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In the `import` syntax of ES6, how is a module evaluated exactly?

Posted by: admin November 30, 2017 Leave a comment

Questions:

Let’s say we have four modules, A, B,C and D

In module A:

console.log("A evaluated")
function AClass {
  console.log("A constructor")
}
var aObj = new AClass()
export default aObj;

In module B:

import aObj from A
export default "B"

In module C:

import aObj from A
export default "C"

In module D:

import b from B
import c from C
import aObj from A

So when module D is evaluated, how many times will the A evaluated and A constructor be printed in the console?

Is this behavior described in ES6 standard? What should I do if I want a module to be evaluated ONLY ONCE no matter how many times is imported directly or indirectly? Does anyone have any ideas about this?

Answers:

When the D module is executed, the console will print this message:

A evaluated
A constructor

Which means that the A module was evaluated only once, even if it was imported multiple times by other modules.

The evaluation rules for ES6 modules is the same as for commonjs format:

  • A module is a piece of code that is executed once it is loaded. It means that if a module is not included in the main bundle, it will not be evaluated
  • Modules are singletons. If a module is imported multiple times, only a single instance of it exists and it is evaluated only once at load

The behaviour of importing the same instance of the module is described HostResolveImportedModule section of the ECMAScript 6 specification.
It mentions:

This operation (import operation) must be idempotent if it completes normally. Each time
it is called with a specific referencingModule, specifier pair (import <a> from <source>) as
arguments it must return the same Module Record instance.

The behaviour of single time evaluation of the module is described in ModuleEvaluation, point 4 and 5 using Evaluated boolean flag.
Each module has Evaluated flag which makes sure to evaluate the module code only once.