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In a JavaScript application, a function is returning an array filled with undefined elements when using Promise.all. This issue arises before the function completes its operations. Here's the problematic function:
<code class="js">classMethods.getQueries = function(models, dbId, dateStart, dateEnd) { return new Promise(function(resolve, reject) { // Fetch database. .then(extractQueries, reject) .then(sortQueries, reject) .then(onlyTen, reject) .then(addText, reject) .then(function(queries) { console.log("getQueries finished", queries); // Array of 10× undefined! resolve(queries); }, reject); // Functions here. }); };</code>
The addText function is identified as the root cause of the problem:
<code class="js">function addText(queries) { // This line is missing the `return` keyword. Promise.all(queries.map(function(query) { models.queries.findById(query.queryId, { raw: true, attributes: [ "query" ] }) .then(function(queryFetched) { query.text = queryFetched.query; console.log(query); // Here, missing the `return` keyword in front of `Promise.resolve(query)`. return Promise.resolve(query); }, function(error) { // Missing `return Promise.reject(error);` }); })); };</code>
To fix the issue, you must remember to return Promises in Promise.all's map callback:
<code class="js">function addText(queries) { return Promise.all(queries.map(function(query) { // Add `return` here to fix the problem. return models.queries .findById(query.queryId, { raw: true, attributes: [ "query" ] }) .then(function(queryFetched) { query.text = queryFetched.query; console.log(query); // Add `return` here. return Promise.resolve(query); }, function(error) { // Add `return` here. return Promise.reject(error); }); })); };</code>
By adding the missing return statements, you now properly wrap each database call in a Promise and return the Promise.all result when all the individual Promises have resolved. This ensures that the getQueries function receives the correct result when all queries are fetched and processed.
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