# Call & Apply

In short, call() and apply() are methods that allow us to explicitly specify the scope that a function should run in. Let's start with an example.

var person1 = {
    name: 'Nicholas', age: 42
};
var person2 = {
    name: 'Dave', age: 34
};
var sayHello = function(){
    console.log('Hello, ' + this.name);
};
var sayGoodbye = function(){
    console.log('Goodbye, ' + this.name);
};
sayHello();
sayGoodbye();

In Javascript, scopes are dynamic and depend on the caller. The calls to sayHello() and sayGoodbye() will give unexpected values since they rely on their scope for the values of this.name. When called without an explicit scope, they run in the scope of the Window Object.

This is where call() and apply() come to the rescue. call() and apply() allow us to execute a method, in the scope of the object passed to them as shown below

sayHello.call(person1);
sayGoodbye.call(person2);

sayHello.apply(person1);
sayGoodbye.apply(person2);

The four calls above achieve the same thing. They execute the function in the scope of the argument supplied to it.

Consider what happens when the function takes multiple arguments.

var update = function(name, age, gender){
    this.name = name;
    this.age = age;
    this.gender = gender;
};
update.call(person1, 'Nicholas', 35, "Male");

No big deal. They're simply passed to the function as subsequent arguments. The problem with call() now becomes apparent. What do we do when we do not know the number of parameter that a function takes?

This is where apply() comes in handy. The second argument to apply is an array that is unpacked into arguments and these are passsed to the function.

var update = function(name, age, gender){
    this.name = name;
    this.age = age;
    this.gender = gender;
};
update.apply(person1, ['Nicholas', 35, "Male"]);

# The Arguments Object

arguments is an array-like object that is accessible inside functions, that contains the values of the arguments passed to the function.

We can illustrate this with an example

function func1(a, b, c) {
  console.log(arguments[0]);
  console.log(arguments[1]);
  console.log(arguments[2]);
}
func1(1, 2, 3);

Output

1
2
3