JavaScript Promise-如何做出多重承诺？

Question

How can I chain the multiple promise? For instance:

var promise = new Promise(function(resolve, reject) {

    // Compose the pull url.
    var pullUrl = 'xxx';

    // Use request library.
    request(pullUrl, function (error, response, body) {
        if (!error && response.statusCode == 200) {
            // Resolve the result.
            resolve(true);
        } else {
            reject(Error(false));
        }
    });

});

promise.then(function(result) {

    // Stop here if it is false.
    if (result !== false) {
        // Compose the pull url.
        var pullUrl = 'xxx';

        // Use request library.
        request(pullUrl, function (error, response, body) {
            if (!error && response.statusCode == 200) {
                resolve(body); // <-- I want to pass the result to the next promise.
            } else {
                reject(Error(false));
            }
        });
    }

}, function(err) {
    // handle error
});

promise.then(function(result) {

    // Stop here if it is false.
    if (result !== false) {
        // handle success.
        console.log(result);
    }
}, function(err) {
    // handle error.
});

Error:

resolve(body); ReferenceError: resolve is not defined

Any ideas?

Answer 1

When chaining Promises, then return value from a function given to then should either be a Promise, or the value to pass along.

In your case, since you're making an async call, you'll just return another promise and call reject or resolve within there. If it wasn't async, you could just return the value, or throw an error, which also gets passed along to the next then or error handler/catch as appropriate.

Also, you need to chain them together, because each then() returns a different Promise.

So, something like this:

var promise = new Promise(function(resolve, reject) {

    // Compose the pull url.
    var pullUrl = 'xxx';

    // Use request library.
    request(pullUrl, function (error, response, body) {
        if (!error && response.statusCode == 200) {
            // Resolve the result.
            resolve(true);
        } else {
            reject(Error(false));
        }
    });

});

promise.then(function(result) {

    // Stop here if it is false.
    if (result !== false) {
        var airportCode = result;

        // Compose the pull url.
        var pullUrl = 'xxx';

        // Use request library.
        return new Promise(function (resolve, reject) {
            request(pullUrl, function (error, response, body) {
                if (!error && response.statusCode == 200) {
                    resolve(body);
                } else {
                    reject(Error(false));
                }
            });
        });
    }

}).then(function(result) {
    // Stop here if it is false.
    if (result !== false) {
        // handle success.
        console.log(result);
    }
}).catch(function (err) {
  // handle error
});

Here is a JSFiddle with a working version: JSFiddle