Question

If I define something in a passed function, e.g.

var name = "Whatever";

Is this now accessible from other parts of the application, or is it limited? What’s the logic behind defining scope in node.js javascript?

Answer 1

Any variable defined in a method is only available in that method, if you use var. If you do

myVar = 1;

myVar will be global.

Node is like other javascript. Node does define a global variable, so you can do things like

global.SOME_CONSTANT = 'A Constant'

and then use that like

SOME_CONSTANT

anywhere in your code/modules.

Because node is asynchronous, you end up defining a lot of callbacks. i.e. you do a lot of

someModule.someMethod(opts, function(data){
   // callback code in here
});

where the function you passed in gets invoked by someMethod when it is done. Let’s say you invoked someModule.someMethod in a function. If you defined a variable in that outer function, so your code looks like

var x = [];
someModule.someMethod(opts, function(data){
   // x is available in this callback because of closures, so you can do
   x.push(data);
});

the variable defined in the outer scope is available in the callback because of closures. I suggest you spend some time reading about closures in javascript.