How do I pass variables between functions in Javascript?

Question

Here are 2 functions in the simplest form. I'm working with jquery.

What is the best way to pass var str from the first function to the second one?

function a() {
    var str = "first";
};

function b() {
    var new = str + " second";
};

Solution 1

You need to either pass it between them, or it seems from your example, just declare it in a higher scope:

var str;
function a(){
  str="first";
}
function b(){
  var something = str +" second"; //new is reserved, use another variable name
}

Solution 2

Use function parameters, like this:

function a() {
   var str = "first";
   b(str);
}

function b(s) {
   var concat = s + " second";
   //do something with concat here...
}

You could just declare a variable higher up in the scope chain, but I opt to use arguments to restrict variable access to only the contexts that absolutely need it.

Oh yeah, isn't that called the principle of least privilege?

Comments

[Jacob Relkin]

@Nick I like passing them around better than leaving them out in the open for others to modify.

[Nick Craver]

@Jacob - You're assuming a has anything to do with b though, it could just be a() in one place, possibly never even called, then b() in another.

[Phrogz]

Man, it's really hard to decide which one of these answers to go with. Both are very, very valid given various uses, and there's no way to tell given the sparse details from the OP.

[david]

The module pattern allows you to do this while still keeping the 'str' variable protected from outside editing. Quick example of a shared var: jsfiddle.net/Ma8Hs

[Nick Craver]

@david - true, but you'd have to proxy the functions outside, creating a closure so they can be accessed, complicated things a bit :)

[David Tang]

@Jacob, from this very limited example it's hard to see the OP's intended use of the variable. Choosing whether to pass as an argument, or declaring at a high scope, or even b.call(new String(str)) for arguments sake, really depends on the situation...

[david]

@Nick, it's only complex if you say it out loud >.>

[Hussein]

What happens if we are referencing $(this) in var str. Will that still work in function b and retain the value of this that was referenced in function a.

[Nick Craver]

@alex - it depends on where the functions are called from, this will refer to the current context, but what that is depends on how they're called