JavaScript Miscellany Jan 14 2019 Properties of functions Functions are objects and have properties length the number of formal parameters arguments the Arguments object which is like an array of actual parameters Note length is not necessarily equal to arguments length caller the function that invoked this one or null if the function was invoked from the top level prototype for constructor functions an object that defines properties and methods of functions created with this constructor 2 The for in loop This kind of loop has the syntax for variable in object statement Each property of the object or if an array each index of the array is assigned in turn to variable Properties will be assigned in an arbitrary order It turns out however that the for in loop does not loop through all properties of an object Built in methods and many built in properties are flagged as nonenumerable All built in properties of functions are nonenumerable There are lots of little surprises like this in JavaScript 3 Global and local variables A variable is local to a function if It is a formal parameter of the function It is declared with var inside the function e g var x 5 Otherwise variables are global Specifically a variable is global if It is declared outside any function with or without var It is declared by assignment inside a function e g x 5 4 Methods I First we construct an object function Point xcoord ycoord this x xcoord keyword this is mandatory this y ycoord myPoint new Point 3 5 A method is a function that is associated with and invoked through an object hence can use this Here is a function that makes no sense by itself function distance x2 y2 function sqr x return x x return Math sqrt sqr this x x2 sqr this y y2 5 Methods II We can turn this function into a method like so Now this inside the function refers to myPoint and we can say document write The distance is dist call myPoint 6 9 Or document write The distance is myPoint dist 6 9 If we don t want to permanently associate the function with myPoint but just use it briefly we can say myPoint dist distance document write The distance is dist apply myPoint 6 9 The difference between these two Function methods is call takes an object and an arbitrary number of actual parameters apply takes an object and an array of actual parameters 6 The End 7
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