = 3'b101
} my_states;
let COMP_STATES = my_states inside {do_add, do_sub};
let WAIT_STATES = my_states inside {wait_X, wait_Y};
let RUN_STATES = my_states inside {run_X, run_Y};
initial repeat(20) begin
std::randomize(my_states);
$write (my_states.name());
case(1)
COMP_STATES, RUN_STATES:
$display("=comp,run");
WAIT_STATES:
$display("-wait");
endcase
end
endmodule
Finally, if you were starting from scratch, I would suggest looking at tagged unions and their corresponding pattern matching conditional statements
I'm having a hard time grokking classes in Lua. Fruitless googling led me to ideas about meta-tables, and implied that third-party libraries are necessary to simulate/write classes.
Here's a sample (just because I've noticed I get better answers when I provide sample code):
public class ElectronicDevice
{
protected bool _isOn;
public bool IsOn { get { return _isOn; } set { _isOn = value; } }
public void Reboot(){_isOn = false; ResetHardware();_isOn = true; }
}
public class Router : ElectronicDevice
{
}
public class Modem :ElectronicDevice
{
public void WarDialNeighborhood(string areaCode)
{
ElectronicDevice cisco = new Router();
cisco.Reboot();
Reboot();
if (_isOn)
StartDialing(areaCode);
}
}
Here is my first attempt to translate the above using the technique suggested by Javier.
I took the advice of RBerteig. However, invocations on derived classes still yield: "attempt to call method 'methodName' (a nil value)"
--Everything is a table
ElectronicDevice = {};
--Magic happens
mt = {__index=ElectronicDevice};
--This must be a constructor
function ElectronicDeviceFactory ()
-- Seems that the metatable holds the fields
return setmetatable ({isOn=true}, mt)
end
-- Simulate properties with get/set functions
function ElectronicDevice:getIsOn() return self.isOn end
function ElectronicDevice:setIsOn(value) self.isOn = value end
function ElectronicDevice:Reboot() self.isOn = false;
self:ResetHardware(); self.isOn = true; end
function ElectronicDevice:ResetHardware() print('resetting hardware...') end
Router = {};
mt_for_router = {__index=Router}
--Router inherits from ElectronicDevice
Router = setmetatable({},{__index=ElectronicDevice});
--Constructor for subclass, not sure if metatable is supposed to be different
function RouterFactory ()
return setmetatable ({},mt_for_router)
end
Modem ={};
mt_for_modem = {__index=Modem}
--Modem inherits from ElectronicDevice
Modem = setmetatable({},{__index=ElectronicDevice});
--Constructor for subclass, not sure if metatable is supposed to be different
function ModemFactory ()
return setmetatable ({},mt_for_modem)
end
function Modem:WarDialNeighborhood(areaCode)
cisco = RouterFactory();
--polymorphism
cisco.Reboot(); --Call reboot on a router
self.Reboot(); --Call reboot on a modem
if (self.isOn) then self:StartDialing(areaCode) end;
end
function Modem:StartDialing(areaCode)
print('now dialing all numbers in ' .. areaCode);
end
testDevice = ElectronicDeviceFactory();
print("The device is on? " .. (testDevice:getIsOn() and "yes" or "no") );
testDevice:Reboot(); --Ok
testRouter = RouterFactory();
testRouter:ResetHardware(); -- nil value
testModem = ModemFactory();
testModem:StartDialing('123'); -- nil value
Here's an example literal transcription of your code, with a helpful Class library that could be moved to another file.
This is by no means a canonical implementation of Class; feel free to define your object model however you like.
Class = {}
function Class:new(super)
local class, metatable, properties = {}, {}, {}
class.metatable = metatable
class.properties = properties
function metatable:__index(key)
local prop = properties[key]
if prop then
return prop.get(self)
elseif class[key] ~= nil then
return class[key]
elseif super then
return super.metatable.__index(self, key)
else
return nil
end
end
function metatable:__newindex(key, value)
local prop = properties[key]
if prop then
return prop.set(self, value)
elseif super then
return super.metatable.__newindex(self, key, value)
else
rawset(self, key, value)
end
end
function class:new(...)
local obj = setmetatable({}, self.metatable)
if obj.__new then
obj:__new(...)
end
return obj
end
return class
end
ElectronicDevice = Class:new()
function ElectronicDevice:__new()
self.isOn = false
end
ElectronicDevice.properties.isOn = {}
function ElectronicDevice.properties.isOn:get()
return self._isOn
end
function ElectronicDevice.properties.isOn:set(value)
self._isOn = value
end
function ElectronicDevice:Reboot()
self._isOn = false
self:ResetHardware()
self._isOn = true
end
Router = Class:new(ElectronicDevice)
Modem = Class:new(ElectronicDevice)
function Modem:WarDialNeighborhood(areaCode)
local cisco = Router:new()
cisco:Reboot()
self:Reboot()
if self._isOn then
self:StartDialing(areaCode)
end
end
If you were to stick to get/set methods for properties, you wouldn't need __index and __newindex functions, and could just have an __index table. In that case, the easiest way to simulate inheritance is something like this:
BaseClass = {}
BaseClass.index = {}
BaseClass.metatable = {__index = BaseClass.index}
DerivedClass = {}
DerivedClass.index = setmetatable({}, {__index = BaseClass.index})
DerivedClass.metatable = {__index = DerivedClass.index}
In other words, the derived class's __index table "inherits" the base class's __index table. This works because Lua, when delegating to an __index table, effectively repeats the lookup on it, so the __index table's metamethods are invoked.
Also, be wary about calling obj.Method(...) vs obj:Method(...). obj:Method(...) is syntactic sugar for obj.Method(obj, ...), and mixing up the two calls can produce unusual errors.
There are a number of ways you can do it but this is how I do (updated with a shot at inheritance):
function newRGB(r, g, b)
local rgb={
red = r;
green = g;
blue = b;
setRed = function(self, r)
self.red = r;
end;
setGreen = function(self, g)
self.green= g;
end;
setBlue = function(self, b)
self.blue= b;
end;
show = function(self)
print("red=",self.red," blue=",self.blue," green=",self.green);
end;
}
return rgb;
end
purple = newRGB(128, 0, 128);
purple:show();
purple:setRed(180);
purple:show();
---// Does this count as inheritance?
function newNamedRGB(name, r, g, b)
local nrgb = newRGB(r, g, b);
nrgb.__index = nrgb; ---// who is self?
nrgb.setName = function(self, n)
self.name = n;
end;
nrgb.show = function(self)
print(name,": red=",self.red," blue=",self.blue," green=",self.green);
end;
return nrgb;
end
orange = newNamedRGB("orange", 180, 180, 0);
orange:show();
orange:setGreen(128);
orange:show();
I don't implement private, protected, etc. although it is possible.
If you don't want to reinvent the wheel, there is a nice Lua library implementing several object models. It's called LOOP.
The way I liked to do it was by implementing a clone() function.
Note that this is for Lua 5.0. I think 5.1 has more built-in object oriented constructions.
clone = function(object, ...)
local ret = {}
-- clone base class
if type(object)=="table" then
for k,v in pairs(object) do
if type(v) == "table" then
v = clone(v)
end
-- don't clone functions, just inherit them
if type(v) ~= "function" then
-- mix in other objects.
ret[k] = v
end
end
end
-- set metatable to object
setmetatable(ret, { __index = object })
-- mix in tables
for _,class in ipairs(arg) do
for k,v in pairs(class) do
if type(v) == "table" then
v = clone(v)
end
-- mix in v.
ret[k] = v
end
end
return ret
end
You then define a class as a table:
Thing = {
a = 1,
b = 2,
foo = function(self, x)
print("total = ", self.a + self.b + x)
end
}
To instantiate it or to derive from it, you use clone() and you can override things by passing them in another table (or tables) as mix-ins
myThing = clone(Thing, { a = 5, b = 10 })
To call, you use the syntax :
myThing:foo(100);
That will print:
total = 115
To derive a sub-class, you basically define another prototype object:
BigThing = clone(Thing, {
-- and override stuff.
foo = function(self, x)
print("hello");
end
}
This method is REALLY simple, possibly too simple, but it worked well for my project.
It's really easy to do class-like OOP in Lua; just put all the 'methods' in the __index field of a metatable:
local myClassMethods = {}
local my_mt = {__index=myClassMethods}
function myClassMethods:func1 (x, y)
-- Do anything
self.x = x + y
self.y = y - x
end
............
function myClass ()
return setmetatable ({x=0,y=0}, my_mt)
Personally, I've never needed inheritance, so the above is enough for me. If it's not enough, you can set a metatable for the methods table:
local mySubClassMethods = setmetatable ({}, {__index=myClassMethods})
local my_mt = {__index=mySubClassMethods}
function mySubClassMethods:func2 (....)
-- Whatever
end
function mySubClass ()
return setmetatable ({....}, my_mt)
update:
There's an error in your updated code:
Router = {};
mt_for_router = {__index=Router}
--Router inherits from ElectronicDevice
Router = setmetatable({},{__index=ElectronicDevice});
Note that you initialize Router, and build mt_for_router from this; but then you reassign Router to a new table, while mt_for_router still points to the original Router.
Replace the Router={} with the Router = setmetatable({},{__index=ElectronicDevice}) (before the mt_for_router initialization).
Your updated code is wordy, but should work. Except, you have a typo that is breaking one of the metatables:
--Modem inherits from ElectronicDevice
Modem = setmetatable({},{__index,ElectronicDevice});
should read
--Modem inherits from ElectronicDevice
Modem = setmetatable({},{__index=ElectronicDevice});
The existing fragment made the Modem metatable be an array where the first element was almost certainly nil (the usual value of _G.__index unless you are using strict.lua or something similar) and the second element is ElectronicDevice.
The Lua Wiki description will make sense after you've grokked metatables a bit more. One thing that helps is to build a little infrastructure to make the usual patterns easier to get right.
I'd also recommend reading the chapter on OOP in PiL. You will want to re-read the chapters on tables and metatables too. Also, I've linked to the online copy of the 1st edition, but owning a copy of the 2nd is highly recommended. There is also a couple of articles in the Lua Gems book that relate. It, too, is recommended.
Another simple approach for subclass
local super = require("your base class")
local newclass = setmetatable( {}, {__index = super } )
local newclass_mt = { __index = newclass }
function newclass.new(...) -- constructor
local self = super.new(...)
return setmetatable( self, newclass_mt )
end
You still can use the functions from superclass even if overwritten
function newclass:dostuff(...)
super.dostuff(self,...)
-- more code here --
end
don't forget to use ONE dot when pass the self to the superclass function