在执行下一个在 NodeJS 中使用该数据的 function 之前，先完成一个 readln function

Question

please some help with this. I want to make a function that creates a nx2 matrix using console inputs (input lines of two spaced integers). The problem is when I want to make use of this data from another function, my data seems to be undefined. For example, the printmatrix function calls the readfunction but results in undefined because it seems that the readfunc has not yet read all the keystrokes yet, and the printmatrix function is already executed with no input data. How I can make sure the function that requires user input finishes before executing the print function or any other function that would require the data to be ready to be read? Thank you for your help.

const readline = require('readline');

const readfunc =  () =>{
    const rl = readline.createInterface({
        input: process.stdin,
        output: process.stdout
    });

    let arr=[];
    let mat=[];

    rl.on('line', (input) => {
    
    let splitAns=input.split(" ");
    arr.push(splitAns[0])
    arr.push(splitAns[1])
    mat.push(arr)
    arr=[];

  });

  rl.on('close', () =>{
      return (mat)
  })
}

const printmatrix =  () =>{

    const result =  readfunc();
    console.log(result);
}

printmatrix(); // prints undefined :(

Answer 1

I didn't test the code but doing a quickly code review i think that you are missing the result because you're not waiting for it. Since the two lines inside printmatrix are executed asynchronous there is no time for readfunc to finish collecting the data into 'mat' variable.

What i suggest is to return a Promise from readsfunc() method and wait for the response before printing it out to console.

So printmatrix will look like this:

readfunc().then((data) => console.log(data));

More info about Promises -> https://developer.mozilla.org/es/docs/Web/JavaScript/Reference/Global_Objects/Promise

One example from this article:

const myPromise = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve('foo');
  }, 300);
});

myPromise
  .then(handleResolvedA => console.log(handleResolvedA)

Will print 'foo' after 300 ms