How to create Map<String,List<String>> in Javascript?

Question

How to create Map<String,List<String>> in Javascript? Push method does not seem to work if we define

aMap = {} and 
if(aMap [key])
{                                      
    aMap[key].push(value);//this is not working for some reason
}

Answer 1

You have to create the key as an array, first.

// Create an empty Object
var aMap = {};
function addValueToKey(key, value) {
    // Shorcut || returns left side if it is "truthy," or the right otherwise.
    // This means that we only assign a new Array to the Object's property
    // if it has not previously been used.
    aMap[key] = aMap[key] || [];
    // Adds a value to the end of the Array
    aMap[key].push(value);
}

---

Once browsers support the new logical nullish assignment, we can write the code more gracefully (this will apply to LWC only):

var aMap = {};
function addValueToKey(key, value) {
  aMap[key] ??= [];
  aMap[key].push(value);
}

Where ??= reads "Assign the value on the right to the left, only if the left is null/undefined", and will become the new standard for someVar[key] = someVar[key] || newValue.