Does Javascript Have Lambdas like Python?

I came across this not long ago. In fact, I was helping to teach an introductory course on Javascript, and there happened to be a veteran programmer in attendance, and we wound up sitting together while my partner taught the class. He (the veteran programmer) had been programming in Java for 20 some years and mentioned to me that he had been using lambdas in Javascript. Well, I had used them in Python, but never Javascript, at least to my knowledge. I asked if I had heard correctly, and he said, yes, they were lambdas he had been using. He had come to the Javascript class because he really didn’t know much about using it, despite having worked in Java for so long, so I thought maybe he had misheard or misunderstood–or maybe I had.

Now, let me say right off that I haven’t been programming in Python for as long as he has been programming in Java. It’s been maybe three years for me, and two of those years have been focused on javascript. But I was pretty intense about Python for most of 2014 and 2015, and I did quite a bit of programming in Python in 2016. So I give myself 3 years experience. And more like two years in Javascript. And Lambda expressios were always one of those things in Python that I figured I’ll get around to understanding better one day soon! But the gist is that a lambda is a way of creating a function expression without a def statement, so you can reuse a function in simple one-line or so expressions. They are intended to be handy little tools, as I understand it. They were not intended to be used to replace def statements. You don’t use a lambda expression where you should declare a function using def. It’s more like a tweezers than a crowbar or refrigerator dolly.

So How Do You Use a Lambda Expression in Python?

Once again W3 Schools has a good explanation.

A lambda expression doesn’t have to be anonymous:
name = lambda arguments : expression or

Robin = lambda str : 'Holy ' + str + " Batman!" 

So now you could say Robin("television addiction") and you would get back, Holy television addiction, Batman! So in this case, the lambda expression is a simple string concatenation. But you could do math or substitution or plenty of other things too. But this is the basic idea. It’s a function one-liner that can also be anonymous.

If you wanted to sum the numbers passed to a function:

l = lambda *args : sum(args)

and now you could say l(1,2,3,4) and you’d get 10 back. sum is a handy Array method in Python. We would have to use reduce in Javascript.

Mind you, there are many more ways than this to use lambda expressions in Python. But I’ll stop here for now.

So How Do You Do This in Javascript?

I’m using a terminal with Node running in it for these examples just like I was running python3 in a terminal for the previous examples. Typing the following in a Node terminal:

let robin = str =>  "Holy "+str+", Batman!" 

You’ll get the above mentioned effect in Node. If you enter robin('television addiction') you will also get Holy television addiction, Batman! just as in Python. Officially, I think you could call this a lambda expression in Javascript.

You could go:

const sumit = (total, num) => total + num 

and then you could say:


But I don’t see that as the same thing. First of all, we don’t have list comprehensions in Javascript. But wait, JS has an arguments variable and the ‘rest’ variable (...args), so I wonder what we could do with that?

const sumit = (...args) => args.reduce((x,y) => x+y ) 

Now, if we call sumit(1,2,3,4) we get the right answer.

So, is this a lambda function in JS? Hmmm… I think maybe it is!

Conclusion: JS Lambda Expressions Like Python

Well, I’m just beginning my exploration of lambda expressions in JS, so I’m not concluding anything. But I now feel like there’s more to this than I thought at the beginning of this article. It’s taken me a couple of hours of struggling at the command line and the input of smarter people than I in my Slack channels to get to this point.

But I think I can say that we do have lambda expressions in Javascript. At least sorta. I’m confident that I can at least say at least we sorta do! Maybe even we really do, but that will require more research!