问题描述
我想创建一个依赖于最后一个结果的函数序列。第一个函数将从数据库中导入数据。
导入值> fctOne(Value) > fctTwo(fctOne(Value)的值) > fctThree(fctTwo(value)的值) >等等...
App.js 需要等待值被导入,所以 fctOne 不能同步。但是,我背后的所有功能是否也需要异步?他们必须按顺序运行。
我的失败代码:有错误:“callBack(result) TypeError: callBack is not a function”
DB.JSON
{"val":5,"square":25,"SquareRoot":5,"dividedByHimself":1}
在 app.js 上
const db = require('./DB.json')
const fs = require('fs')
const path = require('path')
const dbPath = path.join(__dirname,'DB.json')
let data = fs.readFileSync(dbPath)
let userdbupdate = JSON.parse(data)
///function to import data with simulation of the delay to obtain imported value//
const value = (pathDB,callBack) => {
setTimeout((err) => {
if(err){
return callBack(new Error("loading Failed"))
}
const val = db[pathDB]
callBack(val)
},500)
}
//// square function
let square = (pathDB,callBack) => {
value(pathDB,(val) => {
let result = val * val
userdbupdate.square = result
callBack(result)
let dataMAJ = JSON.stringify(userdbupdate)
fs.writeFileSync(dbPath,dataMAJ)
})
}
square('val')
let squareRootOfSquare = (pathDB,callBack) => {
square(pathDB,(val) => {
let result = Math.sqrt(val)
userdbupdate.square = result
callBack(result)
let dataMAJ = JSON.stringify(userdbupdate)
fs.writeFileSync(dbPath,dataMAJ)
})
}
squareRootOfSquare('val')
解决方法
正如 jfriend00 所提到的,尝试使用 promise 重写它。是的,对于串行计算,它更容易、更清晰。
const valuePromise = new Promise((resolve,reject) => {
setTimeout(()=> {
const val = db.val
resolve(val)
})
}).then(function(resultOfCall){
let resultSquare = resultOfCall * resultOfCall
userDBupdate.square = resultSquare
let dataMAJ = JSON.stringify(userDBupdate)
fs.writeFileSync(dbPath,dataMAJ)
return resultSquare
}).then(function(resultOfSquare){
let resultRoot = Math.sqrt(resultOfSquare)
userDBupdate.squareRootOfSquare = resultRoot
let dataMAJ = JSON.stringify(userDBupdate)
fs.writeFileSync(dbPath,dataMAJ)
return resultRoot
}).then(function(resultOfSquareRoot){
let resultOfDivide = resultOfSquareRoot / resultOfSquareRoot
userDBupdate.squareRoot = resultOfDivide
let dataMAJ = JSON.stringify(userDBupdate)
fs.writeFileSync(dbPath,dataMAJ)
return resultOfDivide
})