I am currently watching a tutorial video "30 days to learn jQuery".
I have a question about why the tutor in the video returned a variable from a function.
Here's the code:
This is in the HTML file which just binds an event handler to buttons, calls functions, etc.
(function() {
slider.nav.find('button').on('click', function() {
slider.setCurrent( $(this).data('dir') );
slider.transition();
});
})();
and this is the one function I'm interested in (in the js file):
Slider.prototype.setCurrent = function( dir ) {
var pos = this.current;
pos += ( ~~( dir === 'next' ) || -1 );
this.current = ( pos < 0 ) ? this.imgsLen - 1 : pos % this.imgsLen;
return pos; // <== HERE
};
The only thing I want to figure out is why return pos? I tried removing it and the code still worked.
Was it a mistake or is there sound logic to this?
In a nutshell, setCurrent function is called and setCurrent returns a value. But why?
I can't speculate as to why your tutor did that, but a common practice, especially in jQuery, is to overload a function by having the same function be both a setter and getter based on what parameters are passed. An example is below:
function (dir)
{
//If dir was passed as a parameter
if (typeof dir !== "undefined")
{
//Setter code
value = dir;
}
//Getter (always returns value)
return value;
}
There are tons of functions in jQuery that use this, e.g .val, .height, .css('propertyName', [optionallySetValue]), etc.
Let me know if this makes sense, or if you have any questions :)
It's hard to know without seeing the rest of your code, but often functions that set a value on object will return something, even though that is counterintuitive, since their purpose is to set a value, not to get something.
The most common pattern you'll see is to return the object itself. The allows you to chain together multiple setter calls at once:
object.setColor("red").setSize("large");
This is not specifically going on in your case, but the author may have had a similar use in mind, in the sense that he wanted to do 2 things at once: set a value, and get some information about how that value was set.
Related
In Javascript what is the best way to handle scenarios when you have a set of arrays to perform tasks on sets of data and sometimes you do not want to include all of the arrays but instead a combination.
My arrays are labeled in this small snippet L,C,H,V,B,A,S and to put things into perspective the code is around 2500 lines like this. (I have removed code notes from this post)
if(C[0].length>0){
L=L[1].concat(+(MIN.apply(this,L[0])).toFixed(7));
C=C[1].concat(C[0][0]);
H=H[1].concat(+(MAX.apply(this,H[0])).toFixed(7));
V=V[1].concat((V[0].reduce(function(a,b){return a+b}))/(V[0].length));
B=B[1].concat((MAX.apply(this,B[0])-MIN.apply(this,B[0]))/2);
A=A[1].concat((MAX.apply(this,A[0])-MIN.apply(this,A[0]))/2);
D=D[1].concat((D[0].reduce(function(a,b){return a+b}))/(D[0].length));
S=S[1].concat((S[0].reduce(function(a,b){return a+b}))/(S[0].length));
}
It would seem counter-productive in this case to litter the code with tones of bool conditions asking on each loop or code section if an array was included in the task and even more silly to ask inside each loop iteration with say an inline condition as these would also slow down the processing and also make the code look like a maze or rabbit hole.
Is there a logical method / library to ignore instruction or skip if an option was set to false
All I have come up with so far is kind of pointless inline thing
var op=[0,1,1,0,0,0,0,0]; //options
var L=[],C=[],H=[],V=[],B=[],A=[],D=[],S=[];
op[0]&&[L[0]=1];
op[1]&&[C[0]=1,console.log('test, do more than one thing')];
op[2]&&[H[0]=1];
op[3]&&[V[0]=1];
op[4]&&[B[0]=1];
op[5]&&[A[0]=1];
op[6]&&[A[0]=1];
It works in that it sets only C[0] and H[0] to 1 as the options require, but it fails as it needs to ask seven questions per iteration of a loop as it may be done inside a loop. Rather than make seven versions of the the loop or code section, and rather than asking questions inside each loop is there another style / method?
I have also noticed that if I create an array then at some point make it equal to NaN rather than undefined or null the console does not complain
var L=[],C=[],H=[],V=[],B=[],A=[],D=[],S=[];
L=NaN;
L[0]=1;
//1
console.log(L); //NaN
L=undefined;
L[0]=1
//TypeError: Cannot set property '0' of undefined
L=null
L[0]=1
//TypeError: Cannot set property '0' of null
Am I getting warmer? I would assume that if I performed some math on L[0] when isNaN(L)===true that the math is being done but not stored so the line isn't being ignored really..
If I understand what you want I would do something like this.
var op = [...],
opchoice = {
//these can return nothing, no operation, or a new value.
'true': function(val){ /*operation do if true*/ },
'false': function(val){ /*operation do if false*/ },
//add more operations here.
//keys must be strings, or transformed into strings with operation method.
operation: function(val){
//make the boolean a string key.
return this[''+(val == 'something')](val);
}
};
var endop = [];//need this to prevent infinite recursion(loop).
var val;
while(val = op.shift()){
//a queue operation.
endop.push(opchoice.operation(val));
}
I'm sure this is not exactly what you want, but it's close to fulfilling the want of not having a ton of conditions every where.
Your other option is on every line do this.
A = isNaN(A) ? A.concat(...) : A;
Personally I prefer the other method.
It looks like you repeat many of the operations. These operations should be functions so at least you do not redefine the same function over and over again (it is also an optimization to do so).
function get_min(x)
{
return +(MIN.apply(this, a[0])).toFixed(7);
}
function get_max(x)
{
return +(MAX.apply(this, a[0])).toFixed(7);
}
function get_average(x)
{
return (x[0].reduce(function(a, b) {return a + b})) / (x[0].length);
}
function get_mean(x)
{
return (MAX.apply(this, x[0]) - MIN.apply(this, x[0])) / 2;
}
if(C[0].length > 0)
{
L = L[1].concat(get_min(L));
C = C[1].concat(C[0][0]);
H = H[1].concat(get_max(H));
V = V[1].concat(get_average(V));
B = B[1].concat(get_mean(B));
A = A[1].concat(get_mean(A);
D = D[1].concat(get_average(D));
S = S[1].concat(get_average(S));
}
You could also define an object with prototype functions, but it is not clear whether it would be useful (outside of putting those functions in a namespace).
In regard to the idea/concept of having a test, what you've found is probably the best way in JavaScript.
op[0] && S = S[1].concat(get_average(S));
And if you want to apply multiple operators when op[0] is true, use parenthesis and commas:
op[3] && (V = V[1].concat(get_average(V)),
B = B[1].concat(get_mean(B)),
A = A[1].concat(get_mean(A));
op[0] && (D = D[1].concat(get_average(D)),
S = S[1].concat(get_average(S)));
However, this is not any clearer, to a programmer, than an if() block as shown in your question. (Actually, many programmers may have to read it 2 or 3 times before getting it.)
Yet, there is another solution which is to use another function layer. In that last example, you would do something like this:
function VBA()
{
V = V[1].concat(get_average(V));
B = B[1].concat(get_mean(B));
A = A[1].concat(get_mean(A));
}
function DS()
{
D = D[1].concat(get_average(D));
S = S[1].concat(get_average(S));
}
op = [DS,null,null,VBA,null,null,...];
for(key in op)
{
// optional: if(op[key].hasOwnProperty(key)) ... -- verify that we defined that key
if(op[key])
{
op[key](); // call function
}
}
So in other words you have an array of functions and can use a for() loop to go through the various items and if defined, call the function.
All of that will very much depend on the number of combinations you have. You mentioned 2,500 lines of code, but the number of permutations may be such that writing it one way or the other will possibly not reduce the total number of lines, but it will make it easier to maintain because many lines are moved to much smaller code snippet making the overall program easier to understand.
P.S. To make it easier to read and debug later, I strongly suggest you put more spaces everywhere, as shown above. If you want to save space, use a compressor (minimizer), Google or Yahoo! both have one that do a really good job. No need to write your code pre-compressed.
My intention is to get your thoughts and criticism about the script below, as regards the algorithm's design, performance and cross-browser compatibility.
I have just started getting into JavaScript having missed out on its awesomeness for quite a while. My background and experience is in developing C/C++/PHP based RESTful backends.
In order to understand the language and the right way of using it, I decided to do something which I am sure has been done many times before. But learning to use a new language and paradigm often entails pain anyway.
This is my attempt to create a normal form processing and validation script/ function.
In order to reduce complexity and keep code simple/clean, I decided to use HTML5 Custom Data Attributes (data-*) to assign metadata for each element in the form:
Data-Required: True or False. If set to true, this parameter makes the form-field required and so it cannot be empty. A value set to false indicates that the field is optional. Default is false.>
Data-Type: Type of validation to be performed. Examples include 'email', 'password', 'numbers' or any other 'regexp'.
A fairy simple example of such a form would be:
<form action="postlistings" id="postlistings" enctype='multipart/form-data' method="post" class="postlistings">
<ul class="login-li">
<li>
<input class="title" name="title" type="title" id="title" data-required="true" data-type="title"></a>
</li>
<li>
<textarea name="body" id="elm1" class="elm1" name="elm1" data-type="body" data-required="true" >
</textarea>
</li>
<li>
<span class="nav-btn-question">Add Listing</span>
</li>
</ul>
</form>
Reminder: This is my first piece of JavaScript code.
The idea is to call Form while passing the form name to retrieve and validate all the field values in one loop for performance. The validation involves two steps as can be guessed from the Data-* attributes described above:
i. Check for required form fields.
In case the values fail to meet step 1 requirement, an error message from configuration is pulled for the specific form value. Thus, for all values that fail to meet this requirement, an array of error messages are collected and passed on to the View.
ii. Perform respective validations.
Validations are only performed if all the values passed step 1. Otherwise, they follow the same steps as indicated in 1 above.
function Form(){
var args = Array.prototype.slice.call(arguments),
formName = args[0],
callback = args.pop(),
userError = [{type: {}, param: {}}],
requiredDataParam = 'required',
typeDataParam = 'type',
form = document.forms[formName],
formLength = form.length || null,
formElement = {id: {}, name: {}, value: {}, required: {}, type: {}};
function getFormElements(){
var num = 0;
var emptyContent = false;
for (var i = 0; i < formLength; i += 1) {
var formField = form[i];
formElement.id[i] = inArray('id', formField) ? formField.id : null;
formElement.name[i] = inArray('name', formField) ? formField.name : null;
formElement.value[i] = inArray('value', formField) ? formField.value : null;
formElement.required[i] = getDataAttribute(formField, requiredDataParam);
formElement.type[i] = getDataAttribute(formField, typeDataParam);
if (formElement.required[i] === true){
if(!formElement.type[i]) {
error('Validation rule not defined!');
}
else if (!formElement.value[i]) {
userError[num++] = {'type': 'required', 'param': form[i]};
emptyContent = true;
}
}
if (emptyContent === false) {
// Perform validations only if no empty but required form values were found.
// This is so that we can collect all the empty
// inputs and their corresponding error messages.
}
}
if (userError) {
// Return empty form errors and their corresponding error messages.
}
return formElement;
};
// Removed the getFormParam function that was not used at all.
return {
getFormElements: getFormElements
}
};
Two outside functions that are used in the JS script above (from JQuery source):
var inArray = function(elem, array){
if (array.indexOf){
return array.indexOf(elem);
}
for (var i = 0, length = array.length; i < length; i++){
if (array[i] === elem){
return i;
}
}
return -1;
}
// This is a cross-platform way to retrieve HTML5 custom attributes.
// Source: JQuery
var getDataAttribute = function(elem, key, data) {
if (data === undefined && elem.nodeType === 1) {
data = elem.getAttribute("data-" + key);
if (typeof data === "string") {
data = data === "true" ? true :
data === "false" ? false :
data === "null" ? null :
!CheckType.isNaN ? parseFloat(data) :
CheckType.rbrace.test(data) ? parseJSON(data) :
data;
}
else {
data = undefined;
}
}
return data;
}
An example of Config Error messages can be set as follows:
var errorMsgs = {
ERROR_email: "Please enter a valid email address.",
ERROR_password: "Your password must be at least 6 characters long. Please try another",
ERROR_user_exists: "The requested email address already exists. Please try again."
};
As I post this for your review, please ignore any styling conventions that I might not have followed. My intention is to get your expert reviews on anything I should be doing different or could do better concerning the code itself, and the algorithm.
Besides the styling conventions, all criticism and questions are welcome.
First I'd like to clear up a common misconception. Forgive me if you already understand this clearly; maybe it will be helpful for someone else.
Learning and using jQuery or a similar library does not preclude or conflict with learning the JavaScript language. jQuery is simply a DOM manipulation library which takes away many of the pain points of using the DOM. There's plenty of room to learn and use JavaScript, the language, even if you use a library to abstract away some of the DOM details.
In fact, I would argue that using the DOM directly is likely to teach bad JavaScript coding habits, because the DOM is very much not a "JavaScript-ish" API. It was designed to work identically in JavaScript and Java and potentially other languages, and so it completely fails to make good use of the features of the JavaScript language.
Of course as you said, you're using this as a learning exercise; I just don't want you to fall into the trap that I've seen many people fall into of thinking, "I don't want to learn jQuery, because I want to learn JavaScript instead!" That's a false dichotomy: you have to learn JavaScript in either case, and using jQuery for the DOM doesn't interfere with that at all.
Now some details...
While it's OK to quote property names in an object literal and when you reference the properties, it's customary - and more readable - not to quote them when they are valid JavaScript names. e.g. in your formElement object
formElement = { id: {}, name: {}, value: {}, required: {}, type: {} };
(there was a missing semicolon at the end there too)
and where you use the names you can do:
formElement.id[i] = ...
formElement.name[i] = ...
etc.
Don't run your loops backwards unless the program logic requires it. It doesn't make the code faster except possibly in the case of an extremely tight loop, and it makes it unclear whether you're just prematurely optimizing or actually need the backwards loop.
Speaking of optimization, that loop has several inArray() calls. Since each of those loops through an array, that could be more of a performance impact than the outer loop. I imagine these arrays are probably pretty short? So performance wouldn't matter at all anyway, but this is something to think about in cases where you have longer arrays and objects. In some cases you can use an object with property names and values for a faster lookup - but I didn't look closely enough at what you're doing to suggest anything.
In any case, you're using inArray() wrong! But not your fault, that is a ridiculously named function in jQuery. The name clearly suggests a boolean return value, but the function returns the zero-based array index or -1 if the value is not found. I strongly recommend renaming this function as indexOf() to match the native Array method, or arrayIndex(), or some such.
That same loop has form[i] repeated numerous times. You could do this at the top of the loop:
var field = form[i];
and then use field throughout, e.g. field.id instead of form[i].id. This is generally faster, if it matters (which it probably doesn't here), but more importantly it's easier to read.
Do not use strict boolean comparisons like if( foo === true ) and if( bar === false) unless you really need to - and those cases are rare. The code sends a signal to the reader that there is something going on that's different from the usual boolean test. The only time these particular tests should be used is when you have a variable that may contain a boolean value or may contain some other type of value, and you need to distinguish which is which.
A good example of a case where you should use tests like these is an optional parameter that defaults to true:
// Do stuff unless 'really' is explicitly set to false, e.g.
// stuff(1) will do stuff with 1, but stuff(1,false) won't.
function stuff( value, really ) {
if( really === false ) {
// don't do stuff
}
else {
// do stuff
}
}
That specific example doesn't make a lot of sense, but it should give you the idea.
Similarly, an === true test could be used in a case where need to distinguish an actual boolean true value from some other "truthy" value. Indeed, it looks like this line is a valid case for that:
if (formElement['required'][i] === true){
given that if (formElement['required'][i] comes from the getDataAttribute() function which may return a boolean or other type.
If you are just testing for truthiness, though - and this should be most of the time - simply use if( foo ) or if( ! foo ). Or similarly in a conditional expression: foo ? x : y or !foo ? x : y.
The above was a long-winded way of saying that you should change this:
if (empty_content === false) {
to:
if (!empty_content) {
Your getFormParam() function goes to some work to convert an undefined result to null. There is usually no reason to do this. I don't see any place where that function is called, so I can't advise specifically, but in general you'd be testing for truthiness on something like this, so null and undefined would both be treated as false. Or in cases where you do need to distinguish null/undefined from other values (say, an explicit false), you can easily do it with != null or == null. This is one case where the "looser" comparison performed by == and != is very useful: both null and undefined evaluate the same with these operators.
You asked to ignore coding style, but one little suggestion here: You have a mix of camelCaseNames and names_with_underscores. In JavaScript, camelCaseNames are more idiomatic for function and variable names, with PascalCaseNames for constructor functions. Of course feel free to use underscores where they make more sense, for example if you're writing code that works with database columns in that format you may want your variable names to match the column names.
Hope that helps! Keep up the good work.
Update for your new code
I'm having a bit of trouble following the logic in the code, and I think I know part of the reason. It's a combination of naming conventions and inside-out objects.
First, the name formElement is really confusing. When I see element in JavaScript, I think of either a DOM element (HTMLElement) or an array element. I'm not sure if this formElement represents one or the other or neither.
So I look at the code to figure out what it's doing, and I see it has id:{}, name:{}, ... properties, but the code later treats each of those as an Array and not an Object:
formElement.id[i] = ...
formElement.name[i] = ...
formElement.value[i] = ...
formElement.required[i] = ...
formElement.type[i] = ...
(where i is an integer index)
If that code is right, those should be arrays instead: id:[], name:[], ....
But this is a red flag. When you see yourself creating parallel arrays in JavaScript, you're probably doing it wrong. In most cases you're better off replacing the parallel arrays with a single array of objects. Each of the objects in that array represents a single slice through all your parallel arrays, with a property for each of the previous arrays.
So, this object (where I've made the correction from {} to [] to match its current use):
formElement = { id: [], name: [], value: [], required: [], type: [] };
should be:
formInfo = [];
and then where you have the code that goes:
formElement.id[i] = ...;
formElement.name[i] = ...;
formElement.value[i] = ...;
formElement.required[i] = ...;
formElement.type[i] = ...;
It should be:
var info = {
id: ...,
name: ...,
value: ...,
required: ...,
type: ...
};
formInfo.push( info );
and adjust the rest of the code to suit. For example:
formElement.required[i]
would be:
formInfo[i].required
or even simpler since it's in the same function:
info.required
And note: I'm not saying info and formInfo are great names :-) they are just placeholders so you can think of a better name. The main idea is to create an array of objects instead of a set of parallel arrays.
One last thing and then I'm out of time for now.
That getDataAttribute() function is a complicated little piece of work. You don't need it! It would be simpler would just call the underlying function directly where you need it:
var info = {
...
required: formField.getAttribute('data-required') === 'true',
type: formField.getAttribute('data-type')
};
This also gives you full control of how the attributes are interpreted - as in the === 'true' test above. (This gives you a proper boolean value, so when you test the value later you don't have to use === true on it.)
On a stylistic note, yes, I did hard code the two 'data-xxxx' names right there, and I think that's a better and more clear way to do it.. Don't let your C experience throw you off here. There's no advantage to defining a string "constant" in this particular case, unless it's something that you want to make configurable, which this isn't.
Also, even if you do make a string constant, there's a minor advantage to having the complete 'data-whatever' string instead of just 'whatever'. The reason is that when somebody reads your HTML code, they may see a string in it and search the JS code for that string. But when they search for data-whatever they won't find it if the data- prefix is automagically prepended in the JS code.
Oh, I forgot one last thing. This code:
function Form(){
var args = Array.prototype.slice.call(arguments),
formName = args[0],
callback = args.pop(),
is working way too hard! Just do this instead:
function Form( formName, callback ) {
(and keep the var for the remaining variable declarations of course)
I cannot add comments yet so here is a little tip. I would separate the getFormElements() into smaller private functions. And I would add the errorMsgs to the Form function.
But for a first script in JavaScript, it is very impressive. This is actually the real reason I respond. I think it deserves more upvotes, and I would be very interested in a JS ninja responding to this question.
Good luck!
I'm a javascript newbie so I'm writing ugly code so far sometimes due to my lack of experience and how different it is to the languages I'm used to, so the code I'll post below works, but I'm wondering if I'm doing it the right way or perhaps it works but it's a horrible practice or there is a better way.
Basically, I have a little dude that moves within a grid, he receives from the server an action, he can move in 8 directions (int): 0:up, 1: up-right, 2: right... 7: up-left.
the server will send him this 0 <= action <= 7 value, and he has to take the correct action... now, instead of using a switch-case structure. I created a function goUp(), goLeft(), etc, and loaded them in an array, so I have a method like this:
var getActionFunction = actions[action];
actionFunction();
However, what to set all this up is this:
1) create a constructor function:
function LittleDude(container) {
this.element = container; //I will move a div around, i just save it in field here.
}
LittleDude.prototype.goUp() {
//do go up
this.element.animate(etc...);
}
LittleDude.prototype.actions = [LittleDude.prototype.goUp, LittleDude.prototype.goUpLeft, ...];
//In this array I can't use "this.goUp", because this points to the window object, as expected
LittleDude.prototype.doAction = function(action) {
var actionFunction = this.actions[action];
actionFunction(); //LOOK AT THIS LINE
}
Now if you pay attention, the last line won't work.. because: when i use the index to access the array, it returns a LittleDude.prototype.goUp for instance... so the "this" keyword is undefined..
goUp has a statement "this.element"... but "this" is not defined, so I have to write it like this:
actionFunction.call(this);
so my doAction will look like this:
LittleDude.prototype.doAction = function(action) {
var actionFunction = this.actions[action];
actionFunction.call(this); //NOW IT WORKS
}
I need to know if this is hackish or if I'm violating some sort of "DO NOT DO THIS" rule. or perhaps it can be written in a better way. Since it seems to me kind of weird to add it to the prototype but then treating it like a function that stands on its own.
What you are trying to do is one of the possible ways, but it is possible to make it more simple. Since object property names are not necessary strings, you can use action index directly on prototype. You even don't need doAction function.
LittleDude = function LittleDude(container) {
this.container = container;
}
LittleDude.prototype[0] = LittleDude.prototype.goUp = function goUp() {
console.log('goUp', this.container);
}
LittleDude.prototype[1] = LittleDude.prototype.goUpRight = function goUpRight() {
console.log('goUpRight', this.container);
}
var littleDude = new LittleDude(123),
action = 1;
littleDude[action](); // --> goUpRight 123
littleDude.goUp(); // --> goUp 123
actionFunction.call(this); //NOW IT WORKS
I need to know if this is hackish or if I'm violating some sort of "DO NOT DO THIS" rule. or perhaps it can be written in a better way.
No, using .call() is perfectly fine for binding the this keyword - that's what it's made for.
Since it seems to me kind of weird to add it to the prototype but then treating it like a function that stands on its own.
You don't have to define them on the prototype if you don't use them directly :-) Yet, if you do you might not store the functions themselves in the array, but the method names and then call them with bracket notation:
// or make that a local variable somewhere?
LittleDude.prototype.actions = ["goUp", "goUpLeft", …];
LittleDude.prototype.doAction = function(action) {
var methodName = this.actions[action];
this[methodName](); // calls the function in expected context as well
}
I'm building an application with KnockoutJS with a component that essentially acts as a sequential spreadsheet. On different lines users may define variables or use them to represent a value.
So for example
x =2
x //2
x = 4
x //4
I have this working in the straightforward case of continuing adding new lines. The output function for each line checks and iterates backwards to see if the variable was ever defined previously. If it was it uses the first example it finds and sets that as the value. This works when initially defining the lines, and also works when you edit a line after a previous line has changed.
However, I would like variables to update if a previous definition of that variable has changed, been removed, or been added. That behavior does not exist right now. I have tried adding my own custom dependency handling code using a map to track the variables, but it badly impacted performance. I would like to tap into Knockouts dependency management to solve this, but I'm not sure of the best way to do so. Here is a brief summary of my code structure, I would be happy to add more detail if needed.
calcFramework is the view-model object I bind to the map. It consists of an observable list of Lines, a varMap, and other unrelated properties and functions
Line is a custom object. The relevant code is below
var Line = function (linenum,currline) {
var self = this;
self.varMap = {};
self.input = ko.observable("");
self.linenum = ko.observable(linenum);
self.lnOutput = ko.computed({
read:function(){
return outputFunction(self,self.input());
},
write:function(){},
owner:self
});
};
function outputFunction(self,input) {
try{
var out = EQParser.parse(input,10,self);
return out.toString();
}
catch(ex){
//error handling
}
}
Line.prototype.getVar = function (varName, notCurrentLine) {
if(typeof varName === "undefined"){
return null;
}
//Actually don't want ones set in the current varMap, only past lines
if(varName in this.varMap && notCurrentLine){
return this.varMap[varName];
}
if (this.linenum() > 0) {
var nextLine = calcFramework.lines()[this.linenum() - 1];
return nextLine.getVar(varName,true);
} else {
//eventually go to global
return calcFramework.varMap[varName];
}
};
Line.prototype.setVar = function(varName,value){
this.varMap[varName] = value;
};
SetVar and getVar are passed to eqParser, which gets the value of the expression, calling those functions as needed if a variable is referenced. So the variable value is not explicitly passed to the function and thus knockout does not view it as a dependency. But I'm not sure how I would pass the variable as a parameter without traversing the list every time.
So my question is, given this setup, what is the best way to track changes to a variable assignment (and/or new assignments) and update the lines that reference that variable, while maintaining good performance.
I recognize my question is lengthy and I have attempted to trim out all unnecessary detail. Thanks for your patience in reading.
I would be tempted to use a publish/subscribe model, using something like Peter Higgins' PubSub jquery plugin
Your overall app would subscribe/listen out for lines publishing an event that they have a variable definition. This would store any variable names in a standard javascript hashtable, along with the value. When a variable found event is published by a line, the app would check through all the known variables, and if it finds that it is a change to an existing variable value, it would publish a variable changed event. All the lines would subscribe to that event. They can then check whether they have a variable matching that name, and update the value accordingly.
Here's some untested code to give you an idea of what I mean:
var app = function()
{
var self = this;
self.variables = {};
$.subscribe('/variableAssigned', function (key, value)
{
// I think that this is the best way of checking that there is a variable
// in the object
if(self.variables.hasOwnProperty(key))
{
if(self.variables[key] !== value)
{
$.publish('/variableChanged', [ key, value ]);
}
}
});
}
In your Line object:
$.subscribe('/variableChanged', function (key, value)
{
// loop through varMap and see if any of them need updating.
});
So, I am seeing a curious problem. If I have a function
// counter wraps around to beginning eventually, omitted for clarity.
var counter;
cycleCharts(chartId) {
// chartId should be undefined when called from setInterval
console.log('chartId: ' + chartId);
if(typeof chartId == 'undefined' || chartId < 0) {
next = counter++;
}
else {
next = chartId;
}
// ... do stuff to display the next chart
}
This function can be called explicitly by user action, in which case chartId is passed in as an argument, and the selected chart is shown; or it can be in autoplay mode, in which case it's called by a setInterval which is initialized by the following:
var cycleId = setInterval(cycleCharts, 10000);
The odd thing is, I'm actually seeing the cycleCharts() get a chartId argument even when it's called from setInterval! The setInterval doesn't even have any parameters to pass along to the cycleCharts function, so I'm very baffled as to why chartId is not undefined when cycleCharts is called from the setInterval.
setInterval is feeding cycleCharts actual timing data ( so one can work out the actual time it ran and use to produce a less stilted response, mostly practical in animation )
you want
var cycleId = setInterval(function(){ cycleCharts(); }, 10000);
( this behavior may not be standardized, so don't rely on it too heavily )
It tells you how many milliseconds late the callback is called.
var cycleId = setInterval(cycleCharts, 10000, 4242);
From the third parameter and onwards - they get passed into the function so in my example you send 4242 as the chartId. I know it might not be the answer to the question you posed, but it might the the solution to your problem? I think the value it gets is just random from whatever lies on the stack at the time of passing/calling the method.