Skip to main content

JavaScript: JavaScript has Closures

Python has closures:
def outer():
print "Enter number:",
num = raw_input()
def inner():
print num
return inner

f = outer()
f()
Note that the variable num is available from within inner when outer has already completed.

Did you know that JavaScript has them too?
function testSetTimeout() {
var privateVar = 'A string';
function bar() { alert(privateVar); }
window.setTimeout(bar, 1000);
}

testSetTimeout();
The function bar makes use of privateVar which a variable in testSetTimeout's local scope even though bar is invoked later by setTimeout, i.e. after testSetTimeout has completed.

Comments

Donovan Preston said…
JavaScript even has non-neutered closures, unlike Python. In Python, there is no distinction between creating a name in an inner scope and changing the value stored in that name. JavaScript has the "var" construct (did you know that any variable declared in JavaScript without var is automatically global?) and thus has a way to distinguish between creating a new cell in an inner scope and rebinding an outer cell in a containing scope.

var accum = function() {
var i = 0;
var get = function() { return i };
var set = function() { i++ };
return [get, set];
}

var x = accum()

x[1]()
x[1]()
x[0]()


The last line will return 2.

dp
jjinux said…
Nice tip. It makes total sense. My buddy Brandon L. Golm has argued with me many times wishing that Python's global and local variables were at least declared like with "my" in Perl. I guess this is one small case where it would pay off. I've actually encountered this limitation in my code. If you really need to get around it, wrap the variable with a list, and then modify the first element of that list:

def outer():
num = [5]
def inner():
num[0] += 1
print num[0]
return inner

f = outer()
f()

(Ugh, Blogger won't let me use the pre tag to indent things.)

I saw this trick in Sam Rushing's code, so I can't take credit for it.

Popular posts from this blog

Ubuntu 20.04 on a 2015 15" MacBook Pro

I decided to give Ubuntu 20.04 a try on my 2015 15" MacBook Pro. I didn't actually install it; I just live booted from a USB thumb drive which was enough to try out everything I wanted. In summary, it's not perfect, and issues with my camera would prevent me from switching, but given the right hardware, I think it's a really viable option. The first thing I wanted to try was what would happen if I plugged in a non-HiDPI screen given that my laptop has a HiDPI screen. Without sub-pixel scaling, whatever scale rate I picked for one screen would apply to the other. However, once I turned on sub-pixel scaling, I was able to pick different scale rates for the internal and external displays. That looked ok. I tried plugging in and unplugging multiple times, and it didn't crash. I doubt it'd work with my Thunderbolt display at work, but it worked fine for my HDMI displays at home. I even plugged it into my TV, and it stuck to the 100% scaling I picked for the othe

ERNOS: Erlang Networked Operating System

I've been reading Dreaming in Code lately, and I really like it. If you're not a dreamer, you may safely skip the rest of this post ;) In Chapter 10, "Engineers and Artists", Alan Kay, John Backus, and Jaron Lanier really got me thinking. I've also been thinking a lot about Minix 3 , Erlang , and the original Lisp machine . The ideas are beginning to synthesize into something cohesive--more than just the sum of their parts. Now, I'm sure that many of these ideas have already been envisioned within Tunes.org , LLVM , Microsoft's Singularity project, or in some other place that I haven't managed to discover or fully read, but I'm going to blog them anyway. Rather than wax philosophical, let me just dump out some ideas: Start with Minix 3. It's a new microkernel, and it's meant for real use, unlike the original Minix. "This new OS is extremely small, with the part that runs in kernel mode under 4000 lines of executable code.&quo

Drawing Sierpinski's Triangle in Minecraft Using Python

In his keynote at PyCon, Eben Upton, the Executive Director of the Rasberry Pi Foundation, mentioned that not only has Minecraft been ported to the Rasberry Pi, but you can even control it with Python . Since four of my kids are avid Minecraft fans, I figured this might be a good time to teach them to program using Python. So I started yesterday with the goal of programming something cool for Minecraft and then showing it off at the San Francisco Python Meetup in the evening. The first problem that I faced was that I didn't have a Rasberry Pi. You can't hack Minecraft by just installing the Minecraft client. Speaking of which, I didn't have the Minecraft client installed either ;) My kids always play it on their Nexus 7s. I found an open source Minecraft server called Bukkit that "provides the means to extend the popular Minecraft multiplayer server." Then I found a plugin called RaspberryJuice that implements a subset of the Minecraft Pi modding API for B