在 ES6 的 `import` 语法中，模块是如何准确计算的? [英] In the `import` syntax of ES6, how is a module evaluated exactly?

问题描述

假设我们有四个模块，A、B、C 和 D

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

在模块A中:

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

在模块B中:

import aObj from A
export default "B"

在 C 模块中:

import aObj from A
export default "C"

在 D 模块中:

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

那么当模块D被求值时，A求值和A构造函数会在控制台打印多少次?

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

此行为是否在 ES6 标准中描述?如果我希望一个模块无论直接或间接导入多少次都只评估一次，我该怎么办?有没有人对此有任何想法?

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?

推荐答案

D 模块执行时，控制台会打印此信息:

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

A evaluated
A constructor

这意味着 A 模块只被评估一次，即使它被其他模块多次导入.

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

ES6 模块 的评估规则与commonjs 格式相同:

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

• 模块是一段代码，一旦加载就会执行.这意味着如果一个模块没有包含在主包中，它将不会被评估
• 模块是单例的.如果一个模块被多次导入，它只存在一个 instance 并且在加载时只评估一次

• 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

描述了导入相同模块实例的行为HostResolveImportedModule 部分.
它提到:

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

这个操作(导入操作)如果正常完成就必须是幂等的.每一次它使用特定的引用模块调用，说明符对 (import from ) as参数它必须返回相同的模块记录实例.

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.

模块单次评估的行为在ModuleEvaluation，第 4 点和第 5 点使用 Evaluated 布尔标志.
每个模块都有 Evaluated 标志，确保只对模块代码进行一次评估.

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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