How do you use function in java script?

Answer 1

Functions are one of the absolute best aspects of Javascript (though the topic can be confusing at points). There are essentially three types of function definitions:

variable function declaration

factorial(5); // error

var factorial = function(target){

if (target === 0) {

return 1;

} else {

return target * factorial( target - 1 );

}

}

factorial(5); // 120

notes on usage

calling a "variable" function such as this before it is declared will result in an error as these functions are not "hoisted" (they aren't moved to the top of stack during compilation to ensure they are globally available). This is ideal for scope management of functions.

traditional function declaration

sayhi('Jack'); // Hello Jack

function sayhi(name){

return "Hello " + name;

}

sayhi('Jake'); // Hello Jake

notes on usage

mostly the same as above, except the scope will automatically be global and this will be hoisted

anonymous functions

// immediate invocation style

(function(msg){

return 'Shake hands, shake hands, ' + msg;

})('Shake Shake Shake');

// deferred invocation (demonstrated as callback), first example is not an anomyous function but the function that will accept a function as a parameter

var shake_hands = function(callback){

return "Shake hands, shake hands, " + callback();

}

// anonymous function passed as argument to shake_hands function

shake_hands(function(msg){

return 'Shake hands, shake hands, ' + msg;

});

notes on usage

anonymous functions are typically used as callbacks (functions that are executed once another function has completed). They can be invoked immediately or deferred

Calling functions

There are 3 acceptable ways to call or "use" functions in javascript

Traditional:

factorial(5);

Providing context (setting what the keyword 'this' will refer to in the function):

factorial.call(null, 5);

// apply is just like call, but accepts an array of arguments to the function

factorial.apply(null, [5]);