Implement Callbacks in Loopback 'Before Save' hook

Question

Good day! I am pretty new to Node.js and Loopback. The below is driving me crazy.

I want to write attribute values to my model instance on "before save". I get one value from the calling REST call:

ctx.instance.hash

Then, I need to call a REST API, get the response, and write the value into the model. Getting the value from the API call works, but I get the value from another function.

But I can't get the value back into the original function's scope, to do the following:

tx.instance.somedata = externalData;

I have tried: 1. Making this a global var, but in the original calling function, the value just remains "undef". 2. Doing a "Return" on the value

Both to no avail - the value remains "undefined"

I was thinking that the variable never gets populated, and I need to use callbacks, but I am not sure how to implement the callback function in this case.

Any pointers or assistance will be greatly appreciated, thanks!

module.exports = function(Blockanchor) {

  Blockanchor.observe('before save', function GetHash(ctx, next) {
    if (ctx.instance) {
      //console.log('ctx.instance', ctx.instance)
      var theHash = ctx.instance.hash; //NB - This variable is required for the external API call to get the relevant data

      //Run below functions to call an external API
      //Invoke function
      ExternalFunction(theHash);
      //See comment in external function, I do not know how to get that external variable here, to do the following:
      ctx.instance.somedata = externalData;

    } 
    next();
  }); //Blockanchor.observe

}//module.exports = function(Blockanchor)

Function ExternalFunction(theHash){
  //I successfully get the data from the external API call into the var "externalData"
  var externalData = 'Foo'
  //THIS IS MY PROBLEM, how do I get this value of variable  "externalData" back into the code block above where I called the external function, as I wish to add it to a field before the save occurs
 }

Answer 1

好的，根据我的研究，我需要更改我的应用程序的工作方式，因为上述内容似乎没有实际的解决方案。

Answer 2

You should implement a promise in your external function, then wait for the external API call and return the response using the resolve callback.

module.exports = function(Blockanchor) {

  Blockanchor.observe('before save', function GetHash(ctx, next) {
    if (ctx.instance) {
      //console.log('ctx.instance', ctx.instance)
      var theHash = ctx.instance.hash; //NB - This variable is required for the external API call to get the relevant data

      //Run below functions to call an external API
      //Invoke function
      ExternalFunction(theHash).then(function(externalData){
        ctx.instance.somedata = externalData;

        next();
      })
    } else {
        next();
    }
  }); //Blockanchor.observe

}//module.exports = function(Blockanchor)

function ExternalFunction(theHash){
    return new Promise(function(resolve, reject){
        var externalData = 'Foo'
        resolve(externalData)
    })
 }