Assume, for the sake of this question, that I want to be able to create a function in Javascript that appends all of the elements of one array to another array. One way to achieve this, if you have access to the destination array, is to say:
var destination = [1,2,3];
var source = [4,5];
Array.prototype.push.apply(destination, source);
console.log(destination); // [1,2,3,4,5]
Now, since Array.prototype.push.apply is pretty ugly, I want to alias it to something nicer, like:
var pushAll = Array.prototype.push.apply;
Which I should be able to call with two arguments, the context (destination) and an array of arguments (source). However, when I try to use the alias, this happens:
pushAll(destination, [6,7]);
TypeError: Function.prototype.apply was called on [object global], which
is a object and not a function
So clearly the apply function is not bound to push, which led me to try this:
var pushAll = Function.prototype.apply.bind(Array.prototype.push);
pushAll(destination, [6,7]);
console.log(destination); // [1,2,3,4,5,6,7,8]
Which clearly works fine. My question is, why do I have to bind the push method to apply? Shouldn't Array.prototype.push.apply already be bound to apply? Why does calling it under a different name result in calling it on an unbound context?
why do I have to bind the push method to apply?
It's the other way round: You have to bind the apply method to the Array push function - you can bind it to other functions as well! Otherwise apply doesn't know which method to apply with the arguments.
Function.prototype.apply.bind(Array.prototype.push); does call the bind function on the apply function with push as the argument, the argument on which apply is then bound. The resulting function pushAll will, when called, invoke apply on the push function, and pass it's argument (the array and the arguments array) to it.
Shouldn't Array.prototype.push.apply already be bound to apply?
Nope. JavaScript is designed to bind the context at the call of a function, not already when it's being referred to as a property - there is no implicit binding on property access. Otherwise Array.prototype.push would already be bound to Array.prototype, before you could call any Function methods like bind/apply on it and try to use it with a different context.
Why does calling it under a different name result in calling it on an unbound context?
It's not so much different name, but different style. (Unbound) Functions do get their this value set to the object when they are called as a method on it, i.e. when the reference to the called function is a property access: destination.push().
This allows for great flexibility, you can "borrow" functions from an object and call them on other objects, still being the same (unbound) function. This is rather impossible in languages where function objects are no first-class objects.
If functions (even though they were meant to be methods) are called as plain functions (pushAll()), their this value will be undefined (unless in sloppy mode). Read more on the this keyword at MDN.
Related
In this article js log function, there is a statement:
Function.prototype.apply.call(console.log, console, arguments);
I'm really confused by this statement.
What does it do?
How can I analyse this statement?
Or with some thoughts or tools, I can figure it out step by step?
Can it be simplified to more statements to achieve the same result? for instance: var temp = Function.prototype.call(console.log, console, arguments); Function.prototype.apply(temp);
Thanks for the response.
Apply is a function on the function prototype. Call is also a function on the function prototype. Apply is a function, therefore, it has call on it's prototype. All this is doing is calling the apply function.
Read more about apply here:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/apply
What does it do?
It calls console.log with console as this during the call, passing along the arguments in the
pseudo-array arguments as discrete arguments to the function.
So if arguments had "hi", "there" in it, it would be the same as:
console.log("hi", "there");
How can I analyse this statement?
Or with some thoughts or tools, I can figure it out step by step?
Let's start with what the apply and call functions are: They each have the ability to call a function
using a specific this during the call and passing arguments to that function. apply gets those
arguments from a single array (or anything array-like). call gets those arguments as individual arguments.
Normally, you'd see apply or call used like this:
someFunction.apply(valueForThis, ["arg1", "arg2", "arg3"]);
// or
someFunction.call(valueForThis, "arg1", "arg2", "arg3");
The only difference between apply and call is how they expect to get their arguments (apply = in
an array-like thing, call = as individual arguments).
So why, then, isn't that code doing this?
console.log.apply(console, arguments);
It's being cautious: console.log is a function provided by the host. It may not be a true JavaScript
function, and so it may not have the apply property.
So that code is avoiding relying on console.log having the apply property.
Which is where Function.prototype comes in. It's a reference to the object that is the prototype of all true JavaScript functions.
That prototype is where the apply and call properties on JavaScript functions come from.
So if we're worried that console.log doesn't have it (e.g., in case it doesn't inherit from Function.prototype), we can grab apply from that prototype object directly.
So the code is using call to call apply, and using apply to call console.log.
Can it be simplified to more statements to achieve the same result?
Not really, there's not a lot we can separate. I'll try to use variable names to clarify:
var thisValue = console;
var functionToCall = console.log;
var applyFunction = Function.prototype.apply;
applyFunction.call(functionToCall, thisValue, arguments);
Let's take this one part at a time.
Function.prototype.apply, more commonly seen as myBigFatFunction.apply, lets you call a function with a different this context than it would normally have. The difference between apply and call is that the former takes an array of arguments, the latter takes direct arguments after the first. Example:
myStr.substring(5)
String.prototype.substring.apply(myStr, [5]);
String.prototype.substring.call(myStr, 5);
^ all equivalent
However, for reasons I can't fully explain myself, some browser-native functions accessible to JavaScript don't have this function as a property (eg, console.log.apply). It's still possible to manually call it in the same manner though; so that console is still this, when it calls log, and that's what the given function is doing.
The reason for all that complication is that they want to pass in the arguments special variable. This is a keyword that exists in all functions, which represents all arguments to the function as an array-like object (so, suitable for Function.prototype.apply)
Your variable suggestion would likely simply call console.log once with console as argument one, and arguments as variable two, and return the result, rather than give you a function in a variable. If you want a shortened reference, it's possible you could use Function.prototype.bind, but that could actually lengthen your code.
According to this JavaScript reference:
The value null is a JavaScript literal representing null or an "empty"
value, i.e. no object value is present. It is one of JavaScript's
primitive values.
function getMax(arr){
return Math.max.apply(null, arr);
}
Wouldn't explicitly passing the keyword this be clearer, or at least more readable? Then again, at this point I may not understand why you would use null.
Why would you pass 'null' to 'apply' or 'call'?
When there is no value you wish to specify for the this pointer inside the function and the function you're calling is not expecting a particular this value in order to function properly.
Wouldn't explicitly passing the keyword this be clearer? Or at least
more human readable. Then again at this point I may not understand why
you would use null.
In your specific case, probably the best thing to pass is the Math object:
function getMax(arr){
return Math.max.apply(Math, arr);
}
While it turns out that it doesn't matter what you pass as the first argument for Math.max.apply(...) (only because of the implementation specifics of Math.max()), passing Math sets the this pointer to the exact same thing that it would be set to when calling it normally like Math.max(1,2,3) so that is the safest option since you are best simulating a normal call to Math.max().
Why would you pass 'null' to 'apply' or 'call'?
Here are some more details... When using .call() or .apply(), null can be passed when you have no specific value that you want to set the this pointer to and you know that the function you are calling is not expecting this to have any specific value (e.g. it does not use this in its implementation).
Note: Using null with .apply() or .call() is only usually done with functions that are methods for namespace reasons only, not for object-oriented reasons. In other words, the function max() is a method on the Math object only because of namespacing reasons, not because the Math object has instance data that the method .max() needs to access.
If you were doing it this way:
function foo() {
this.multiplier = 1;
}
foo.prototype.setMultiplier = function(val) {
this.multiplier = val;
}
foo.prototype.weightNumbers = function() {
var sum = 0;
for (var i = 0; i < arguments.length; i++) {
sum += (arguments[i] * this.multiplier);
}
return sum / arguments.length;
}
var x = new foo();
x.setMultiplier(3);
var numbers = [1, 2, 3]
console.log(x.weightNumbers.apply(x, numbers));
When the method you are calling .apply() on needs to access instance data, then you MUST pass the appropriate object as the first argument so that the method has the right this pointer to do its job as expected.
Calling apply with null as the first argument is like calling the function without providing any object for the this.
What does the apply method do?
The apply() method calls a function with a given this value and
arguments provided as an array (or an array-like object).
fun.apply(thisArg, [argsArray])
thisArg
The value of this provided for the call to fun. Note that this may not
be the actual value seen by the method: if the method is a function in
non-strict mode code, null and undefined will be replaced with the
global object, and primitive values will be boxed.
Further documentation can be found here.
One case where I have found this useful is when the function I'm calling is already bound to a particular context.
Because bound functions cannot be rebound, and they will always be called with the thisArg that was passed into bind, there is no use in passing a thisArg into call or apply. From source:
The bind() function creates a new bound function (BF).... When bound function is called, it calls internal method [[Call]] on [[BoundTargetFunction]], with following arguments Call(boundThis, args).
Here's an example:
class C {
constructor() {
this.a = 1;
}
}
function f(n, m) {
console.log(this.a + n + m);
}
let c = new C();
var boundF = f.bind(c, 2); // the context `c` is now bound to f
boundF.apply(null, [3]); // no reason to supply any context, since we know it's going to be `c`
I am bit late to answer this. I will try to give a long descriptive explanation here.
What is null in JavaScript?
The value null is a literal (not a property of the global object like undefined can be). It is one of JavaScript's primitive values.
In APIs, null is often retrieved in place where an object can be expected but no object is relevant.
fun.apply(thisArg, [argsArray])
thisArg: The value of this provided for the call to fun. Note that this may not be the actual value seen by the method: if the method is a function in non-strict mode code, null and undefined will be replaced with the global object, and primitive values will be boxed.
argsArray: An array-like object, specifying the arguments with which fun should be called, or null or undefined if no arguments should be provided to the function. Starting with ECMAScript 5 these arguments can be a generic array-like object instead of an array. See below for browser compatibility information.
If you are using 'strict mode', then it is advisable to pass the this or
Math as the parameter.
Apply is useful when you want to pass along the responsibility for doing something to a function that is determined at run time, and pass a variable number of arguments to that function. You may or may not have any appropriate "this" context when you're doing that.
For example I use a library I wrote to facilitate listening for and raising application events that uses apply.
I wanted to be able to be able to raise an event like this:
EventManager.raise('some:event-name', arg1, arg2, arg3, ..);
..and have all of the registered handlers for that event get called with that list of arguments (arg1, arg2, etc). So in the raise function, it goes through the handlers that are registered for that event name and calls them, passing all the passed in arguments except for the event name, like this:
var args = [];
Array.prototype.push.apply(args, arguments);
args.shift();
for (var l in listeners) {
var listener = listeners[l];
listener.callback.apply(listener.context, args);
}
When a registered handler (listener.callback) is called, apply is used to pass along a variable number of arguments. Here I have allowed the listener to supply a this context for its event handler when the listener is defined, but that context might not be defined or it might be null, and that's perfectly fine.
For a long time the raise function didn't even facilitate using any callback context. I eventually came across a need for it, so I put in support for it, but most of the time I don't really need or use it.
This question already has answers here:
Why should use "apply"?
(5 answers)
Closed 8 years ago.
when read the heatmap code, I find a code like
this._store.addData.apply(this._store, arguments);
I am very confused about this usage of 'apply'.Is there any different in the code below
this._store.addData(arguments);
thanks!
The point of using .apply here is that apparently the function arguments are in the array arguments. The equivalent code without .apply would be:
this._store.addData(arguments[0], arguments[1], ..)
Which is obviously troublesome if arguments is of unknown length.
Passing this._store again is only a necessity, it's not the point of this code.
The two are not the same.
.apply() as documented here on MDN, sets the value of this when the function is executed and passes an array of arguments to that function which will be expanded into normal function arguments.
Your second code example passes an array-like object of arguments to the actual function rather than an expanded list of arguments so they are not the same.
As an example, if there are three items in the arguments pseudo-array, then the first option will call your function like this:
this._store.addData(arg1, arg2, arg3);
And, your second code block will call it like this:
this._store.addData([arg1, arg2, arg3]);
Though technically, it's not even passing an array, it's passing a pseudo-array that doesn't actually have the array methods.
The reason for using .apply() is when you have a list of arguments in an array or array-like structure and you want to call a function using that list of arguments as the arguments to the function (passed to the function as normal arguments). It is very commonly used when you want to just forward the arguments from one function to the next or when you want to modify the arguments (e.g. remove one or change one) and then call another function with all the rest of the arguments.
Or sometimes .apply() is used only to control the this pointer and no arguments are passed (though .call() can also do that).
As a corollary, .call() is used when you know exactly how many arguments there are (e.g. you know there are two arguments AND you want to control the this pointer or .call() is often used when you just want to control the this pointer and don't have any arguments. An advantage of .apply() is that you don't have to know how many arguments there are - you can just pass on whatever arguments there were (that's why it's particularly useful when forward the args from one function to the next) like when calling a base class constructor or when proxying for a function like what .bind() does.
This this pointer will be the same in the two methods because calling obj.method() will set the value of this to obj which in both of your cases is this._store.
Note, that these two would be the same:
this._store.addData(arguments);
this._store.addData.call(this._store, arguments);
Recently I read a tutorial which says if define the function like below.
function Animal() { }
On the surface, this code seems to create a function called Animal.
But with JavaScript, the full truth is slightly more complicated. What
actually happens when this code executes is that two objects are
created. The first object, called Animal, is the constructor function
itself. The second object, called Animal.prototype, has a property
called Animal.prototype.constructor, which points to Animal. Animal
has a property which points back to its prototype, Animal.prototype.
But I have little confuse about it .What about the Function object ?What is the use for the Animal object?
And If I write code like below .
var test= new Function();
and I inspected the variable test in the Developer tool of the Chrome.
I found test is nothing to do with the Function. Can someone tell me why ? thanks.
Updated
The diagram below is the objects relationship when the code is executed, please review it.
If my understanding is wrong. please correct me. thanks.
That blog post goes into a lot of detail that's interesting but unnecessarily confusing for most people most of the time.
First, let's talk about functions; forget about prototypes for a minute. When you create a function:
function Whatever() {
// ...
}
you've created an object. That is, all functions are objects, and they're constructed via the Function built-in constructor. The symbol "Whatever" in this example will have as its value a reference to that object.
Given a reference to a function, it's possible to call it:
Whatever(); // call the function
It's possible to take that value (the reference to the function object) and assign it to another variable, or pass it as a parameter to another function, or to use it just like any other value in JavaScript.
var another = Whatever;
another(); // also calls the "Whatever" function
Constructing a function via the Function constructor explicitly is something that's rarely done, but it gives you a function that's otherwise unremarkable. (In the OP, the constructed function doesn't do anything because no code was passed to the Function constructor.)
Now, things get interesting when a function is invoked as part of a new expression.
var test = new Whatever();
By using new, a new object is instantiated and associated with the "Whatever" function. The "Whatever" function is the constructor for that new object.
Every function object, whether it's ever used as a constructor or not, has an associated "prototype" object. When a function is used as a constructor, then objects it constructs (that is, objects made in new expressions that invoke the function) are implicitly associated with that prototype object.
The prototype object becomes interesting when an object property reference expression is evaluated. Object property references look like this:
obj.name
obj[ nameExpression ]
In such an expression, the property name (either the identifier used in the . expression or the value of the expression inside [ ]) is checked against the set of properties on the object directly. If the name is not the same as one of the object's properties, then the runtime consults the prototype object associated with the constructor function used to make the object.
For most code that most people write, that relationship and some of its direct implications are the only things to worry about. You don't have to fool around with the (non-standard, at this time) "proto" property of objects unless you're putting together some sort of library or framework.
Finally, it might be instructive to look at the Object.create() function and in particular the "polyfill" shown in that documentation.
I've seen it done differently in code out there, but is there any benefit or reason to doing a (blank params) .call / .apply over a regular () function execution.
This of course is an over-simplified example
var func = function () { /* do whatever */ };
func.call();
func.apply();
VERSUS just the simple parenthesis.
func();
Haven't seen any information on this anywhere, I know why call/apply are used when params are passed.
When you call a method with func();, this variable inside the method points to window object.
Where as when you use call(...)/apply(...) the first parameter passed to the method call becomes this inside the method. If you are not passing any arguments/pass null or undefined then this will become global object in non strict mode.
Yes, there is an important difference in some cases. For example, dealing with callbacks. Let's assume you have a class with constructor that accepts callback parameter and stores it under this.callback. Later, when this callback is invoked via this.callback(), suddenly the foreign code gets the reference to your object via this. It is not always desirable, and it is better/safer to use this.callback.call() in such case.
That way, the foreign code will get undefined as this and won't try to modify your object by accident. And properly written callback won't be affected by this usage of call() anyways, since they would supply the callback defined as an arrow function or as bound function (via .bind()). In both such cases, the callback will have its own, proper this, unaffected by call(), since arrow and bound functions just ignore the value, set by apply()/call().