Cycles

When there are circular require() calls, a module might not have finished executing when it is returned. Consider this situation:

 a.js:

console.log(‘a starting’);

exports.done = false;

const b = require(‘./b.js’);

console.log(‘in a, b.done = %j’, b.done);

exports.done = true;

console.log(‘a done’);

 b.js:

console.log(‘b starting’);

exports.done = false;

const a = require(‘./a.js’);

console.log(‘in b, a.done = %j’, a.done);

exports.done = true;

console.log(‘b done’);

 main.js:

console.log(‘main starting’);

const a = require(‘./a.js’);

const b = require(‘./b.js’);

console.log(‘in main, a.done = %j, b.done = %j’, a.done, b.done);

 When main.js loads a.js, then a.js in turn loads b.js. At that point, b.js tries to load a.js. In order to prevent an infinite loop, an unfinished copy of the a.js exports object is returned to the b.js module. b.js then finishes loading, and its exports object is provided to the a.js module.

 By the time main.js has loaded both modules, they’re both finished. The output of this program would thus be:

 $ node main.js

main starting

a starting

b starting

in b, a.done = false

b done

in a, b.done = true

a done

in main, a.done = true, b.done = true

 Careful planning is required to allow cyclic module dependencies to work correctly within an application.