How does require work with new operator in node.js

问题:

Let's have a file.js with this code:

module.exports.func = function(txt) {
    this.a = 1;
    this.b = 2;
    console.log(txt, this);
    return this;
}

Now we have another JS file where we do following:

var r1 = new (require('./file')).func('r1');
var r2 = new require('./file').func('r2');

In r1 case it works as intended - r1 contains reference to the newly created object.

In r2 case it does not work - r2 gets reference to module.exports from within the file.js.

The intention was to create a new object by calling func() constructor. Sure, I can do it also this way which is equal to r1:

var r3 = require('./file');
var r4 = new r3.func('r1');

However, I do not understand why r2 does not behave the same way as r1.

How do the extra parenthesis around require('./file') make a difference?

回答1:

These two versions are fundamentally different.

This one:

new (require('./file')).func('r1');

Executes the require, returning the exports of ./file and then calling the new operator on the results .

This one:

var r2 = new require('./file').func('r2');

Invokes require as a constructor.

---

Let's look at a more isolated and simple example:

new Date() // creates a new date object
new (Date()) // throws a TypeError: string is not a function