Develop Reference

javascript method is undefined

I'm trying to learn javascript. As part of that effort, I am writing a basic minimax AI. I have the following methods:
Computer.prototype.expand = function(node) {
/* adds all state action pairs to the node.successors array */
};
Computer.prototype.getMove = function(boardAr) {
console.log("getMove");
var b2 = boardAr.slice();
var i;
var action;
this.root = new TNode(b2, this.mark);
this.root.AIPlayedLast = false;
this.expand(this.root);
this.root.successors.forEach(this.minVal);
action = maxNode(root.successors);
this.draw(action);
registerMove(action, this.mark);
};
Computer.prototype.minVal = function(node) {
if (node.isTerminal) {
return;
} else {
this.expand(node);
node.successors.forEach(maxVal);
node.utility = this.minNode(node.successors).utility;
}
};
When the getMove method is called the subsequent call to expand goes as expected. But, when expand is called from the minVal method I get: Uncaught TypeError: undefined is not a function. I'm utterly perplexed by this. Any help/suggestions would be greatly appreciated.

I think the reason is in this row:
this.root.successors.forEach(this.minVal);
You pass minVal as contextless reference, it will not be called in a context of your Computer instance (this)
Here is how you can improve it:
var self = this;
this.root.successors.forEach(function() {
self.minVal.apply(self,arguments);
})

The simplest and quickest solution is just to change
this.root.successors.forEach(this.minVal);
to
this.root.successors.forEach(this.minVal.bind(this))
This solves the problem in the same as the other answers, but in a way some might consider more compact.
Or, you can pass a "this" to the forEach function as the second argument, a somewhat under-utilized feature of forEach:
this.root.successors.forEach(this.minVal, this)
This feature is also available on other Array prototype methods that take functions, including map, filter, some, every (but not reduce and reduceRight).
ES6 arrow functions handle this differently, so you can do
this.root.successors(forEach(e => this.minVal(e)));

The forEach() method might be called for each of the successors. So, you pass the Computer::minVal method (this.minVal), but with the TNode(?) as this-pointer. Try:
var that = this;
this.root.successors.forEach(function(node) {
that.minVal(node));
});

need help understanding closures usage in this code

Here is a simplified snippet from some code I wrote for managing tablet gestures on canvas elements
first a function that accepts an element and a dictionary of callbacks and register the events plus adding other features like 'hold' gestures:
function registerStageGestures(stage, callbacks, recieverArg) {
stage.inhold = false;
stage.timer = null;
var touchduration = 1000;
var reciever = recieverArg || window;
stage.onLongTouch = function(e) {
if (stage.timer) clearTimeout(stage.timer);
stage.inhold = true;
if (callbacks.touchholdstart) callbacks.touchholdstart.call(reciever, e);
};
stage.getContent().addEventListener('touchstart', function(e) {
e.preventDefault();
calcTouchEventData(e);
stage.timer = setTimeout(function() {
stage.onLongTouch(e);
}, touchduration);
if (callbacks.touchstart) callbacks.touchholdstart.call(reciever, e);
});
stage.getContent().addEventListener('touchmove', function(e) {
e.preventDefault();
if (stage.timer) clearTimeout(stage.timer);
if (stage.inhold) {
if (callbacks.touchholdmove) callbacks.touchholdmove.call(reciever, e);
} else {
if (callbacks.touchmove) callbacks.touchmove.call(reciever, e);
}
});
stage.getContent().addEventListener('touchend', function(e) {
e.preventDefault();
if (stage.timer) clearTimeout(stage.timer);
if (stage.inhold) {
if (callbacks.touchholdend) callbacks.touchholdend.call(reciever, e);
} else {
if (callbacks.touchend) callbacks.touchend.call(reciever, e);
}
stage.inhold = false;
});
}
later I call registerStageGestures on a few elements (represented by 'View' objects) in the same page. Something like:
function View() {
var self=this;
..
function InitView() {
...
registerStageGestures(kineticStage, {
touchstart: function(e) {
// do something
},
touchmove: function(e) {
// do something
},
touchendunction(e) {
// do something
},
touchholdstart: function(e) {
// do something
},
touchholdmove: function(e) {
// do something
},
touchholdend: function(e) {
// do something
},
}, self);
Everything works fine, however I'm left wondering about two things in the implementation of registerStageGestures:
First, is it necessary to make inhold, timer and onLongTouch members of the stage ? or will closures make everything works well if they are local vars in registerStageGestures ?
Second, is it necessary to call the callbacks with '.call(receiver,' syntax ? I'm doing this to make sure the callback code will run in the context of the View but I'm not sure if it's needed ?
any input is much appreciated
Thanks!

First, is it necessary to make inhold, timer and onLongTouch members
of the stage ? or will closures make everything works well if they are
local vars in registerStageGestures ?
As far as registerStageGestures() is concerned, var inhold, var timer and function onLongTouch(e) {...}. would suffice. The mechanism by which an inner function has automatic access to its outer function's members is known as "closure". You would only need to set stage.inhold, stage.timer and stage.onLongTouch if some other piece of code needs access to these settings as properties of stage.
Second, is it necessary to call the callbacks with '.call(receiver,'
syntax ? I'm doing this to make sure the callback code will run in the
context of the View but I'm not sure if it's needed ?
Possibly, depending on how those callbacks are written. .call() and .apply() are sometimes used when calling functions that use this internally. In both cases, the first parameter passed defines the object to be interpreted as this. Thus, javascript gives you the means of defining general purpose methods with no a priori assumption about the object to which those methods will apply when called. Similarly, you can call a method of an object in such a way that it acts on another object.
EDIT:
For completeness, please note that even in the absence of this in a function, .apply() can be very useful as it allows multiple parameters to be specified as elements of a single array, eg the ubiquitous jQuery.when.apply(null, arrayOfPromises)...

There are some simple answers, here.
First, closure:
Closure basically says that whatever is defined inside of a function, has access to the rest of that function's contents.
And all of those contents are guaranteed to stay alive (out of the trash), until there are no more objects left, which ere created inside.
A simple test:
var testClosure = function () {
var name = "Bob",
recallName = function () { return name; };
return { getName : recallName };
};
var test = testClosure();
console.log(test.getName()); // Bob
So anything that was created inside can be accessed by any function which was also created inside (or created inside of a function created in a function[, ...], inside).
var closure_2x = function () {
var name = "Bob",
innerScope = function () {
console.log(name);
return function () {
console.log("Still " + name);
}
};
return innerScope;
};
var inner_func = closure_2x();
var even_deeper = inner_func(); // "Bob"
even_deeper(); // "Still Bob"
This applies not only to variables/objects/functions created inside, but also to function arguments passed inside.
The arguments have no access to the inner-workings(unless passed to methods/callbacks), but the inner-workings will remember the arguments.
So as long as your functions are being created in the same scope as your values (or a child-scope), there's access.
.call is trickier.
You know what it does (replaces this inside of the function with the object you pass it)...
...but why and when, in this case are harder.
var Person = function (name, age) {
this.age = age;
this.getAge = function () {
return this.age;
};
};
var bob = new Person("Bob", 32);
This looks pretty normal.
Honestly, this could look a lot like Java or C# with a couple of tweaks.
bob.getAge(); // 32
Works like Java or C#, too.
doSomething.then(bob.getAge);
? Buh ?
We've now passed Bob's method into a function, as a function, all by itself.
var doug = { age : 28 };
doug.getAge = bob.getAge;
Now we've given doug a reference to directly use bobs methid -- not a copy, but a pointer to the actual method.
doug.getAge(); // 28
Well, that's odd.
What about what came out of passing it in as a callback?
var test = bob.getAge;
test(); // undefined
The reason for this, is, as you said, about context...
But the specific reason is because this inside of a function in JS isn't pre-compiled, or stored...
this is worked out on the fly, every time the function is called.
If you call
obj.method();
this === obj;
If you call
a.b.c.d();
this === a.b.c;
If you call
var test = bob.getAge;
test();
...?
this is equal to window.
In "strict mode" this doesn't happen (you get errors really quickly).
test.call(bob); //32
Balance restored!
Mostly...
There are still a few catches.
var outerScope = function () {
console.log(this.age);
var inner = function () {
console.log("Still " + this.age);
};
inner();
};
outerScope.call(bob);
// "32"
// "Still undefined"
This makes sense, when you think about it...
We know that if a function figures out this at the moment it's called -- scope has nothing to do with it...
...and we didn't add inner to an object...
this.inner = inner;
this.inner();
would have worked just fine (but now you just messed with an external object)...
So inner saw this as window.
The solution would either be to use .call, or .apply, or to use function-scoping and/or closure
var person = this,
inner = function () { console.log(person.age); };
The rabbit hole goes deeper, but my phone is dying...

how to add an argument to a method stored in an array that is called later

This is a follow-up to this question (although this is self-contained) trying to `call` three methods but not working correctly with jQuery map.
I am trying to store a set of methods in an array but there is a set that might have arguments like below (the initial methods are in before_methods and the proposed methods are in lm_methods). I'm sure it's pretty self explanatory what I want but I'd like to be able to merge in the arguments into a reasonable call to f (specifically the arc.pLikedByTerm). I currently have the following:
// signature
pLikedByTerm:function(term, ne, sw, m){
....
}
// code before_methods just to show
this.before_methods=[arc.pLocations,arc.pLikedLocations,arc.pLikedItems];
this.lm_methods=[arc.pLocations,arc.pLikedLocations,arc.pLikedItems, arc.pLikedByTerm('surfing'),arc.pLikedByTerm('sailing')];
$.each(this.lm_methods, function(i,f){
f(ne,sw,m);
});
How would I do this or is this bad design? What would be the idiomatic way? My brain is fried.
thx in advance
Update 1
Playing around with answer below, it looks like this works which might the simplest things:
var fns=[logStuff("this is msg"), logMoreArgs("a term","a you msg")];
for (var i=0; i<fns.length; i++) {
fns[i];
}

Having an array of functions is common practice when used often. For example, consider this Callback class.
function Callback(){
this.callbacks = [];
}
Callback.prototype.run = function(cb) {
for (var i=0; i<this.callbacks.length; i++) {
this.callbacks[i]();
}
};
We can then add some callbacks.
function logStuff(msg) {
jsprint(msg || "No message");
}
obj = new Callback();
obj.callbacks.push(logStuff);
obj.callbacks.push(logStuff);
obj.run();
If we run this we see that it's only logging our default value. So if we want to bind some data, we can use the bind function.
Function.prototype.bind
thisArg
The value to be passed as the this parameter to the target
function when the bound function is called. The value is ignored if
the bound function is constructed using the new operator.
arg1, arg2, ...
Arguments to prepend to arguments provided to the bound function
when invoking the target function.
Our new code sets the first parameter to different strings, which we then see. You can bind any number of parameters.
obj = new Callback();
obj.callbacks.push(logStuff.bind(null, "My message"));
obj.callbacks.push(logStuff.bind(null, "My other message"));
obj.run();
end result

The way you are doing would work just ok. Just remove the arguments and parens:
Instead of:
this.lm_methods=[arc.pLocations,arc.pLikedLocations,arc.pLikedItems,
arc.pLikedByTerm('surfing'),arc.pLikedByTerm('sailing')];
Do:
this.lm_methods=[arc.pLocations,arc.pLikedLocations,arc.pLikedItems,
arc.pLikedByTerm,arc.pLikedByTerm];
Example:
function say(txt) {
console.log("say" + txt);
}
function shout(txt) {
console.log("shout" + txt);
}
function whisper(txt) {
console.log("whisper" + txt);
}
var funcArr = [say, shout, whisper];
$.each(funcArr, function(i, f) {
f("hello");
});
would print:
sayhello
shouthello
whisperhello

Confused by this - getting error "this.myfuntion() is not a function"

Background: I am trying to edit a zen cart horizontal pop out menu to make the popout open inline within the menu. The problem I am having is that I am struggling to get my head around the javascript/jquery that came with it.
Without posting the whole thing the structure of the code is something like this:
(declare some vars)
//some functions like this:
function funcname(obj) {
//do something
}
//then one big master function like this:
function bigfunc(arg1, arg2, arg3, arg4, arg5) {
//declare some vars based on this
this.varname1=varname1;
this.varname2=varname2;
//declare some functions inside the big function
this.innerfunc1= function() {
//do stuff
}
this.innerfunc2= function() {
//do stuff
}
}//end of big function
//then goes on to declare init function
function initfunc(){
//this creates new bigfunc(arg1 arg2 arg3...) for each main menu item
}
//finally calls init function with
window.onload = initfunc();
Now on to my confusion -
1) firstly for clarification, am I correct in thinking based on all the this's floating about in bigfunc() and the fact that it is called with new bigfunc() that this is creating an object?
2)My current problem is with one of the functions inside bigfunc() which looks like this:
this.slideChildMenu = function() {
var divref = this.children[0].div;
var ulref = this.children[0].ul;
var maxwidth = this.children[0].width;
var nextWidth;
if (this.isMouseOnMe || this.isMouseOnChild()) {
nextWidth = divref.offsetWidth + slideSpeed_out;
if (nextWidth >= maxwidth) {
this.finishOpeningChild(divref, ulref, maxwidth);
} else {
ulref.style.left = nextWidth - maxwidth + "px";
divref.style.width = nextWidth + "px";
setTimeout("slideChildMenu('" + this.getId() + "')", slideTimeout_out);
}
}
Now my plan is to alter this to use jquery show to open the element so I tried this:
this.slideChildMenu = function() {
var divref = this.children[0].div;
var ulref = this.children[0].ul;
if (this.isMouseOnMe || this.isMouseOnChild()) {
$(divref).show(function(){
this.finishOpeningChild(divref, ulref);
});
}
}
But I am getting this-> TypeError: this.finishOpeningChild is not a function
Now, there is a lot of other stuff going on in this js so I wouldnt dream of asking someone on here to do my work for me, but I am hoping that if someone can explain to me why this function is not a function I may be able to work the rest out.
NOTE: I thought this was to do with the scope of "this" but the value of this appears to be exactly the same in both versions of the code.
I know this is a long one but your help is greatly appreciated.

The value of this in a function is called the "context" in which the function runs. In general, whenever you pass a callback function as an argument (as you do with $(divref).show(function() {...})), the function can run the callback in whatever context it wants. In this case, the jQuery show function chooses to run its callback in the context of the element being animated.
However, you want access to the value of this at the time the anonymous callback function is defined, rather than when it is run. The solution here is to store the outer value of this in a variable (traditionally called self) which is included in the scope of the newly-defined function:
this.slideChildMenu = function() {
//...
var self = this;
$(divref).show(function(){
self.finishOpeningChild(divref, ulref);
});
}

I am thinking that the jQuery selector has changed the scope of this.
In your example $(this); would refer to object being animated per jQuery api docs:
If supplied, the callback is fired once the animation is complete. This can be useful for stringing different animations together in sequence. The callback is not sent any arguments, but this is set to the DOM element being animated. If multiple elements are animated, it is important to note that the callback is executed once per matched element, not once for the animation as a whole.
If the object in question is instantiated you can call it with dot notation without using this like bigFunc.finishOpeningChild(divref, ulref);

You're probably a little confused about scope, it's not always easy keeping track, but doing something more like this:
var site = {
init: function(elm) {
self=site;
self.master.funcname2(self.varname1, elm); //call function in master
},
funcname: function(obj) {
//do something
},
varname1: 'some string',
varname2: 3+4,
master: function() {
this.varname3 = sin(30);
this.funcname2 = function(stuff, element) {
site.funcname(element); //call function in 'site'
var sinus = site.master.varname3; //get variable
}
}
}
window.onload = function() {
var elm = document.getElementById('elementID');
site.init(elm); //call init function
}
usually makes it a little easier to keep track.

Overriding a JavaScript function while referencing the original

I have a function, a(), that I want to override, but also have the original a() be performed in an order depending on the context. For example, sometimes when I'm generating a page I'll want to override like this:
function a() {
new_code();
original_a();
}
and sometimes like this:
function a() {
original_a();
other_new_code();
}
How do I get that original_a() from within the over-riding a()? Is it even possible?
Please don't suggest alternatives to over-riding in this way, I know of many. I'm asking about this way specifically.

You could do something like this:
var a = (function() {
var original_a = a;
if (condition) {
return function() {
new_code();
original_a();
}
} else {
return function() {
original_a();
other_new_code();
}
}
})();
Declaring original_a inside an anonymous function keeps it from cluttering the global namespace, but it's available in the inner functions.
Like Nerdmaster mentioned in the comments, be sure to include the () at the end. You want to call the outer function and store the result (one of the two inner functions) in a, not store the outer function itself in a.

The Proxy pattern might help you:
(function() {
// log all calls to setArray
var proxied = jQuery.fn.setArray;
jQuery.fn.setArray = function() {
console.log( this, arguments );
return proxied.apply( this, arguments );
};
})();
The above wraps its code in a function to hide the "proxied"-variable. It saves jQuery's setArray-method in a closure and overwrites it. The proxy then logs all calls to the method and delegates the call to the original. Using apply(this, arguments) guarantees that the caller won't be able to notice the difference between the original and the proxied method.

Thanks guys the proxy pattern really helped.....Actually I wanted to call a global function foo..
In certain pages i need do to some checks. So I did the following.
//Saving the original func
var org_foo = window.foo;
//Assigning proxy fucnc
window.foo = function(args){
//Performing checks
if(checkCondition(args)){
//Calling original funcs
org_foo(args);
}
};
Thnx this really helped me out

You can override a function using a construct like:
function override(f, g) {
return function() {
return g(f);
};
}
For example:
a = override(a, function(original_a) {
if (condition) { new_code(); original_a(); }
else { original_a(); other_new_code(); }
});
Edit: Fixed a typo.

Passing arbitrary arguments:
a = override(a, function(original_a) {
if (condition) { new_code(); original_a.apply(this, arguments) ; }
else { original_a.apply(this, arguments); other_new_code(); }
});

The answer that #Matthew Crumley provides is making use of the immediately invoked function expressions, to close the older 'a' function into the execution context of the returned function. I think this was the best answer, but personally, I would prefer passing the function 'a' as an argument to IIFE. I think it is more understandable.
var a = (function(original_a) {
if (condition) {
return function() {
new_code();
original_a();
}
} else {
return function() {
original_a();
other_new_code();
}
}
})(a);

The examples above don't correctly apply this or pass arguments correctly to the function override. Underscore _.wrap() wraps existing functions, applies this and passes arguments correctly. See: http://underscorejs.org/#wrap

In my opinion the top answers are not readable/maintainable, and the other answers do not properly bind context. Here's a readable solution using ES6 syntax to solve both these problems.
const orginial = someObject.foo;
someObject.foo = function() {
if (condition) orginial.bind(this)(...arguments);
};

I had some code written by someone else and wanted to add a line to a function which i could not find in the code. So as a workaround I wanted to override it.
None of the solutions worked for me though.
Here is what worked in my case:
if (typeof originalFunction === "undefined") {
originalFunction = targetFunction;
targetFunction = function(x, y) {
//Your code
originalFunction(a, b);
//Your Code
};
}

I've created a small helper for a similar scenario because I often needed to override functions from several libraries. This helper accepts a "namespace" (the function container), the function name, and the overriding function. It will replace the original function in the referred namespace with the new one.
The new function accepts the original function as the first argument, and the original functions arguments as the rest. It will preserve the context everytime. It supports void and non-void functions as well.
function overrideFunction(namespace, baseFuncName, func) {
var originalFn = namespace[baseFuncName];
namespace[baseFuncName] = function () {
return func.apply(this, [originalFn.bind(this)].concat(Array.prototype.slice.call(arguments, 0)));
};
}
Usage for example with Bootstrap:
overrideFunction($.fn.popover.Constructor.prototype, 'leave', function(baseFn, obj) {
// ... do stuff before base call
baseFn(obj);
// ... do stuff after base call
});
I didn't create any performance tests though. It can possibly add some unwanted overhead which can or cannot be a big deal, depending on scenarios.

So my answer ended up being a solution that allows me to use the _this variable pointing to the original object.
I create a new instance of a "Square" however I hated the way the "Square" generated it's size. I thought it should follow my specific needs. However in order to do so I needed the square to have an updated "GetSize" function with the internals of that function calling other functions already existing in the square such as this.height, this.GetVolume(). But in order to do so I needed to do this without any crazy hacks. So here is my solution.
Some other Object initializer or helper function.
this.viewer = new Autodesk.Viewing.Private.GuiViewer3D(
this.viewerContainer)
var viewer = this.viewer;
viewer.updateToolbarButtons = this.updateToolbarButtons(viewer);
Function in the other object.
updateToolbarButtons = function(viewer) {
var _viewer = viewer;
return function(width, height){
blah blah black sheep I can refer to this.anything();
}
};

Not sure if it'll work in all circumstances, but in our case, we were trying to override the describe function in Jest so that we can parse the name and skip the whole describe block if it met some criteria.
Here's what worked for us:
function describe( name, callback ) {
if ( name.includes( "skip" ) )
return this.describe.skip( name, callback );
else
return this.describe( name, callback );
}
Two things that are critical here:
We don't use an arrow function () =>.
Arrow functions change the reference to this and we need that to be the file's this.
The use of this.describe and this.describe.skip instead of just describe and describe.skip.
Again, not sure it's of value to anybody but we originally tried to get away with Matthew Crumley's excellent answer but needed to make our method a function and accept params in order to parse them in the conditional.