What is the difference between call and apply?

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What is the difference between using call and apply to invoke a function?

var func = function(){ alert('hello!'); }; 

func.apply();

vs

func.call(); 

Are there performance differences between the two methods? When is it best to use call over apply and vice versa?

This Question Has 19 Answeres | Orginal Question | John Duff

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.

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" 

Another example with Call, Apply and Bind. The difference between Call and Apply is evident, but Bind works like this:

  1. Bind returns an instance of a function that can be executed
  2. First Parameter is 'this'
  3. 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 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()); return('Hello reader'); } console.log(reader.getName()); 

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.

Difference between these to methods are, how you want to pass the parameters.

“A for array and C for comma” is a handy mnemonic.

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 // Output: // My name is John and I am a fireman. // My name is Susan and I am a school teacher. // My name is Claude and I am a mathematician. // My name is Matthew and I am a physicist. 

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...)

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 

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.

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.

The main difference is using call we can change the scope and pass arguments as normal, but apply let you call the using arguments as an Array(pass them as array). but in term 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 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 argumenets:

Call:

function.call(thisArg, arg1, arg2, ...); 

Apply:

function.apply(thisArg, [argsArray]); 

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/

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"]); 

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.

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*

The difference is that call() takes the function arguments separately, and apply() takes the function arguments in an array.

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.

Fundamental difference is that call() accepts an argument list, while apply() accepts a single array of arguments.

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.


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Sajjad Hossain

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