// b.js
let count = 1
let plusCount = () => {
 count++
}
setTimeout(() => {
 console.log('b.js-1', count)
}, 1000)
module.exports = {
 count,
 plusCount
}

// a.js
let mod = require('./b.js')
console.log('a.js-1', mod.count)
mod.plusCount()
console.log('a.js-2', mod.count)
setTimeout(() => {
 mod.count = 3
 console.log('a.js-3', mod.count)
}, 2000)

node a.js
a.js-1 1
a.js-2 1
b.js-1 2 // 1秒后
a.js-3 3 // 2秒后

// 其他代码相同
module.exports = {
 get count () {
 return count
 },
 plusCount
}

node a.js
a.js-1 1
a.js-2 1
b.js-1 2 // 1秒后
a.js-3 2 // 2秒后， 由于没有定义setter，因此无法对值进行设置。所以还是返回2

// b.js
let obj = {
 count: 1
}
let plusCount = () => {
 obj.count++
}
setTimeout(() => {
 console.log('b.js-1', obj.count)
}, 1000)
setTimeout(() => {
 console.log('b.js-2', obj.count)
}, 3000)
module.exports = {
 obj,
 plusCount
}

// a.js
var mod = require('./b.js')
console.log('a.js-1', mod.obj.count)
mod.plusCount()
console.log('a.js-2', mod.obj.count)
setTimeout(() => {
 mod.obj.count = 3
 console.log('a.js-3', mod.obj.count)
}, 2000)

node a.js
a.js-1 1
a.js-2 2
b.js-1 2
a.js-3 3
b.js-2 3

3, 4, 5可以使用同一个例子说明

// b.js
exports.done = false
let a = require('./a.js')
console.log('b.js-1', a.done)
exports.done = true
console.log('b.js-2', '执行完毕')

// a.js
exports.done = false
let b = require('./b.js')
console.log('a.js-1', b.done)
exports.done = true
console.log('a.js-2', '执行完毕')

// c.js
let a = require('./a.js')
let b = require('./b.js')

console.log('c.js-1', '执行完毕', a.done, b.done)

node c.js
b.js-1 false
b.js-2 执行完毕
a.js-1 true
a.js-2 执行完毕
c.js-1 执行完毕 true true

// b.js
export let counter = {
 count: 1
}
setTimeout(() => {
 console.log('b.js-1', counter.count)
}, 1000)

// a.js
import { counter } from './b.js'
counter = {}
console.log('a.js-1', counter)

// Syntax Error: "counter" is read-only

// a.js
import { counter } from './b.js'
counter.count++
console.log(counter)

// 2

// b.js
import {foo} from './a.js';
export function bar() {
 console.log('bar');
 if (Math.random() > 0.5) {
 foo();
 }
}

// a.js
import {bar} from './b.js';
export function foo() {
 console.log('foo');
 bar();
 console.log('执行完毕');
}
foo();

node a.js
foo
bar
执行完毕

// 执行结果也有可能是
foo
bar
foo
bar
执行完毕
执行完毕