How it is possible to use a variable as a function in javascript?

问题

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?

回答1:

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 function

Is 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 function

You 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.

回答2:

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!

回答3:

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

来源：https://stackoverflow.com/questions/66075802/how-it-is-possible-to-use-a-variable-as-a-function-in-javascript