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javascriptasynchronoustypescriptasync-await
JWP
JWP

Reputation: 6963

TypeScript and Promise wrapper with Async/Await. How do I tie in legacy code?

I'm trying to create a JavaScript/typescript function that wraps a promise, by returning a new Promise, adding a try catch and a call back for user to write the code. This is the concept:

function XPromise(code) {
    return new Promise((resolve, reject) => {
        try {
           resolve(code());
        } catch (exception) {
            reject(exception);
        }
    });
}

But how would I use the snippet above in something like this:

async function GetData(testClient, project, testPlan, suiteId) {
    return XPromise(code => {
        console.debug("GetData");
        testClient.getData(project, testPlan, suiteId)
            .then(data => {
                if (data.length === 0) reject(data);
                resolve(data);
            });
}

The legacy code uses the .then construct which is ideal place to put reject and resolve, but no function exists to do that.

If I do this:

function XPromise(code, resolve, reject) {
    return new Promise((resolve, reject) => {
        try {
            resolve(code());
        } catch (exception) {
            reject(exception);
        }
    });
}

async function GetData(testClient, project, testPlan, suiteId) {
    return XPromise(code => {
        console.debug("GetData");
        testClient.getData(project, testPlan, suiteId)
            .then(data => {
                if (data.length === 0) reject(data);
                resolve(data);
            });
    },
    resolve => { },
    reject => { }
}

I don't know how to get the then logic to do it's thing in the "lower" functions.

The reason I want the new Promise wrapper is that I have tons of these things to implement... I don't want to do this throughout the code. This code compiles, but requires me to write New Promises and Try Catch statements for each function I implement.

async function GetData(testClient, project, testPlan, suiteId) {
    return new Promise((resolve, reject) => {
        console.debug("GetPoints");
        try {
            testClient.getData(project, testPlan, suiteId)
                .then(data => {
                    if (data.length === 0) reject(data);
                    resolve(data);
                });
        } catch (exception) { reject(exception); }
    });
}

Upvotes: 3

Views: 3046

Answers (1)

Vignesh
Vignesh

Reputation: 506

you should not pass value params resolve,reject to new Promise(). They are given by native promise object while invoking your callback . you just have to invoke it to make that promise either resolve or reject.

new Promise((resolve,reject)=>{
  if(condition){ 
   resolve();// resolve is callback sent by promise object. you just invoke it
  }else{
   reject(); like resolve reject makes this promise to fail
  }
});

i think your last mentioned logic is correct

 async function GetData(testClient, project, testPlan, suiteId) {
    return new Promise((resolve, reject) => {
        console.debug("GetPoints");
        try {
            testClient.getData(project, testPlan, suiteId)
                .then(data => {
                    if (data.length === 0) reject(data);
                    resolve(data);
                });
        } catch (exception) { reject(exception); }
    });
}

note : async/await is not a replacement for promise . if you want to create a promise sure , you have to create it. you can use async/await to only avoid then chains and your code looks like synchronous and easy to understand. but remain they are still asynchronous.

but in your case you can use Promise.resolve() , Promise.reject() to avoid new Promise(). see below,

    async function GetData(testClient, project, testPlan, suiteId) {    
        console.debug("GetPoints");
        try {
          const data = await testClient.getData(project, testPlan, suiteId);                
          if (data.length === 0) return Promise.reject(data);
            return Promise.resolve(data);

        } catch (exception) { return Promise.reject(exception); }    
}

Upvotes: 5

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