Filling the pit - node callback function

Recently I wrote some node scripts, and I feel that the callback function is a big pit, which is briefly summarized here.

The most direct manifestation of node asynchronous programming is callback

The difference between synchronous and asynchronous: Blocking occurs when reading file information synchronously, but not asynchronously.

1. Callback for reading and writing files (asynchronous)

var fs=require("fs") //Introduce the fs module
var fileName = "./haha" // file path
 
// start synchronization
var data = fs.readFileSync(fileName);
console.log(data.toString) //Convert the read file into a string
Another way to convert to string
var data = fs.readFileSync(fileName,{flag: 'r+', encoding: 'utf8'})
console.log(data)

// start asynchronously
fs.readFile(__dirname + '/test.txt', {flag: 'r+', encoding: 'utf8'}, function (err, data) {
    if(err) {
     console.error(err);
     return;
    }
    console.log(data);
});

 2, add the callback parameter to the function to call back

    var a=0;

(1) The simplest kind of callback function

function setNumber(cb) {
    var a=1;
    var b=a+1;
    cb(b)
}
function getNumber(data) {
    console.log(data)
}
setNumber (getNumber)

 (2) I am raising a chestnut

function findByStaId(staId,cb) {
        getMongosData({staId: staId},function (result) { //call a function
            if (!result[0]) {
                cb({status: false})
            }
            cb({status: true, stationConfig: result[0]}
        })

    }
findByStaId(staId,function(result){
    console.log(result)
})