JavaScript Mapping for Dictionaries

Here is the definition of our EmployeeMap once more:

Slice

dictionary<int, Employee> EmployeeMap;

In the JavaScript mapping, dictionaries using a JavaScript built-in type as the key type are mapped to the JavaScript Map type. This is true for all Slice built-in types except long:

JavaScript

let em = new Map();

let e = new Employee();
e.number = 31;
e.firstName = "James";
e.lastName = "Gosling";

em.set(e.number, e);

For cases where the key type does not correspond with a JavaScript built-in type, the dictionary is mapped to HashMap. This is true for Slice dictionaries where the key type is long or a Slice structure that qualifies as a legal dictionary key:

Slice

dictionary<long, Employee> EmployeeMap;

In these cases an extra constructor is generated that initializes the HashMap with the desired comparison operator.

JavaScript

let em = new EmployeeMap();

let e = new Employee();
e.number = new Ice.Long(31);
e.firstName = "James";
e.lastName = "Gosling";

em.set(e.number, e);

HashMap supports the same API as the standard JavaScript Map object. It provides the following additional properties and functions:

HashMap(keyComparator, valueComparator)
This version of the constructor accepts optional comparator functions that the map uses to compare keys and values for equality. If you instantiate a map directly using new Ice.HashMap() without specifying comparator functions, the default comparators use the === operator to compare keys and values. As an example, the following map compares its keys and values using equals methods:
JavaScript
```
function compareEquals(a, b)
{
    return a.equals(b);
}
var m = new Ice.HashMap(compareEquals, compareEquals);
```
The valueComparator function is only used when comparing two maps for equality.

The type-specific constructor generated for a Slice dictionary supplies comparator functions appropriate for its key and value types.

equals(other, valueComparator)
Returns true if this map compares equal to the given map, false otherwise. You can optionally supply a function for valueComparator that the map uses when comparing values; this function takes precedence over the comparator supplied to the map's constructor.

clone()
Returns a shallow copy of the map.

Legal key types for HashMap include JavaScript's primitive types along with null, NaN, and any object that defines a hashCode method. The generated code for a Slice structure that qualifies as a legal dictionary key type includes a hashCode method. Suppose we define another dictionary type:

Slice

dictionary<Employee, string> EmployeeDeptMap;

The Slice compiler generates a constructor function equivalent to the following code:

JavaScript

class EmployeeDeptMap extends Ice.HashMap
{
    constructor(h)
    {
        let keyComparator = ...;
        let valueComparator = ...;
        super(h || keyComparator, valueComparator);
    }
}

Since the key is a user-defined structure type, the map requires a comparator that properly compares keys. Instantiating a map using new EmployeeDeptMap automatically sets the comparators, whereas calling new Ice.HashMap in this case would require you to supply your own comparators.

Slice dictionaries that map to a HashMap must be instantiated using the generated constructor.

Notes

Attempting to use the map[key] = value syntax to add an element to the map will not have the desired effect; you must use the set function instead.

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