What is a Lambda in Javascript ?
jquery - JavaScript lambda functions - Stack Overflow
[AskJS] Why are lambda functions called lambda functions everywhere except in JS
Need example of calling AWS Lambda from JavaScript - Stack Overflow
Why does JavaScript use so many lambda functions?
Why use the lambda function?
Can I use lambda in JavaScript?
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In JavaScript these are called function expressions (using function as an operator as opposed to declarations, using function as a statement), and can be named or anonymous.
It's really as simple as doing something that tells the compiler it is an expression (e.g. var x =) then writing a function normally, and adding a delimiter ; on the end.
function invoke(lam) {
console.log(
lam()
);
}
var lambda = function () {return 'foo';};
invoke(lambda); // "foo" logged
As with any function in JavaScript, the scope is inherited from where it is defined, not where it is invoked.
Self-invoking and anonymous functions are nearly always function expressions. For self-invoking functions, the ( before function means the code is interpreted as an expression, then the (preferred) ()); or (alternate) )(); invokes it immediately.
You may need to remember that a function expression by itself is not hoisted. If you need hoisting for it to avoid a Reference Error, combine with var. The function itself will still not be fully available until the code passes the line where it is defined.
For a named function expression the name is only available inside the function and not outside (some old versions of IE leaked the name though). To describe this, I'll use two examples, one self invoking and one vard;
// self-invoked
(function foo() {
console.log('inside:', foo); // foo is defined here
}());
console.log('outside:', foo); // ReferenceError: foo is not defined
// var
var bar = function foobar() {
console.log('inside:', foobar); // foobar is defined here
console.log('inside:', bar); // bar is defined here too (=== foobar)
};
bar(); // invoke
console.log('outside:', bar); // bar is also defined here, but..
console.log('outside:', foobar); // ReferenceError: foobar is not defined
A lambda function (anonymous function) is really just a function declaration without a name (it can be assigned to a variable later and still technically be a lambda). One common example is a self-executing function:
(function(){
/*
do stuff
*/
})();
Another common example is passing a function as a parameter for an AJAX or JSONP callback, a timeout, or a sort():
setTimeout(
function() {
console.log('lambda!');
},
100
);
On the "receiving" end of a lambda, you invoke the function parameter as you would any other function:
functionThatUsesLamba(function(s) { console.log(s); });
function functionThatUsesLambda(logFn) {
if (typeof(logFn) == 'function') {
logFn('lambda');
} else {
throw "logFn must be a function!!!";
}
}
Why most js developers call them arrow functions instead of lambda functions
Since you need to run Lambda from the browser, you have two options you can achieve it.
Use
AWS Javascript SDK, set it up with user via static configuration orCognitowithIAM Permissionsto yourLambda. You can also consider subscribing yourLambdafunctions toSNS Topicand run theLambdaby sending a message to the topic. This SNS approach would also require you to store and retrieve the submission state via separate call.Use
AWS API Gatewayto create RESTful endpoint with proper CORS configuration that you can ping from the browser using AJAX.
Both options have their pros and cons. More information about your use-case would be necessary to properly evaluate which one suits you best.
I see people have used AWS SDK for Javascript but it is not required specially since you need to create Amazon Cognito identity pool with access enabled for unauthenticated identities (Atleast for beginners like me). Below code works fine for me -
<html>
<head>
<script>
function callAwsLambdaFunction() {
var xhttp = new XMLHttpRequest();
xhttp.onreadystatechange = function() {
if (this.readyState == 4 && this.status == 200) {
document.getElementById("myDiv").innerHTML = this.responseText;
}
};
xhttp.open("GET", "https://test123.ap-south-1.amazonaws.com/dev", true);
xhttp.send();
}
</script>
<title>Hello World!</title>
</head>
<body>
<h1>Hello world!</h1>
<h1>Click below button to call API gatway and display result below!</h1>
<h1><div id="myDiv"></div></h1>
<button onclick="callAwsLambdaFunction()">Click me!</button><br>
Regards,<br/>
Aniket
</body>
</html>
Above is sample index.html that I have added to my S3 bucket and made a static site. Couple of points to note -
- Make your index.html open from outside if you are using S3 for static site hosting.
- Make sure you turn on CORS for your API gateway if your website domain is not same as API gateway domain. Else you might get -
Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at https://test123.ap-south-1.amazonaws.com/dev. (Reason: CORS header ‘Access-Control-Allow-Origin’ missing).
Wikipedia says "In computer programming, an anonymous function (function literal, lambda abstraction, lambda function, lambda expression or block) is a function definition that is not bound to an identifier."
This makes things relatively easy. For your purposes, "lambda function" and "anonymous function" are effectively synonymous. Therefore, everything you express via the => syntax is a lambda function/anonymous function, and everything you define with the function syntax isn't.
A callback is simply code that is passed to other code to be called at some later time. As you've seen, you can use both named and unnamed functions as callbacks.
The important thing to remember is that "callback" is a role that a function takes on in a specific context. It's entirely possible to call a function as a normal function and also use it as a callback elsewhere.
Callback
A Callback function is any function passed as a parameter to another function to be executed when some condition occurs. In your example, when the Promise returned by fetch is fulfilled.
A callback may be anonymous or named, or defined using function or () => {}.
That is, I would define a callback as 'a function that is called else where in your application'. By that definition, the lambda functions passed into an Array.prototype method are callback functions.
Yup! The first parameter to Array.prototype.forEach is even named callbackFn.
Anonymous vs lambda
In software engineering in general, a lambda function and an anonymous function are the same thing. Here is the definition of anonymous function from the C2 wiki.
In a programming language, an unnamed function object (also: "function literal").
Example (in PseudoCode): "lambda(x,y){ x>y }" is an anonymous function object representing the function that tells whether its first argument is greater than its second argument.
A lambda function is understood to be the same thing because of lambda calculus, which involves anonymous functions, and because the lambda keyword is often used in specific language constructs implementing support for anonymous functions.
When we drill down a bit into Javascript specifically, there are two language constructs that implement anonymous functions.
The first one is an anonymous function expression
function() { console.log("Doing stuff") }
The second is an arrow function expression
() => console.log("Doing stuff")
While these provide language support for making anonymous functions, you can still assign names to the result.
const myFunction = function() { console.log("Doing stuff") }
In other languages, such as Java and C#, lambda function refers to a syntax similar to arrow functions. While Javascript doesn't really have a language construct with that name, arrow functions would probably spring to mind for many people because of the similarity.
In conclusion, anonymous functions and lambda functions can be said to be the same thing from a software engineering perspective, but they can also refer to specific language constructs which are not equivalent.
The code below
function printCurrentValue(value) {
console.log("the value is: " + value)
}
Is then not an anonymous function, nor a lambda function. But if it had been
const printCurrentValue = function(value) {
console.log("the value is: " + value)
}
Then it's still not an anonymous function, but you could say it's defined using an anonymous function expression.
As for
fetch('/user')
.then((res) => res.json())
.then((json) => console.log(json));
(res) => res.json() is
- Anonymous
- A callback
- An arrow function
And you could say it's a lambda function, both referring to it being anonymous and referring to it being an arrow function.