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In this function, I am trying to run 2 different ajax requests (3 options, only 2 will run depending on if statements). addDep(), and addCust() functions both have an ajax request in them and get sent an url. However, I need that ajax function to run and complete before the code in the .then function being called after it runs. Right now the code runs but has null values because it hasn't got data1 or data2 back from the ajax function. How do I fix this?

function builder(){
    var beforeDate = dateConv(document.getElementById('Before').value);
    var afterDate = dateConv(document.getElementById('After').value);

    var url = "http://localhost:8181/GRAIN/grain_map?start="
    url += afterDate;
    url += "&end=";
    url += beforeDate;
    url += "&attributes=DATE-SHIPPED+DESTINATION-NAME+DESTINATION";

    ajaxReq(url).then(function (jsonString) {
        for (i=0; i < jsonString.grain_map.length; i++){
            destinationArray.push(jsonString.grain_map[i].DESTINATION);
        }

        for (i = 0; i < destinationArray.length; i++){
            if (destinationArray[i] == ""){
                continue;
            }
            if (destinationArray[i].length == 3 || destinationArray[i].length == 2){
                var destCode = destinationArray[i];
                addDep(destCode).then(function (data1) {
                    var address = "";
                    var depArray = data1.dep_address[0].ADDRESS;
                    for(i = 0; i < depArray.length; i++){
                        address += data1.dep_address[0].ADDRESS[i] + " ";
                    }
                    addressArray.push(address);
                }).catch(function (err) {
                    console.error(err);
                });

            }else if (destinationArray[i].length == 6){
                var destCode = destinationArray[i];
                addCust(destCode).then(function (data2) {
                    var address = "";
                    var depArray = data2.customer_address[0].ADDRESS;
                    for(i = 0; i < depArray.length; i++){
                        address += data2.customer_address[0].ADDRESS[i] + " ";
                    }
                    addressArray.push(address);
                }).catch(function (err) {
                    console.error(err);
                });
            }
        }
    }).catch(function (err) {
        console.error(err);
    });
}

As it stands:

• builder() returns undefined - it should return Promise.
• the outer then's callback returns undefined - it should return Promise.
• the two inner thens' callbacks return undefined - they could (and should) return String.
• the inner Promises returned by addDep() or addCust() are not aggregated - there's no Promise.all().
• there are two mysterious, undeclared arrays, destinationArray and addressArray, resulting in some guesswork on my part.

Array methods .map(), .filter() and .join() allow the code to be simplified

function builder() {
    var beforeDate = dateConv(document.getElementById('Before').value);
    var afterDate = dateConv(document.getElementById('After').value);
    var url = `http://localhost:8181/GRAIN/grain_map?start=${afterDate}&end=${beforeDate}&attributes=DATE-SHIPPED+DESTINATION-NAME+DESTINATION`;
    return ajaxReq(url)
//  ^^^^^^
    .then(jsonString => {
        var promises = jsonString.grain_map
        .map(item => {
            var destcode = item.DESTINATION || '';
            switch(destCode.length) {
                case 2:
                case 3:
                    return addDep(destCode)
                //  ^^^^^^
                    .then(data => data.dep_address[0].ADDRESS.join(' '))
                    .catch(error => `addDep: ${destCode}: ${error.message}`); // inject message in place of missing address
                break;
                case 6:
                    return addCust(destCode)
                //  ^^^^^^
                    .then(data => data.customer_address[0].ADDRESS.join(' '))
                    .catch(error => `addCust: ${destCode}: ${error.message}`); // inject message in place of missing address
                break;
                default:
                    return null;
                //  ^^^^^^
            }
        })
        .filter(p => !!p); // filter out any nulls leaving just Promises
        return Promise.all(promises);
    });
}

Back in the caller ...

builder()
.then(addressArray => {
    // do something with addressArray.
})
.catch(err => {
    console.error(err);
    throw error; // unless this promise chain is at the topmost level of an event thread.
});

Notes:

• The question's intermediate destinationArray disappears completely - it was (probably) never necessary.
• Arrow functions are used for compactness.
• What you do in those catch handlers depends on exactly what you want. As written above, an error message is injected in place of each missing address. You could alternatively choose to re-throw the error and allow the Promise returned by Promise.all(promises) to follow its error path on first error encountered.
• addressArray is now delivered by the Promise returned by builder().
• the switch/case structure is not absolutely necessary but makes for neater code (IMHO).