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?
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:
instance of it exists and it is evaluated only once at loadThe 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.
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