Display Conditional Alert with Javascript

Question

I have a custom button that displays an alert message when some fields on my object are missing. I would like to now also display the fields that are missing as part of that alert. I tried the code below, but it only shows the last field missing. How can I get the 1st alert along with any of the other alerts that are applicable? For example, I would want the alert to read:

Please complete the Approval Request Information section before submitting this Quote for approval. The following fields are missing:

Missing Offer Scope Missing Justification for Discounting

Javascript Button:

{!RequireScript("/soap/ajax/38.0/connection.js")} 
if('{!SBQQ__Quote__c.Offer_Scope__c }' == '' || '{! SBQQ__Quote__c.Justification_for_Discounting__c}' == '' || '{! SBQQ__Quote__c.Client_Background__c }' == '' || '{! SBQQ__Quote__c.Non_Standard_Terms__c }' == '' ) 
{ 
alert('Please complete the Approval Request Information section before submitting this Quote for approval. The following fields are missing:'); 

/* if('{!SBQQ__Quote__c.Offer_Scope__c }' == ''){ 
alert('Missing Offer Scope'); 
} 
if('{!SBQQ__Quote__c.Justification_for_Discounting__c }' == ''){ 
alert('Missing Justification for Discounting'); 
} 
if('{!SBQQ__Quote__c.Client_Background__c }' == ''){ 
alert('Missing Client Background'); 
} 
if('{!SBQQ__Quote__c.Non_Standard_Terms__c }' == ''){ 
alert('Missing Non-Standard Terms Indicator'); 
}*/ 
} 
else { 
window.top.location.href = "{!URLFOR($Action.SBQQ__Quote__c.Submit, SBQQ__Quote__c.Id )}"; 
}

Answer 1

The issue is that you are calling the alert function before you determined which fields are empty. You need to create a variable where you build up the message containing the missing fields, then at the end call alert and pass in the message variable:

{!RequireScript("/soap/ajax/38.0/connection.js")} 
if('{!SBQQ__Quote__c.Offer_Scope__c }' == '' || '{! SBQQ__Quote__c.Justification_for_Discounting__c}' == '' || '{! SBQQ__Quote__c.Client_Background__c }' == '' || '{! SBQQ__Quote__c.Non_Standard_Terms__c }' == '' ) 
{
    var message = 'Please complete the Approval Request Information section before submitting this Quote for approval. The following fields are missing: ';

    if('{!SBQQ__Quote__c.Offer_Scope__c }' == ''){ 
        message += '\nMissing Offer Scope';
    } 
    if('{!SBQQ__Quote__c.Justification_for_Discounting__c }' == ''){ 
        message += '\nMissing Justification for Discounting';
    } 
    if('{!SBQQ__Quote__c.Client_Background__c }' == ''){ 
        message += '\nMissing Client Background'; 
    } 
    if('{!SBQQ__Quote__c.Non_Standard_Terms__c }' == ''){ 
        message += '\nMissing Non-Standard Terms Indicator';
    }
    alert(message);
} 
else { 
    window.top.location.href = "{!URLFOR($Action.SBQQ__Quote__c.Submit, SBQQ__Quote__c.Id )}"; 
}