I'm trying to learn express.js and then i encountered these statements.
const express = require('express');
const app = express();
So now how is it possible to write this variable "express" as "express()". Is it possible to do this with any type of variable?
Functions are first-class objects in JavaScript, they aren't special like they are in some other languages. They're just objects that are callable. Since they're objects, you can pass them around:
function example() {
console.log("Hi there");
}
const e = example;
e(); // "Hi there"You can also put additional properties on them (something that Express also does). Here's an example:
function example() {
console.log("Hi there");
}
example.prop = 42;
const e = example;
e(); // "Hi there"
console.log(example.prop); // 42
console.log(e.prop); // 42 -- since `e` and `example` both refer to the same functionIs it possible to do this with any type of variable?
JavaScript variables don't have a type as they do in strongly-typed languages. The value the variable holds has a type, but the variable doesn't; a variable can hold a number one moment, a string the next, a function after that...
function example() {
console.log("Hi there");
}
let e = example;
e(); // "Hi there"
e = 42;
console.log(e); // 42
e = "Hi there";
console.log(e); // "Hi there"If you want variables, parameters, and such to have types (so you can't put a string in a variable that's expected to contain a number), consider using TypeScript, Flow, or even just JSDoc combined with robust IDE support. These layer a type system on top of JavaScript (to varying degrees).
Getting back to your question: You can do it with any variable, but not with any type of value. You can't call a number:
let e = 42;
e(); // TypeError: e is not a functionYou can only call functions or other host-provided callable objects¹ (which don't absolutely have to be true functions, but it's uncommon now for hosts to provide callable objects that aren't actual functions, it was problematic).
¹ Specifically, in spec terms, any object that supports the [[Call]] internal operation.
express function creates and returns an Application object, and that's what you want to use when building your Express app (mostly; the express function itself has some methods you might also use, but the primary ones are on the Application object, not the express function).It is not possible to do this with any variable but there is something called arrow functions for example
const test = () => {}
and you can call this function by saying
test()
within () it takes input and within {} you specify what it does or returns
require fetches whatever you specified so i'm guessing it is equal to a function
To do this with any variable it must be an arrow function. Hope I helped you!
Definitely yes, you are talking about JavaScript inline functions, check this out!
For example, you can do:
function example() {
return 1;
}
const execute = example();
console.log(execute) // returns 1
