Instantiating javascript class in Node.js

Question

Why is the following code gives me an error?

// In Foo.js
class Foo {
    constructor(a) {
        this.a = a;
    }
}

module.exports = Foo

// In Index.js
var foo = new require('path/Foo.js')('param');

This gives me Class constructor Foo cannot be invoked without 'new'.

Thanks.

Because the code shown is invoking the require function with new rather than your constructor? — nnnnnn
– nnnnnn, Commented Dec 5, 2017 at 2:46
@nnnnnn I forgot to do module.exports at the end. If i do this, require('path/Foo.js) returns Foo class isn't it? — eChung00
– eChung00, Commented Dec 5, 2017 at 2:48

nnnnnn · Accepted Answer · 2017-12-05 02:59:24Z

3

I believe that in your code the new operator is applied to the require() function, not to what require() returns. That is, this:

var foo = new require('path/Foo.js')('param');

...is like doing this:

var foo = ( new require('path/Foo.js') )('param');

...or:

var temp = new require('path/Foo.js');
var foo = temp('param');

Try the following instead, so that new is applied to your class:

var Foo = require('path/Foo.js');
var foo = new Foo('param');

edited Dec 5, 2017 at 2:59

answered Dec 5, 2017 at 2:52

nnnnnn

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1 Comment

eChung00 Over a year ago

Thank you! I thought "require("path/Foo.js")" is same as "Foo".

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