异步/等待和承诺

Question

我在使用节点v8.1使用async / await时遇到问题。看来我的问题是我没有从异步函数返回一个promise。这导致程序流程无序运行。我想通过创建函数async该函数自动返回一个promise，但事实并非如此。

我希望下面的程序输出：

Validating xlsx file...

(text from validateParsedXlsx)

Adding users to cognito...

(text from addUsersToCognito)

Adding users to dynamodb...

(text from addUsersToDynamodb)

相反，我得到：

Validating xlsx file...

Adding users to cognito...

Adding users to dynamodb...

(text from validateParsedXlsx) 

(text from addUsersToCognito)  

(text from addUsersToDynamodb)

问题似乎非常明显，validateParsedXlsx() addUsersToCognito()和addUsersToDynamodb()没有返回承诺。我再次认为，通过使用async关键字，该函数会自动处理此问题。

感谢您的帮助。

这是我的剧本：

const xlsx = require('xlsx');
const AWS = require('aws-sdk');

AWS.config.update({region: 'us-west-2'});
const documentClient = new AWS.DynamoDB.DocumentClient({convertEmptyValues: true});

async function main(){

    if (!process.argv[2]) {
        console.log('\nAbsolute filepath missing. Pass the absolute filepath in as command line argument.\n')
        process.exit(1);
    }

    const xlsxFilePath = process.argv[2];
    let parsedXlsx = [];
    try {
        parsedXlsx = parseXlsx(xlsxFilePath);
    } catch (error) {
        if(error.code === 'ENOENT') {
            console.log(`\nThe file path: ${process.argv[2]} cannot be resolved\n`)
        } else {
            console.log(error);
        }
    }

    console.log('\n\nValidating xlsx file...\n');
    await validateParsedXlsx(parsedXlsx);
    console.log('\n\nAdding users to cognito...\n');
    await addUsersToCognito(parsedXlsx);
    console.log('\n\nAdding users to dynamodb...\n');
    await addUsersToDynamodb(parsedXlsx);

}

function parseXlsx(filePath) {

    const workbook = xlsx.readFile(filePath);
    const sheetNameList = workbook.SheetNames;

    const parsedXlsxSheets = sheetNameList.map(function (y) {
        const worksheet = workbook.Sheets[y];
        const headers = {};
        const data = [];

        for (z in worksheet) {
            if(z[0] === '!') continue;
            //parse out the column, row, and value
            const col = z.substring(0,1);
            const row = parseInt(z.substring(1));
            const value = worksheet[z].v;

            //store header names
            if(row == 1) {
                headers[col] = value;
                continue;
            }

            if(!data[row]) data[row] = {};
            data[row][headers[col]] = value;
        }
        //drop those first two rows which are empty
        data.shift();
        data.shift();
        return data;
    });

    return parsedXlsxSheets[0]
}

async function validateParsedXlsx(users) {

    let error = false;
    users.forEach(async (user, index) => {
        if (!user.email) {
            console.log(`User at row ${index + 2} doesn't have 'email' entry in xlsx file.`);
            error = true;
        }
        if (!user.displayName) {
            console.log(`User at row ${index + 2} doesn't have 'displayName' entry in xlsx file.`);
            error = true;  
        }
        if (!user.serviceProviderId) {
            console.log(`Userat row ${index + 2} doesn't have 'displayName' entry in xlsx file.`);
            error = true;
        } else {
            const params = {
                TableName: 'service-providers',
                Key: {
                    serviceProviderId: user.serviceProviderId
                }
            }

            const response = await documentClient.get(params).promise();
            if (!response.Item) {
                console.log(`User at row ${index +2} does not have a valid serviceProviderId.`);
                error = true;
            } else {
                console.log(`User ${user.email} is valid, assigned to service provider: ${response.Item.displayName}`);
            }
        }

        if (error) {
            console.log(`Every user in xlsx file must have these attributes, spelled correctly: email, displayName, and serviceProviderId\n\nIn addition, make sure the serviceProviderId is correct by checking the service-providers dynanomdb table.`);
            process.exit(1);
        }
    });

}

async function addUsersToCognito(users) {

    const cognitoIdentityServiceProvider = new AWS.CognitoIdentityServiceProvider();

    const results = await cognitoIdentityServiceProvider.listUserPools({MaxResults: 10}).promise();

    let serviceProviderUserPoolId = '';

    results.UserPools.forEach((userPool) => {
        if(userPool.Name === 'service-provider-users') {
            serviceProviderUserPoolId = userPool.Id;
        }
    });

    users.forEach(async (user) => {

        const params = {
            UserPoolId: serviceProviderUserPoolId,
            Username: user.email,
            DesiredDeliveryMediums: ['EMAIL'],
            TemporaryPassword: 'New_User1',
            UserAttributes: [
                {
                    Name: 'email',
                    Value: user.email
                },
                {
                    Name: 'custom:service_provider_id',
                    Value: user.serviceProviderId
                }
            ]
        }

        try {
            await cognitoIdentityServiceProvider.adminCreateUser(params).promise();
            console.log(`Added user ${user.email} to cognito user pool`);
        } catch (error) {
            if (error.code === 'UsernameExistsException') {
                console.log(`Username: ${user.email} already exists. No action taken.`);
            }
            else {
                console.log(error);
            }
        }
    });

}

async function addUsersToDynamodb(users) {

    users.forEach(async (user) => {
        const params = {
            TableName: 'service-provider-users',
            Item: {
                serviceProviderId: user.serviceProviderId,
                userId: user.email,
                displayName: user.displayName,
                isActive: false,
                role: 'BASIC'
            },
            ConditionExpression: 'attribute_not_exists(userId)'
        }

        try {
            await documentClient.put(params).promise();
            console.log(`Added user ${user.email} to dynamodb user table`);
        } catch (error) {
            if (error.code === 'ConditionalCheckFailedException') {
                console.log(`User ${user.email} already in the dynamodb table service-provider-users`);
            } else {
                console.log(error);
            }
        }
    });

}

main();

Answer 1

  users.forEach(async (user, index) => {

这开始了一些有希望的行动，但从未等待他们。可以这样做：

  await Promise.all(users.map(async (user, index) => {

...并行执行它们或执行此操作：

  await users.reduce((chain, user, index) => async (user, index) => {
   await chain;
   //...
  }, Promise.resolve());

一个接一个地执行它们。

PS：使用process.exit应该是结束程序的最后一个选项