note 1) 'pointer' and 'reference' are not synonymous. Although they should be, c++ and languages evolved from it have changed the definition such that they are not.
Unfortunately; 'reference' when used by C++ people means a pointer to the value being passed; but hides the details that it is a pointer by making '.' operator behave like '->'. transparently, and just setting 'a = new thing()' actually be '(*a) = new thing()', again, transparently. ( I really wanted to use 'reference' in the place of 'pointer' at several places in this; and have instead used 'pointer to a value' when I want to say 'a reference of a value' which, since C++ does not mean the same thing at all really) there's a few good, diferent explanations of the terms but I'll add my own. https://stackoverflow.com/questions/373419/whats-the- difference-between-passing-by-reference-vs-passing-by-value by there are really 3 options, pass by value, pass by reference, pass by pointer; and in this case the world should not be so black and white. any primitive value passes by value. 1, 2,3; "apple", "banana" null, etc. a copy of the value is passed to the called function. Changing values does not change the original value... if you add to a string the caller does not get its string updated. pass by pointer, a pointer to the value is passed. Changing the pointer in the callee does not change the caller's pointer. changing the content of what the pointer is pointing does change the content in the caller's pointer. pass by reference, setting a reference to a new value updates the (pointer) in the caller. In javascript there is no good way to pass by reference. So given the choice of 'pass by value' or 'pass by reference' the only choice is that javascript is always 'pass by value' because changing the values that are passed do not change the caller's original variables. Wish I had more knowledge of what sort of language background you have. in C... pass by value ( A pretty obvious thing) ``` void f( int a ) { a = 3; // this does not change the caller's variable. } f( 86 ); ``` pass by pointer ``` void f( int *a ) { (*a) = 5; // de'referenc'ing the pointer, allows changing the value in the caller a = (some other pointer to int); // this would not change the caller's value. } int value = 3; f( &value ); ``` pass by reference (and this, being a double pointer might be tough to wrap your head around) ``` void f( int **a ) { (*a[0] ) = 5; // change the value in a ( *a ) = New( int ); // New being a macro that allocates a new int, updates the 'reference' in caller. a = (some other change); // this is never reflected back to the caller. } int a = 5; int *b = &a; f( &b ); // gives the function the ability to change the pointer that is b, or the value a ``` In the third case the double-pointer is hidden in C++ so you never have the choice to try and affect the local copy of the variable, and all changes update the variables that the caller has passed. ------------- in C# there is a parameter attribute 'ref' that you can pass pointers values. Structures are simple values like numbers, so if they are passed to a function, they are copied so the callee's changes to the content of the structures does not update in the caller. (and this is not something javascript has, nor should have) ``` struct Point { int x, int y }; Point p; void byValue( p ) { p = new Point( 1,2 ); // updates to 'p' do not update the caller p.x = 44; // updates to values in structures do not update to the caller } Point a = { 1, 3 }; byValue( a ); ``` so in the previous case, since no changes to the arguments are given to the caller, this is 'pass by value' ``` struct Point { int x, int y }; Point p; void byValue( Point ref p ) { p = new Point( 1,2 ); // updates to 'p' DO update the caller's variable p.x = 44; // updates to values in structures DO update the caller's values } Point a = { 1, 3 }; byValue( ref a ); ``` in this case, a doesn't need to be allocated by the caller, and instead the called function can create a new point, or update values in the referred to pointer. This is much more like C++ 'reference' in which there is no way to make a change in the called function the does not update to the caller. a class type in C# is passed by pointer (a copy of the pointer is passed) but the contents of a class can update. Although you can also use the 'ref' parameter argument to pass the pointer to the caller's class (hmm pointer-value), so the callee can update the caller's class pointer. (there are no pointers in c# and a C# purist will flame me for saying so, but I'm trying really hard to not use 'reference to a value; see note 1') ----- Java behaves a lot like C#; but doesn't have structure type to cloud the issue. Passing by reference allows the callee to update the caller's value that references a class instance. by normal calls in that regard are pass by value, so changes to the argument value itself does not update the caller; although of course if there are values contained within the argument passed (a pointer to a class) changes to contents do propagate.... I don't know; I shouldn't talk about java. ------ So at this point, I'm going to change my rules and allow myself to say 'reference' because there are no pointers in javascript; and there are no c++-esque references. So in javascript, a function itself is a value, it is passed by reference, updating the argument of a function never modifies the caller's values. Updating what a parameter references does change the values in the caller's values. ``` var thing = { a : 5 }; function f( arg ) { arg.a = 6; // will update the caller arg = { a : 9 }; // does not update the caller } f( thing ); ``` now I have diverged a lot from the original question....(going back to read what it was) `I recently had an exchange which started out with my "correcting" someone who said "Functions are values" by saying "Function *references* are values. Functions are objects." He/she replied that objects are also values, and after much back and forth, cited [this part of the JavaScript spec][1]:` ................. after considering my reply and the question there's actually no equate.................. ``` var f = function() { } f.x = 3; ``` function references are values, not sure you can really have any context that you're not dealing with a reference to a function. functions can have properties too; so they are objects... but that's not really whether they are values or references. basically every variable in javascript is a pointer though; not a reference or a value. On Sat, Aug 19, 2017 at 11:44 AM, Naveen Chawla <naveen.c...@gmail.com> wrote: > But references are also values. It's not a mutually exclusive thing > > On Sat, 19 Aug 2017, 11:32 p.m. kdex <k...@kdex.de> wrote: > >> Yes, objects/functions are values, but they are not *primitive* values. >> >> The current draft uses the phrasing "object value" several times. >> >> Section 4.3.3 explains that objects are "members of type Object", and a >> type >> is, by definition, a set of values. Therefore, objects must be values. >> >> On Saturday, August 19, 2017 7:34:45 PM CEST T.J. Crowder wrote: >> > I recently had an exchange which started out with my "correcting" >> someone >> > who said "Functions are values" by saying "Function *references* are >> > values. Functions are objects." He/she replied that objects are also >> > values, and after much back and forth, cited [this part of the >> JavaScript >> > >> > spec][1]: >> > > An ECMAScript *language type* corresponds to values that are directly >> > >> > manipulated by an ECMAScript programmer using the ECMAScript language. >> The >> > ECMAScript language types are Undefined, Null, Boolean, String, Symbol, >> > Number, and Object. An *ECMAScript language value* is a value that is >> > characterized by an ECMAScript language type. >> > >> > ...and said "So objects are values." >> > >> > Is he/she right? [Wikpedia][2] isn't much help, at least not to me. I >> asked >> > a friend who, unlike me, did get comp sci theory at Uni, and he said >> > "...you're not wrong, but it's very Humpty Dumpty: When I use a word, it >> > means just what I choose it to mean -- neither more nor less." >> > >> > I've used the term "value" in explanations of variables, properties, and >> > function arguments and in particular when explaining why `b = a` copies >> an >> > object reference, not an object, from `a` to `b`. (Saying the *object >> > reference* is the value.) It's been an invaluable aid to helping people >> > "get" object references. >> > >> > I'd be very sorry to hear that I was misusing a term of art. But I'd >> rather >> > know. :-) >> > >> > If I'm misusing "value," what should I use instead for that thing that >> > actually gets stored in memory? How to explain the `b = a` thing with >> > object references to beginners? >> > >> > This is slightly off-topic for the thread, but also not, as I spend a >> lot >> > of time explaining things to JavaScript beginners, and the authors of >> the >> > text being used to tell me I'm wrong are on this list. :-) >> > >> > Thanks, >> > >> > -- T.J. Crowder >> > >> > [1]: >> > http://www.ecma-international.org/ecma-262/8.0/index.html# >> sec-ecmascript-lan >> > guage-types [2]: https://en.wikipedia.org/wiki/Value_(computer_science) >> _______________________________________________ >> es-discuss mailing list >> es-discuss@mozilla.org >> https://mail.mozilla.org/listinfo/es-discuss >> > > _______________________________________________ > es-discuss mailing list > es-discuss@mozilla.org > https://mail.mozilla.org/listinfo/es-discuss > >
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