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I used C++ before and I realized that pointers were very helpful. Is there anything in javascript that acts like a pointer? Does javascript have pointers? I like to use pointers when I want to use something like:
var a = 1;
var b = "a";
document.getElementById(/* value pointed by b */).innerHTML="Pointers";
I know that this is an extremely simple example and I could just use a, but there are several more complex examples where I would find pointers very useful. Any ideas?
No, JS doesn't have pointers.
Objects are passed around by passing a copy of a reference. The programmer cannot access any C-like "value" representing the address of an object.
Within a function, one may change the contents of a passed object via that reference, but you cannot modify the reference that the caller had because your reference is only a copy:
var foo = {'bar': 1};
function tryToMungeReference(obj) {
obj = {'bar': 2}; // won't change caller's object
}
function mungeContents(obj) {
obj.bar = 2; // changes _contents_ of caller's object
}
tryToMungeReference(foo);
foo.bar === 1; // true - foo still references original object
mungeContents(foo);
foo.bar === 2; // true - object referenced by foo has been modified
You bet there are pointers in JavaScript; objects are pointers.
//this will make object1 point to the memory location that object2 is pointing at
object1 = object2;
//this will make object2 point to the memory location that object1 is pointing at
function myfunc(object2){}
myfunc(object1);
If a memory location is no longer pointed at, the data there will be lost.
Unlike in C, you can't see the actual address of the pointer nor the actual value of the pointer, you can only dereference it (get the value at the address it points to.)
I just did a bizarre thing that works out, too.
Instead of passing a pointer, pass a function that fills its argument into the target variable.
var myTarget;
class dial{
constructor(target){
this.target = target;
this.target(99);
}
}
var myDial = new dial((v)=>{myTarget = v;});
This may look a little wicked, but works just fine. In this example I created a generic dial, which can be assigned any target in form of this little function "(v)=>{target = v}". No idea how well it would do in terms of performance, but it acts beautifully.
due to the nature of JS that passes objects by value (if referenced object is changed completely) or by reference (if field of the referenced object is changed) it is not possible to completely replace a referenced object.
However, let's use what is available: replacing single fields of referenced objects. By doing that, the following function allows to achieve what you are asking for:
function replaceReferencedObj(refObj, newObj) {
let keysR = Object.keys(refObj);
let keysN = Object.keys(newObj);
for (let i = 0; i < keysR.length; i++) {
delete refObj[keysR[i]];
}
for (let i = 0; i < keysN.length; i++) {
refObj[keysN[i]] = newObj[keysN[i]];
}
}
For the example given by user3015682 you would use this function as following:
replaceReferencedObj(foo, {'bar': 2})
Assigning by reference and arrays.
let pizza = [4,4,4];
let kebab = pizza; // both variables are references to shared value
kebab.push(4);
console.log(kebab); //[4,4,4,4]
console.log(pizza); //[4,4,4,4]
Since original value isn't modified no new reference is created.
kebab = [6,6,6,6]; // value is reassigned
console.log(kebab); //[6,6,6,6]
console.log(pizza); //[4,4,4,4]
When the compound value in a variable is reassigned, a new reference is created.
Technically JS doesn't have pointers, but I discovered a way to imitate their behavior ;)
var car = {
make: 'Tesla',
nav: {
lat: undefined,
lng: undefined
}
};
var coords: {
center: {
get lat() { return car.nav.lat; }, // pointer LOL
get lng() { return car.nav.lng; } // pointer LOL
}
};
car.nav.lat = 555;
car.nav.lng = 777;
console.log('*** coords: ', coords.center.lat); // 555
console.log('*** coords: ', coords.center.lng); // 777
What is the difference between using Function.prototype.apply() and Function.prototype.call() to invoke a function?
var func = function() {
alert('hello!');
};
func.apply(); vs func.call();
Are there performance differences between the two aforementioned methods? When is it best to use call over apply and vice versa?
The difference is that apply lets you invoke the function with arguments as an array; call requires the parameters be listed explicitly. A useful mnemonic is "A for array and C for comma."
See MDN's documentation on apply and call.
Pseudo syntax:
theFunction.apply(valueForThis, arrayOfArgs)
theFunction.call(valueForThis, arg1, arg2, ...)
There is also, as of ES6, the possibility to spread the array for use with the call function, you can see the compatibilities here.
Sample code:
function theFunction(name, profession) {
console.log("My name is " + name + " and I am a " + profession +".");
}
theFunction("John", "fireman");
theFunction.apply(undefined, ["Susan", "school teacher"]);
theFunction.call(undefined, "Claude", "mathematician");
theFunction.call(undefined, ...["Matthew", "physicist"]); // used with the spread operator
K. Scott Allen has a nice writeup on the matter.
Basically, they differ on how they handle function arguments.
The apply() method is identical to call(), except apply() requires an array as the second parameter. The array represents the arguments for the target method."
So:
// assuming you have f
function f(message) { ... }
f.call(receiver, "test");
f.apply(receiver, ["test"]);
To answer the part about when to use each function, use apply if you don't know the number of arguments you will be passing, or if they are already in an array or array-like object (like the arguments object to forward your own arguments. Use call otherwise, since there's no need to wrap the arguments in an array.
f.call(thisObject, a, b, c); // Fixed number of arguments
f.apply(thisObject, arguments); // Forward this function's arguments
var args = [];
while (...) {
args.push(some_value());
}
f.apply(thisObject, args); // Unknown number of arguments
When I'm not passing any arguments (like your example), I prefer call since I'm calling the function. apply would imply you are applying the function to the (non-existent) arguments.
There shouldn't be any performance differences, except maybe if you use apply and wrap the arguments in an array (e.g. f.apply(thisObject, [a, b, c]) instead of f.call(thisObject, a, b, c)). I haven't tested it, so there could be differences, but it would be very browser specific. It's likely that call is faster if you don't already have the arguments in an array and apply is faster if you do.
Here's a good mnemonic. Apply uses Arrays and Always takes one or two Arguments. When you use Call you have to Count the number of arguments.
While this is an old topic, I just wanted to point out that .call is slightly faster than .apply. I can't tell you exactly why.
See jsPerf, http://jsperf.com/test-call-vs-apply/3
[UPDATE!]
Douglas Crockford mentions briefly the difference between the two, which may help explain the performance difference... http://youtu.be/ya4UHuXNygM?t=15m52s
Apply takes an array of arguments, while Call takes zero or more individual parameters! Ah hah!
.apply(this, [...])
.call(this, param1, param2, param3, param4...)
Follows an extract from Closure: The Definitive Guide by Michael Bolin. It might look a bit lengthy, but it's saturated with a lot of insight. From "Appendix B. Frequently Misunderstood JavaScript Concepts":
What this Refers to When a Function is Called
When calling a function of the form foo.bar.baz(), the object foo.bar is referred to as the receiver. When the function is called, it is the receiver that is used as the value for this:
var obj = {};
obj.value = 10;
/** #param {...number} additionalValues */
obj.addValues = function(additionalValues) {
for (var i = 0; i < arguments.length; i++) {
this.value += arguments[i];
}
return this.value;
};
// Evaluates to 30 because obj is used as the value for 'this' when
// obj.addValues() is called, so obj.value becomes 10 + 20.
obj.addValues(20);
If there is no explicit receiver when a function is called, then the global object becomes the receiver. As explained in "goog.global" on page 47, window is the global object when JavaScript is executed in a web browser. This leads to some surprising behavior:
var f = obj.addValues;
// Evaluates to NaN because window is used as the value for 'this' when
// f() is called. Because and window.value is undefined, adding a number to
// it results in NaN.
f(20);
// This also has the unintentional side effect of adding a value to window:
alert(window.value); // Alerts NaN
Even though obj.addValues and f refer to the same function, they behave differently when called because the value of the receiver is different in each call. For this reason, when calling a function that refers to this, it is important to ensure that this will have the correct value when it is called. To be clear, if this were not referenced in the function body, then the behavior of f(20) and obj.addValues(20) would be the same.
Because functions are first-class objects in JavaScript, they can have their own methods. All functions have the methods call() and apply() which make it possible to redefine the receiver (i.e., the object that this refers to) when calling the function. The method signatures are as follows:
/**
* #param {*=} receiver to substitute for 'this'
* #param {...} parameters to use as arguments to the function
*/
Function.prototype.call;
/**
* #param {*=} receiver to substitute for 'this'
* #param {Array} parameters to use as arguments to the function
*/
Function.prototype.apply;
Note that the only difference between call() and apply() is that call() receives the function parameters as individual arguments, whereas apply() receives them as a single array:
// When f is called with obj as its receiver, it behaves the same as calling
// obj.addValues(). Both of the following increase obj.value by 60:
f.call(obj, 10, 20, 30);
f.apply(obj, [10, 20, 30]);
The following calls are equivalent, as f and obj.addValues refer to the same function:
obj.addValues.call(obj, 10, 20, 30);
obj.addValues.apply(obj, [10, 20, 30]);
However, since neither call() nor apply() uses the value of its own receiver to substitute for the receiver argument when it is unspecified, the following will not work:
// Both statements evaluate to NaN
obj.addValues.call(undefined, 10, 20, 30);
obj.addValues.apply(undefined, [10, 20, 30]);
The value of this can never be null or undefined when a function is called. When null or undefined is supplied as the receiver to call() or apply(), the global object is used as the value for receiver instead. Therefore, the previous code has the same undesirable side effect of adding a property named value to the global object.
It may be helpful to think of a function as having no knowledge of the variable to which it is assigned. This helps reinforce the idea that the value of this will be bound when the function is called rather than when it is defined.
End of extract.
It is useful at times for one object to borrow the function of another object, meaning that the borrowing object simply executes the lent function as if it were its own.
A small code example:
var friend = {
car: false,
lendCar: function ( canLend ){
this.car = canLend;
}
};
var me = {
car: false,
gotCar: function(){
return this.car === true;
}
};
console.log(me.gotCar()); // false
friend.lendCar.call(me, true);
console.log(me.gotCar()); // true
friend.lendCar.apply(me, [false]);
console.log(me.gotCar()); // false
These methods are very useful for giving objects temporary functionality.
Another example with Call, Apply and Bind.
The difference between Call and Apply is evident, but Bind works like this:
Bind returns an instance of a function that can be executed
First Parameter is 'this'
Second parameter is a Comma separated list of arguments (like Call)
}
function Person(name) {
this.name = name;
}
Person.prototype.getName = function(a,b) {
return this.name + " " + a + " " + b;
}
var reader = new Person('John Smith');
reader.getName = function() {
// Apply and Call executes the function and returns value
// Also notice the different ways of extracting 'getName' prototype
var baseName = Object.getPrototypeOf(this).getName.apply(this,["is a", "boy"]);
console.log("Apply: " + baseName);
var baseName = Object.getPrototypeOf(reader).getName.call(this, "is a", "boy");
console.log("Call: " + baseName);
// Bind returns function which can be invoked
var baseName = Person.prototype.getName.bind(this, "is a", "boy");
console.log("Bind: " + baseName());
}
reader.getName();
/* Output
Apply: John Smith is a boy
Call: John Smith is a boy
Bind: John Smith is a boy
*/
I'd like to show an example, where the 'valueForThis' argument is used:
Array.prototype.push = function(element) {
/*
Native code*, that uses 'this'
this.put(element);
*/
}
var array = [];
array.push(1);
array.push.apply(array,[2,3]);
Array.prototype.push.apply(array,[4,5]);
array.push.call(array,6,7);
Array.prototype.push.call(array,8,9);
//[1, 2, 3, 4, 5, 6, 7, 8, 9]
**details: http://es5.github.io/#x15.4.4.7*
Call() takes comma-separated arguments, ex:
.call(scope, arg1, arg2, arg3)
and apply() takes an array of arguments, ex:
.apply(scope, [arg1, arg2, arg3])
here are few more usage examples:
http://blog.i-evaluation.com/2012/08/15/javascript-call-and-apply/
From the MDN docs on Function.prototype.apply() :
The apply() method calls a function with a given this value and
arguments provided as an array (or an array-like object).
Syntax
fun.apply(thisArg, [argsArray])
From the MDN docs on Function.prototype.call() :
The call() method calls a function with a given this value and arguments provided individually.
Syntax
fun.call(thisArg[, arg1[, arg2[, ...]]])
From Function.apply and Function.call in JavaScript :
The apply() method is identical to call(), except apply() requires an
array as the second parameter. The array represents the arguments for
the target method.
Code example :
var doSomething = function() {
var arr = [];
for(i in arguments) {
if(typeof this[arguments[i]] !== 'undefined') {
arr.push(this[arguments[i]]);
}
}
return arr;
}
var output = function(position, obj) {
document.body.innerHTML += '<h3>output ' + position + '</h3>' + JSON.stringify(obj) + '\n<br>\n<br><hr>';
}
output(1, doSomething(
'one',
'two',
'two',
'one'
));
output(2, doSomething.apply({one : 'Steven', two : 'Jane'}, [
'one',
'two',
'two',
'one'
]));
output(3, doSomething.call({one : 'Steven', two : 'Jane'},
'one',
'two',
'two',
'one'
));
See also this Fiddle.
Here's a small-ish post, I wrote on this:
http://sizeableidea.com/call-versus-apply-javascript/
var obj1 = { which : "obj1" },
obj2 = { which : "obj2" };
function execute(arg1, arg2){
console.log(this.which, arg1, arg2);
}
//using call
execute.call(obj1, "dan", "stanhope");
//output: obj1 dan stanhope
//using apply
execute.apply(obj2, ["dan", "stanhope"]);
//output: obj2 dan stanhope
//using old school
execute("dan", "stanhope");
//output: undefined "dan" "stanhope"
Fundamental difference is that call() accepts an argument list, while apply() accepts a single array of arguments.
The difference is that call() takes the function arguments separately, and apply() takes the function arguments in an array.
Summary:
Both call() and apply() are methods which are located on Function.prototype. Therefore they are available on every function object via the prototype chain. Both call() and apply() can execute a function with a specified value of the this.
The main difference between call() and apply() is the way you have to pass in arguments into it. In both call() and apply() you pass as a first argument the object you want to be the value as this. The other arguments differ in the following way:
With call() you have to put in the arguments normally (starting from the second argument)
With apply() you have to pass in array of arguments.
Example:
let obj = {
val1: 5,
val2: 10
}
const summation = function (val3, val4) {
return this.val1 + this.val2 + val3 + val4;
}
console.log(summation.apply(obj, [2 ,3]));
// first we assign we value of this in the first arg
// with apply we have to pass in an array
console.log(summation.call(obj, 2, 3));
// with call we can pass in each arg individually
Why would I need to use these functions?
The this value can be tricky sometimes in javascript. The value of this determined when a function is executed not when a function is defined. If our function is dependend on a right this binding we can use call() and apply() to enforce this behaviour. For example:
var name = 'unwantedGlobalName';
const obj = {
name: 'Willem',
sayName () { console.log(this.name);}
}
let copiedMethod = obj.sayName;
// we store the function in the copiedmethod variable
copiedMethod();
// this is now window, unwantedGlobalName gets logged
copiedMethod.call(obj);
// we enforce this to be obj, Willem gets logged
We can differentiate call and apply methods as below
CALL : A function with argument provide individually.
If you know the arguments to be passed or there are no argument to pass you can use call.
APPLY : Call a function with argument provided as an array. You can use apply if you don't know how many argument are going to pass to the function.
There is a advantage of using apply over call, we don't need to change the number of argument only we can change a array that is passed.
There is not big difference in performance. But we can say call is bit faster as compare to apply because an array need to evaluate in apply method.
The main difference is, using call, we can change the scope and pass arguments as normal, but apply lets you call it using arguments as an Array (pass them as an array). But in terms of what they to do in your code, they are pretty similar.
While the syntax of this function is almost identical to that of
apply(), the fundamental difference is that call() accepts an argument
list, while apply() accepts a single array of arguments.
So as you see, there is not a big difference, but still, there are cases we prefer using call() or apply(). For example, look at the code below, which finding the smallest and largest number in an array from MDN, using the apply method:
// min/max number in an array
var numbers = [5, 6, 2, 3, 7];
// using Math.min/Math.max apply
var max = Math.max.apply(null, numbers);
// This about equal to Math.max(numbers[0], ...)
// or Math.max(5, 6, ...)
var min = Math.min.apply(null, numbers)
So the main difference is just the way we passing the arguments:
Call:
function.call(thisArg, arg1, arg2, ...);
Apply:
function.apply(thisArg, [argsArray]);
Difference between these to methods are, how you want to pass the parameters.
“A for array and C for comma” is a handy mnemonic.
Call and apply both are used to force the this value when a function is executed. The only difference is that call takes n+1 arguments where 1 is this and 'n' arguments. apply takes only two arguments, one is this the other is argument array.
The advantage I see in apply over call is that we can easily delegate a function call to other function without much effort;
function sayHello() {
console.log(this, arguments);
}
function hello() {
sayHello.apply(this, arguments);
}
var obj = {name: 'my name'}
hello.call(obj, 'some', 'arguments');
Observe how easily we delegated hello to sayHello using apply, but with call this is very difficult to achieve.
Even though call and apply achive the same thing, I think there is atleast one place where you cannot use call but can only use apply. That is when you want to support inheritance and want to call the constructor.
Here is a function allows you to create classes which also supports creating classes by extending other classes.
function makeClass( properties ) {
var ctor = properties['constructor'] || function(){}
var Super = properties['extends'];
var Class = function () {
// Here 'call' cannot work, only 'apply' can!!!
if(Super)
Super.apply(this,arguments);
ctor.apply(this,arguments);
}
if(Super){
Class.prototype = Object.create( Super.prototype );
Class.prototype.constructor = Class;
}
Object.keys(properties).forEach( function(prop) {
if(prop!=='constructor' && prop!=='extends')
Class.prototype[prop] = properties[prop];
});
return Class;
}
//Usage
var Car = makeClass({
constructor: function(name){
this.name=name;
},
yourName: function() {
return this.name;
}
});
//We have a Car class now
var carInstance=new Car('Fiat');
carInstance.youName();// ReturnsFiat
var SuperCar = makeClass({
constructor: function(ignore,power){
this.power=power;
},
extends:Car,
yourPower: function() {
return this.power;
}
});
//We have a SuperCar class now, which is subclass of Car
var superCar=new SuperCar('BMW xy',2.6);
superCar.yourName();//Returns BMW xy
superCar.yourPower();// Returns 2.6
Let me add a little detail to this.
these two calls are almost equivalent:
func.call(context, ...args); // pass an array as list with spread operator
func.apply(context, args); // is same as using apply
There’s only a minor difference:
The spread operator ... allows passing iterable args as the list to call.
The apply accepts only array-like args.
So, these calls complement each other. Where we expect an iterable, call works, where we expect an array-like, apply works.
And for objects that are both iterable and array-like, like a real array, we technically could use any of them, but apply will probably be faster because most JavaScript engines internally optimize it better.
I just want to add a simple example to a well explained post by flatline, which makes it easy to understand for beginners.
func.call(context, args1, args2 ); // pass arguments as "," separated value
func.apply(context, [args1, args2]); // pass arguments as "Array"
we also use "Call" and "Apply" method for changing reference as defined in code below
let Emp1 = {
name: 'X',
getEmpDetail: function(age, department) {
console.log(`Name: ${this.name} Age: ${age} Department: ${department}`)
}
}
Emp1.getEmpDetail(23, 'Delivery')
// 1st approach of changing "this"
let Emp2 = {
name: 'Y',
getEmpDetail: Emp1.getEmpDetail
}
Emp2.getEmpDetail(55, 'Finance')
// 2nd approach of changing "this" using "Call" and "Apply"
let Emp3 = {
name: 'Emp3_Object',
}
Emp1.getEmpDetail.call(Emp3, 30, 'Admin')
// here we have change the ref from **Emp1 to Emp3** object
// now this will print "Name = Emp3_Object" because it is pointing to Emp3 object
Emp1.getEmpDetail.apply(Emp3, [30, 'Admin'])
The call() method calls a function with a given this value and a second parameter which are arguments separated by comma.
object.someMethod.call( someObject, arguments )
The apply() method is the same as call except the fact that the second argument it takes is an array of arguments .
object.someMethod.apply( someObject, arrayOfarguments )
var car = {
name: "Reno",
country: "France",
showBuyer: function(firstName, lastName) {
console.log(`${firstName} ${lastName} just bought a ${this.name} from ${this.country}`);
}
}
const firstName = "Bryan";
const lastName = "Smith";
car.showBuyer(firstName, lastName); // Bryan just bought a Reno from France
const obj = { name: "Maserati", country: "Italy" };
car.showBuyer.call(obj, firstName, lastName); // Bryan Smith just bought a Maserati from Italy
car.showBuyer.apply(obj, [firstName, lastName]); // Bryan Smith just bought a Maserati from Italy
What is the difference between using Function.prototype.apply() and Function.prototype.call() to invoke a function?
var func = function() {
alert('hello!');
};
func.apply(); vs func.call();
Are there performance differences between the two aforementioned methods? When is it best to use call over apply and vice versa?
The difference is that apply lets you invoke the function with arguments as an array; call requires the parameters be listed explicitly. A useful mnemonic is "A for array and C for comma."
See MDN's documentation on apply and call.
Pseudo syntax:
theFunction.apply(valueForThis, arrayOfArgs)
theFunction.call(valueForThis, arg1, arg2, ...)
There is also, as of ES6, the possibility to spread the array for use with the call function, you can see the compatibilities here.
Sample code:
function theFunction(name, profession) {
console.log("My name is " + name + " and I am a " + profession +".");
}
theFunction("John", "fireman");
theFunction.apply(undefined, ["Susan", "school teacher"]);
theFunction.call(undefined, "Claude", "mathematician");
theFunction.call(undefined, ...["Matthew", "physicist"]); // used with the spread operator
K. Scott Allen has a nice writeup on the matter.
Basically, they differ on how they handle function arguments.
The apply() method is identical to call(), except apply() requires an array as the second parameter. The array represents the arguments for the target method."
So:
// assuming you have f
function f(message) { ... }
f.call(receiver, "test");
f.apply(receiver, ["test"]);
To answer the part about when to use each function, use apply if you don't know the number of arguments you will be passing, or if they are already in an array or array-like object (like the arguments object to forward your own arguments. Use call otherwise, since there's no need to wrap the arguments in an array.
f.call(thisObject, a, b, c); // Fixed number of arguments
f.apply(thisObject, arguments); // Forward this function's arguments
var args = [];
while (...) {
args.push(some_value());
}
f.apply(thisObject, args); // Unknown number of arguments
When I'm not passing any arguments (like your example), I prefer call since I'm calling the function. apply would imply you are applying the function to the (non-existent) arguments.
There shouldn't be any performance differences, except maybe if you use apply and wrap the arguments in an array (e.g. f.apply(thisObject, [a, b, c]) instead of f.call(thisObject, a, b, c)). I haven't tested it, so there could be differences, but it would be very browser specific. It's likely that call is faster if you don't already have the arguments in an array and apply is faster if you do.
Here's a good mnemonic. Apply uses Arrays and Always takes one or two Arguments. When you use Call you have to Count the number of arguments.
While this is an old topic, I just wanted to point out that .call is slightly faster than .apply. I can't tell you exactly why.
See jsPerf, http://jsperf.com/test-call-vs-apply/3
[UPDATE!]
Douglas Crockford mentions briefly the difference between the two, which may help explain the performance difference... http://youtu.be/ya4UHuXNygM?t=15m52s
Apply takes an array of arguments, while Call takes zero or more individual parameters! Ah hah!
.apply(this, [...])
.call(this, param1, param2, param3, param4...)
Follows an extract from Closure: The Definitive Guide by Michael Bolin. It might look a bit lengthy, but it's saturated with a lot of insight. From "Appendix B. Frequently Misunderstood JavaScript Concepts":
What this Refers to When a Function is Called
When calling a function of the form foo.bar.baz(), the object foo.bar is referred to as the receiver. When the function is called, it is the receiver that is used as the value for this:
var obj = {};
obj.value = 10;
/** #param {...number} additionalValues */
obj.addValues = function(additionalValues) {
for (var i = 0; i < arguments.length; i++) {
this.value += arguments[i];
}
return this.value;
};
// Evaluates to 30 because obj is used as the value for 'this' when
// obj.addValues() is called, so obj.value becomes 10 + 20.
obj.addValues(20);
If there is no explicit receiver when a function is called, then the global object becomes the receiver. As explained in "goog.global" on page 47, window is the global object when JavaScript is executed in a web browser. This leads to some surprising behavior:
var f = obj.addValues;
// Evaluates to NaN because window is used as the value for 'this' when
// f() is called. Because and window.value is undefined, adding a number to
// it results in NaN.
f(20);
// This also has the unintentional side effect of adding a value to window:
alert(window.value); // Alerts NaN
Even though obj.addValues and f refer to the same function, they behave differently when called because the value of the receiver is different in each call. For this reason, when calling a function that refers to this, it is important to ensure that this will have the correct value when it is called. To be clear, if this were not referenced in the function body, then the behavior of f(20) and obj.addValues(20) would be the same.
Because functions are first-class objects in JavaScript, they can have their own methods. All functions have the methods call() and apply() which make it possible to redefine the receiver (i.e., the object that this refers to) when calling the function. The method signatures are as follows:
/**
* #param {*=} receiver to substitute for 'this'
* #param {...} parameters to use as arguments to the function
*/
Function.prototype.call;
/**
* #param {*=} receiver to substitute for 'this'
* #param {Array} parameters to use as arguments to the function
*/
Function.prototype.apply;
Note that the only difference between call() and apply() is that call() receives the function parameters as individual arguments, whereas apply() receives them as a single array:
// When f is called with obj as its receiver, it behaves the same as calling
// obj.addValues(). Both of the following increase obj.value by 60:
f.call(obj, 10, 20, 30);
f.apply(obj, [10, 20, 30]);
The following calls are equivalent, as f and obj.addValues refer to the same function:
obj.addValues.call(obj, 10, 20, 30);
obj.addValues.apply(obj, [10, 20, 30]);
However, since neither call() nor apply() uses the value of its own receiver to substitute for the receiver argument when it is unspecified, the following will not work:
// Both statements evaluate to NaN
obj.addValues.call(undefined, 10, 20, 30);
obj.addValues.apply(undefined, [10, 20, 30]);
The value of this can never be null or undefined when a function is called. When null or undefined is supplied as the receiver to call() or apply(), the global object is used as the value for receiver instead. Therefore, the previous code has the same undesirable side effect of adding a property named value to the global object.
It may be helpful to think of a function as having no knowledge of the variable to which it is assigned. This helps reinforce the idea that the value of this will be bound when the function is called rather than when it is defined.
End of extract.
It is useful at times for one object to borrow the function of another object, meaning that the borrowing object simply executes the lent function as if it were its own.
A small code example:
var friend = {
car: false,
lendCar: function ( canLend ){
this.car = canLend;
}
};
var me = {
car: false,
gotCar: function(){
return this.car === true;
}
};
console.log(me.gotCar()); // false
friend.lendCar.call(me, true);
console.log(me.gotCar()); // true
friend.lendCar.apply(me, [false]);
console.log(me.gotCar()); // false
These methods are very useful for giving objects temporary functionality.
Another example with Call, Apply and Bind.
The difference between Call and Apply is evident, but Bind works like this:
Bind returns an instance of a function that can be executed
First Parameter is 'this'
Second parameter is a Comma separated list of arguments (like Call)
}
function Person(name) {
this.name = name;
}
Person.prototype.getName = function(a,b) {
return this.name + " " + a + " " + b;
}
var reader = new Person('John Smith');
reader.getName = function() {
// Apply and Call executes the function and returns value
// Also notice the different ways of extracting 'getName' prototype
var baseName = Object.getPrototypeOf(this).getName.apply(this,["is a", "boy"]);
console.log("Apply: " + baseName);
var baseName = Object.getPrototypeOf(reader).getName.call(this, "is a", "boy");
console.log("Call: " + baseName);
// Bind returns function which can be invoked
var baseName = Person.prototype.getName.bind(this, "is a", "boy");
console.log("Bind: " + baseName());
}
reader.getName();
/* Output
Apply: John Smith is a boy
Call: John Smith is a boy
Bind: John Smith is a boy
*/
I'd like to show an example, where the 'valueForThis' argument is used:
Array.prototype.push = function(element) {
/*
Native code*, that uses 'this'
this.put(element);
*/
}
var array = [];
array.push(1);
array.push.apply(array,[2,3]);
Array.prototype.push.apply(array,[4,5]);
array.push.call(array,6,7);
Array.prototype.push.call(array,8,9);
//[1, 2, 3, 4, 5, 6, 7, 8, 9]
**details: http://es5.github.io/#x15.4.4.7*
Call() takes comma-separated arguments, ex:
.call(scope, arg1, arg2, arg3)
and apply() takes an array of arguments, ex:
.apply(scope, [arg1, arg2, arg3])
here are few more usage examples:
http://blog.i-evaluation.com/2012/08/15/javascript-call-and-apply/
From the MDN docs on Function.prototype.apply() :
The apply() method calls a function with a given this value and
arguments provided as an array (or an array-like object).
Syntax
fun.apply(thisArg, [argsArray])
From the MDN docs on Function.prototype.call() :
The call() method calls a function with a given this value and arguments provided individually.
Syntax
fun.call(thisArg[, arg1[, arg2[, ...]]])
From Function.apply and Function.call in JavaScript :
The apply() method is identical to call(), except apply() requires an
array as the second parameter. The array represents the arguments for
the target method.
Code example :
var doSomething = function() {
var arr = [];
for(i in arguments) {
if(typeof this[arguments[i]] !== 'undefined') {
arr.push(this[arguments[i]]);
}
}
return arr;
}
var output = function(position, obj) {
document.body.innerHTML += '<h3>output ' + position + '</h3>' + JSON.stringify(obj) + '\n<br>\n<br><hr>';
}
output(1, doSomething(
'one',
'two',
'two',
'one'
));
output(2, doSomething.apply({one : 'Steven', two : 'Jane'}, [
'one',
'two',
'two',
'one'
]));
output(3, doSomething.call({one : 'Steven', two : 'Jane'},
'one',
'two',
'two',
'one'
));
See also this Fiddle.
Here's a small-ish post, I wrote on this:
http://sizeableidea.com/call-versus-apply-javascript/
var obj1 = { which : "obj1" },
obj2 = { which : "obj2" };
function execute(arg1, arg2){
console.log(this.which, arg1, arg2);
}
//using call
execute.call(obj1, "dan", "stanhope");
//output: obj1 dan stanhope
//using apply
execute.apply(obj2, ["dan", "stanhope"]);
//output: obj2 dan stanhope
//using old school
execute("dan", "stanhope");
//output: undefined "dan" "stanhope"
Fundamental difference is that call() accepts an argument list, while apply() accepts a single array of arguments.
The difference is that call() takes the function arguments separately, and apply() takes the function arguments in an array.
Summary:
Both call() and apply() are methods which are located on Function.prototype. Therefore they are available on every function object via the prototype chain. Both call() and apply() can execute a function with a specified value of the this.
The main difference between call() and apply() is the way you have to pass in arguments into it. In both call() and apply() you pass as a first argument the object you want to be the value as this. The other arguments differ in the following way:
With call() you have to put in the arguments normally (starting from the second argument)
With apply() you have to pass in array of arguments.
Example:
let obj = {
val1: 5,
val2: 10
}
const summation = function (val3, val4) {
return this.val1 + this.val2 + val3 + val4;
}
console.log(summation.apply(obj, [2 ,3]));
// first we assign we value of this in the first arg
// with apply we have to pass in an array
console.log(summation.call(obj, 2, 3));
// with call we can pass in each arg individually
Why would I need to use these functions?
The this value can be tricky sometimes in javascript. The value of this determined when a function is executed not when a function is defined. If our function is dependend on a right this binding we can use call() and apply() to enforce this behaviour. For example:
var name = 'unwantedGlobalName';
const obj = {
name: 'Willem',
sayName () { console.log(this.name);}
}
let copiedMethod = obj.sayName;
// we store the function in the copiedmethod variable
copiedMethod();
// this is now window, unwantedGlobalName gets logged
copiedMethod.call(obj);
// we enforce this to be obj, Willem gets logged
We can differentiate call and apply methods as below
CALL : A function with argument provide individually.
If you know the arguments to be passed or there are no argument to pass you can use call.
APPLY : Call a function with argument provided as an array. You can use apply if you don't know how many argument are going to pass to the function.
There is a advantage of using apply over call, we don't need to change the number of argument only we can change a array that is passed.
There is not big difference in performance. But we can say call is bit faster as compare to apply because an array need to evaluate in apply method.
The main difference is, using call, we can change the scope and pass arguments as normal, but apply lets you call it using arguments as an Array (pass them as an array). But in terms of what they to do in your code, they are pretty similar.
While the syntax of this function is almost identical to that of
apply(), the fundamental difference is that call() accepts an argument
list, while apply() accepts a single array of arguments.
So as you see, there is not a big difference, but still, there are cases we prefer using call() or apply(). For example, look at the code below, which finding the smallest and largest number in an array from MDN, using the apply method:
// min/max number in an array
var numbers = [5, 6, 2, 3, 7];
// using Math.min/Math.max apply
var max = Math.max.apply(null, numbers);
// This about equal to Math.max(numbers[0], ...)
// or Math.max(5, 6, ...)
var min = Math.min.apply(null, numbers)
So the main difference is just the way we passing the arguments:
Call:
function.call(thisArg, arg1, arg2, ...);
Apply:
function.apply(thisArg, [argsArray]);
Difference between these to methods are, how you want to pass the parameters.
“A for array and C for comma” is a handy mnemonic.
Call and apply both are used to force the this value when a function is executed. The only difference is that call takes n+1 arguments where 1 is this and 'n' arguments. apply takes only two arguments, one is this the other is argument array.
The advantage I see in apply over call is that we can easily delegate a function call to other function without much effort;
function sayHello() {
console.log(this, arguments);
}
function hello() {
sayHello.apply(this, arguments);
}
var obj = {name: 'my name'}
hello.call(obj, 'some', 'arguments');
Observe how easily we delegated hello to sayHello using apply, but with call this is very difficult to achieve.
Even though call and apply achive the same thing, I think there is atleast one place where you cannot use call but can only use apply. That is when you want to support inheritance and want to call the constructor.
Here is a function allows you to create classes which also supports creating classes by extending other classes.
function makeClass( properties ) {
var ctor = properties['constructor'] || function(){}
var Super = properties['extends'];
var Class = function () {
// Here 'call' cannot work, only 'apply' can!!!
if(Super)
Super.apply(this,arguments);
ctor.apply(this,arguments);
}
if(Super){
Class.prototype = Object.create( Super.prototype );
Class.prototype.constructor = Class;
}
Object.keys(properties).forEach( function(prop) {
if(prop!=='constructor' && prop!=='extends')
Class.prototype[prop] = properties[prop];
});
return Class;
}
//Usage
var Car = makeClass({
constructor: function(name){
this.name=name;
},
yourName: function() {
return this.name;
}
});
//We have a Car class now
var carInstance=new Car('Fiat');
carInstance.youName();// ReturnsFiat
var SuperCar = makeClass({
constructor: function(ignore,power){
this.power=power;
},
extends:Car,
yourPower: function() {
return this.power;
}
});
//We have a SuperCar class now, which is subclass of Car
var superCar=new SuperCar('BMW xy',2.6);
superCar.yourName();//Returns BMW xy
superCar.yourPower();// Returns 2.6
Let me add a little detail to this.
these two calls are almost equivalent:
func.call(context, ...args); // pass an array as list with spread operator
func.apply(context, args); // is same as using apply
There’s only a minor difference:
The spread operator ... allows passing iterable args as the list to call.
The apply accepts only array-like args.
So, these calls complement each other. Where we expect an iterable, call works, where we expect an array-like, apply works.
And for objects that are both iterable and array-like, like a real array, we technically could use any of them, but apply will probably be faster because most JavaScript engines internally optimize it better.
I just want to add a simple example to a well explained post by flatline, which makes it easy to understand for beginners.
func.call(context, args1, args2 ); // pass arguments as "," separated value
func.apply(context, [args1, args2]); // pass arguments as "Array"
we also use "Call" and "Apply" method for changing reference as defined in code below
let Emp1 = {
name: 'X',
getEmpDetail: function(age, department) {
console.log(`Name: ${this.name} Age: ${age} Department: ${department}`)
}
}
Emp1.getEmpDetail(23, 'Delivery')
// 1st approach of changing "this"
let Emp2 = {
name: 'Y',
getEmpDetail: Emp1.getEmpDetail
}
Emp2.getEmpDetail(55, 'Finance')
// 2nd approach of changing "this" using "Call" and "Apply"
let Emp3 = {
name: 'Emp3_Object',
}
Emp1.getEmpDetail.call(Emp3, 30, 'Admin')
// here we have change the ref from **Emp1 to Emp3** object
// now this will print "Name = Emp3_Object" because it is pointing to Emp3 object
Emp1.getEmpDetail.apply(Emp3, [30, 'Admin'])
The call() method calls a function with a given this value and a second parameter which are arguments separated by comma.
object.someMethod.call( someObject, arguments )
The apply() method is the same as call except the fact that the second argument it takes is an array of arguments .
object.someMethod.apply( someObject, arrayOfarguments )
var car = {
name: "Reno",
country: "France",
showBuyer: function(firstName, lastName) {
console.log(`${firstName} ${lastName} just bought a ${this.name} from ${this.country}`);
}
}
const firstName = "Bryan";
const lastName = "Smith";
car.showBuyer(firstName, lastName); // Bryan just bought a Reno from France
const obj = { name: "Maserati", country: "Italy" };
car.showBuyer.call(obj, firstName, lastName); // Bryan Smith just bought a Maserati from Italy
car.showBuyer.apply(obj, [firstName, lastName]); // Bryan Smith just bought a Maserati from Italy
How can I create an object like that
var sum = {
a : 5,
b : 7,
sumar : function()
{
return (this.a+this.b);
},
sumar : function(a, b)
{
return (a+b);
}
}
and then use any of the methods declared like this?
sumar0 = sum.sumar(); //use the method without parameters
sumar1 = sum.sumar(6,7); //use the method with parameters.
Just like "overriding" the methods? is this possible?
Thanks in advance and sorry for my bad english
In Javascript, you do NOT declare two methods of the same name with differnt args like you do in some other languages. Instead, you declare only a single method with the name and then you examine the arguments when the function is called to decide which behavior you should follow.
When you do declare two properties with the exact same name, then one of them is ignored by the interpreter since there can only be one value for any given property in Javascript.
There is a long description of overloading in Javascript with a number of examples here:
How to overload functions in javascript?
In your specific case, you could test how many arguments were passed to the method and branch accordingly:
var sum = {
a : 5,
b : 7,
sumar : function(a, b)
{
if (arguments.length < 2) {
// no arguments passed, use instance variables
return (this.a+this.b);
} else {
// arguments were passed, use them
return (a+b);
}
}
}
document.write(sum.sumar() + "<br>");
document.write(sum.sumar(6, 7) + "<br>");
Though, I must say, this is a particularly odd method that sometimes operates on the instance properties and sometimes doesn't.
I have a list of arguments such as var args = ['blah', 1, 3.9] and I want to apply it to something that needs to be newed like new bleh.Thinggy(a, b, c).
I want to do the following var m = {}; bleh.Thinggy.apply(m, args);
I am worried there is something I am not thinking of does anyone know if this is safe?
Your current method is flawed, because prototype inheritance will not work as expected.
The equivalent of method.apply(context, args) for constructors is:
// Given a list of arguments `args`:
var bindArgs = [Constructor.prototype].concat(args);
new (Function.prototype.bind.apply(Constructor, bindArgs));
The roles of Function.prototype.bind and Function.prototype.apply are explained at the corresponding documentation. Remember: .bind returns a function!
To keep it simple, I'll explain how to use .bind for a fixed number of arguments, say two. Then, the following have the same effect:
Math.max.apply(Math, 2, 3);
Math.max.bind(Math, 2, 3)();
And
Math.max.apply(Math, 2, 3, 4, 5);
Math.max.bind(Math, 2, 3)(4, 5);
If you've even glanced at the documentation, you'll certainly understand the first form. The second form is trickier though. It works in this case, because the position of an argument in Math.max is not relevant. The maximum value of all arguments is considered, where all arguments are treated identically.
Now, here follows an example with a custom function:
function echoMe(name, age) {
console.log('Hello ' + name + '. Your age is ' + age);
console.log('this is ', this);
}
echoMe('Rob', '19');
// "Hello Rob. Your age is 19"
// "this is [object DOMWindow]" (non-strict mode)
var echoYou = echoMe.bind(null, "Oops");
echoYou("Peter", "19");
// "Hello Oops. Your age is Peter"
// "this is null"
Because the position of arguments is significant in this case, the last example showed something weird. Indeed, the first argument is bound to "Oops" by the .bind method. The arguments passed to the bound function echoYou are appended to the arguments list. Additionally, you notice that the context this was changed to null.
Interesting.. Let's try to change the context using .apply:
function printThisFood() {
console.log("this.food is " + this.food);
}
printThisFood.apply({food: "Fish"});
// "this.food is fish"
var locked = printThisFood.bind({food: "Strong"});
locked.apply({food: "Weak"});
// "This.food is Strong"
As you can see, this.food still points to method from the context as defined through .bind!
So, we know how to lock the context of a function, as well as passing an arbitrary number of fixed arguments to a function. This can be applied to constructors, resulting in the function which I presented on top of the answer. To verify that it works as expected:
function Constructor() {
console.log(this instanceof Constructor); // true
console.log(this, arguments); // Convince yourself via console
}
var bindArgs = [Constructor.prototype].concat([1, 2]);
// is equal to [Constructor.prototype, 1, 2]
var BoundConstructor = Function.prototype.bind.apply(Constructor, bindArgs);
var instance = new BoundConstructor();
// Eliminated intermediate variable, and put on one line
var instance = new (Function.prototype.bind.apply(Constructor, bindArgs));
Note: I omitted parentheses () in the one-liner, because constructors can be initialized without these. new Image and new Image() are behaving identically.
To immediately read a property (or invoke a method) from the constructed method, you can either wrap the whole expression in parentheses or append () to remove ambiguity:
(new (Function.prototype.bind.apply(Constructor, bindArgs))).method()
new (Function.prototype.bind.apply(Constructor, bindArgs))().method();
Note 2: It still holds that additional arguments are appended to the argument list. This property can also be used to "preset" the first arguments of a given constructor:
function Stupid(obvious1, obvious2, foo) { this.interesting = foo; }
Stupid.prototype.onlymethod = function() { return this.interesting};
var saveKeyStroke = Function.prototype.bind.call(Stupid, Stupid.prototype, 1, 2);
// Or, equivalent:
//var saveKeyStroke=Function.prototype.bind.apply(Stupid,[Stupid.prototype,1,2]);
new saveKeyStroke('Fourth argument').onlymethod(); // "Fourth argument"
new saveKeyStroke().onlymethod(); // undefined
(new saveKeyStroke).onlymethod(); // undefined
Apply is safe,but it would not return any instance of bleh.Thinggy.
If you want instance of bleh.Thinggy then you have to create it before using bleh.Thinggy.apply.
code:
var m = new bleh.Thinggy; // m is instance of bleh.Thinggy.
bleh.Thinggy.apply(m, args);
... use m as instance of bleh.Thinggy.apply