Question

I’m new to JavaScript, and have a simple (I presume) question regarding best practices for accessing variables in functions:

When should I declare a global variable, as opposed to simple passing a value into a function?

Answer 1

Declaring a global variable should only be used as an option of last resort.

Global variables are bad in general and especially so in javascript. There is simply no way to prevent another piece of javascript from clobbering your global. The clobbering will happen silently and lead to runtime errors.

Take the following as an example.

// Your code
myParam = { prop: 42 };
function operateOnMyParam() {
  console.log(myParam.prop);
}

Here i’ve declared 2 global variables

• myParam
• operateOnMyParam

This might work fine while testing your javascript in isolation. However what happens if after testing a user combines your javascript library with my javascript library that happens to have the following definitions

// My code
function myParam() {
  console.log("...");
} 

This also defines a global value named myParam which clashes with your myParam. Which one wins depends on the order in which the scripts were imported. But either way one of us is in trouble because one of our global objects is dead.