Locally var.s affect global ones

[Andrew Poulos]

Say I have script that looks a bit like this:


Array.prototype.getValue = function(ind, dir) {
if (dir !=0) ind[ind.length-1] += dir;
// more script
}

screens = [...// a big array
fnd = [1,2,3];
// 1.
a = findStart(fnd);
// 2.
b = findEnd(fnd);

findStart = function(cs) {
return screens.getValue(cs, 1);
}

findEnd = function(cs) {
return screens.getValue(cs, -1)
}


As I go from 1. to 2. the value of fnd changes. I don't understand how a
locally scoped variable might be affecting a global one.

Andrew Poulos

 

[Martin Honnen]

fnd = [1,2,3];
// 1.
a = findStart(fnd);
// 2.
b = findEnd(fnd);

As I go from 1. to 2. the value of fnd changes. I don't understand how a
locally scoped variable might be affecting a global one.

What you have above is a variable fnd which contains a reference to an
array object and whereever you pass fnd to you pass in the reference to
that array object and any function then operates on the array object fnd
references.
So it is not suprising in JavaScript that an object or array object is
manipulated if you pass it to a function or method, no copies of objects
respectively array objects are created, instead references to objects
are passed around. Pretty much the same as in Java where you have value
types and reference types.