Invoking Functions

JavaScript functions can be invoked in four ways:

• as functions,
• as methods,
• as constructors, and
• indirectly through their call() and apply() methods.

Function Invocation

    var total = distance(0,0,2,1) + distance(2,1,3,5);

Method Invocation

A method is nothing more than a JavaScript function that is stored in a property of an object.

    var calculator = { 
    operand1: 1,
    operand2: 1, 
    add: function() {
        // Note the use of the this keyword to refer to this object.
        this.result = this.operand1 + this.operand2; 
        }
    };
    calculator.add(); 

Constructor Invocation

If a function or method invocation is preceded by the keyword new, then it is a constructor invocation.

    var o = new Object();

Indirect Invocation

call() and apply() allow you to indirectly invoke a function as if it were a method of some other object.

To invoke the function f() as a method of the object o (passing no arguments), you could use either call() or apply():

    f.call(o); 
    f.apply(o);

to pass two numbers to the function f() and invoke it as if it were a method of the object o, you could use code like this:

    f.call(o, 1, 2);

The apply() method is like the call() method, except that the arguments to be passed to the function are specified as an array:

    f.apply(o, [1,2]);