Julian on JavaScript: Functions Julian M Bucknall, CTO

Functions are objects −Is an encapsulation of some code −Can have properties and methods of it’s own −Can appear as a parameter, can be returned from another function −Like all objects, functions are passed by reference not value

Defining a function −Two main ways: −Function literal −var f = function(arguments) { … }; −var f = function optName(arguments) { … }; −Function statement −function f(arguments) { … }

Return value? −All functions return something −There is no void type −“return;” will return undefined −If no return statement −Function returns undefined −Unless it’s a constructor, called via new

Invocation −When invoked, all functions get two extra variables −this − −arguments −The parameter values of the call as an array-like object

Invocation patterns −Method −Function −Constructor −“apply”

Method invocation −Defined as a method on an object −Called via that object − this points to the object containing the method −Like C#, really

Function invocation −Defined as a global function −Or, defined as an internal function −Called without reference to an object −this points to the Global Object −Catches everyone out

The Global Object −Has no name −Is where all unowned variables (primitives, objects, functions) end up as public properties −are visible to all code! −Browser sets up a special property called window to point to the Global Object −(window is a property of the Global Object)

Constructor invocation −Defined as normal function −Convention: Name has initial uppercase letter −Called with new keyword −this points to object being constructed −constructor property already set −Object constructed is returned by default −No need for return statement

“apply” invocation −Defined however you want −Called via the function’s apply method −Alternately: use the call method −You get to define the this object

Scope −Scope in JavaScript is by function −NOT braces as in C# −No block scope here −The this object stays with the outer function, inner functions get their own this (usually the Global Object) −Watch out for callbacks

Scope 2 −A variable declared in a function is visible throughout the function −Even before it’s lexically defined

Closures −The “yay!” to scope’s “ow!”

Thank You Julian M Bucknall ∙ CTO ∙