Creating Multiple Promises in JavaScript: A Comprehensive Guide

Question

Creating Multiple Promises in JavaScript: A Comprehensive Guide

How can I link multiple promises together? For example:

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 suggestions on what might be causing this issue?

javascript express promise es6-promise

Answer 1

Answer №1

When connecting Promises in a chain, the return value from a function passed to `then` should be either a Promise or a value to pass on.

In your situation, since you are making an asynchronous call, you will need to return another promise and utilize `reject` or `resolve` within it. If the operation was synchronous, you could simply return the value or throw an error, which would then be passed to the next `then` or error handler accordingly.

It is important to chain the promises together as each call to `then()` results in a new Promise.

For example:

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

    // Create 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 result is false.
    if (result !== false) {
        var airportCode = result;

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

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

Check out this working version on JSFiddle: JSFiddle

Answer 2

When connecting Promises in a chain, the return value from a function passed to `then` should be either a Promise or a value to pass on.

In your situation, since you are making an asynchronous call, you will need to return another promise and utilize `reject` or `resolve` within it. If the operation was synchronous, you could simply return the value or throw an error, which would then be passed to the next `then` or error handler accordingly.

It is important to chain the promises together as each call to `then()` results in a new Promise.

For example:

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

    // Create 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 result is false.
    if (result !== false) {
        var airportCode = result;

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

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

Check out this working version on JSFiddle: JSFiddle

Creating Multiple Promises in JavaScript: A Comprehensive Guide

Answer №1

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