如何过滤DynamoDB Scan结果数组，仅保留有效返回值

The core issue is that your Promise.all is collecting all resolved values—including the invalid 'x' or undefined entries. To get only the valid ['a','b'] arrays, you just need to filter the final result set and fix a common closure bug in your loop.

Step-by-Step Fixes: #

1. Fix the loop variable: Use let instead of var for index in your for loop. This ensures each iteration has its own copy of the index (avoids a bug where all scan operations end up using the final index value).
2. Filter invalid results: After Promise.all resolves, filter out any values that aren't valid arrays matching your desired format.

Modified Code: #

var results = [];
var array = ["a", "b"];

// Use let for index to fix closure issue
for (let index = 1; index <= 5; ++index) {
    var params1 = {
        TableName: "MyCatTable",
        FilterExpression: "#rule=:rule",
        ExpressionAttributeNames: { "#rule": "rule" },
        ExpressionAttributeValues: { ":rule": String(index) }
    };

    results.push(
        dynamodb.scan(params1).promise()
            .then(function (data) {
                if (data.Items.length == 2) {
                    var uw = [];
                    console.log("Entered");
                    for (var i = 0; i < 2; i++) {
                        if (array.indexOf(data.Items[i].category) >= 0) {
                            console.log("pushing");
                            uw.push(data.Items[i].category);
                        }
                    }
                    // Return valid array only if it has both elements
                    return uw.length === 2 ? uw : null;
                } else {
                    // Return null for invalid cases
                    return null;
                }
            })
    );
}

Promise.all(results)
    .then((data) => {
        // Filter out null/undefined and keep only valid arrays
        const validResults = data.filter(result => Array.isArray(result) && result.length === 2);
        console.log("----------------------");
        console.log(validResults); // Output: [ ['a','b'], ['a','b'] ]
    })
    .catch(err => {
        console.log(err);
    });

Explanation: #

• Loop Variable Fix: Using let in the for loop creates a new binding for index in each iteration, so each scan operation uses the correct value of index when it runs.
• Filtering: The filter method checks if each result is an array with exactly 2 elements (matching your valid ['a','b'] format). This removes any null, 'x', or undefined entries from the final output.

If you prefer to keep returning 'x' instead of null, adjust the filter to:

const validResults = data.filter(result => result !== 'x' && Array.isArray(result));

内容的提问来源于stack exchange，提问作者user3872094