How to set variable from callback function?

Question

I have function below.

function sh(cmd) {
    exec(cmd, (err, stdout, stderr) => {
      if (err) {
        console.log(err);
        std_value.flush();
        std_value.set("error");
      } else {
        console.log({stdout, stderr});
        std_value.flush();
        std_value.set(stdout);
        console.log('std_value in ' + std_value.result);
      }
    });
    console.log('std_value out ' + std_value.result);
    return std_value.result;
}

I want to sh(cmd) return output of cmd. So i am using the following(it is little bit shitty). But exec is async function, so outer std_value.result will be the value from last call. Is it normal way to create sh(cmd), which wiil return stdout or I should use promises?

Duplicate of [How do I return the response from an asynchronous call?](https://stackoverflow.com/questions/14220321/how-do-i-return-the-response-from-an-asynchronous-call) — Guy Incognito, Jun 19 '20 at 09:50

score 1 · Accepted Answer · answered Jun 19 '20 at 09:36

You can make use of promise here since you can not return value from the callback/async function.

function sh(cmd) {
  return new Promise((resolve, reject) => {
    exec(cmd, (err, stdout, stderr) => {
      if (err) {
        console.log(err);
        std_value.flush();
        std_value.set("error");
        reject(err);
      } else {
        console.log({stdout, stderr});
        std_value.flush();
        std_value.set(stdout);
        console.log('std_value in ' + std_value.result);

        resolve(std_value.result);
      }
    });
  })
}

sh().then(val => {
  console.log(val); //this should console the value of std_value.result
})

Note: I don't know where did you get std_value.

score 1 · Answer 2 · answered Jun 19 '20 at 09:39

function sh(cmd, callback) {
    exec(cmd, (err, stdout, stderr) => {
      if (err) {
        console.log(err);
        std_value.flush();
        std_value.set("error");
        if(!!callback) {
          callback(err);
        }
      } else {
        console.log({stdout, stderr});
        std_value.flush();
        std_value.set(stdout);        
        console.log('std_value in ' + std_value.result);
        if(!!callback) {
          callback(std_value.result);
        }
      }
    });
}

sh("run", function(result) {
  console.log(result);
});

You can implement the same with callback like this.

How to set variable from callback function?

2 Answers2