constructor(
    key? = '',
        // assigned to this.key

    value?: Object | any[]  = undefined
)
    // If `key` is not an empty string and `value` is defined, the item 
    // is stored immediately.

key: string // the unique ID for the stored object or array.
    
className: string // read-only

set(value: Object | any[]): void
    // Saves item `value` in storage.  Replaces previous value, if any.

get(): Object | any[]
    // Returns the stored object or array.

getAsJSON(): string
    // Returns stored object or array as JSON.

modify(changes: Object | any[]): void
    // `changes` does not replace the current value.  It is merged into the current value.

remove(): void
    // After calling this, both the key and value are no longer in
    // storage.  You can store the item again by calling this.set(value)

protected   _createGetterAndOrSetterForEach(
		propertyNames: string[],
		configuration: IGetterSetterConfiguration
	   ) : void
    /*********************
    Use this method when you have a bunch of properties that need getter and/or 
    setter functions that all do the same thing. You pass in an array of string 
    names of those properties, and the method attaches the same getter and/or 
    setter function to each property.
    IGetterSetterConfiguration is this object:
    {
        get_setterFunction?: (
             propertyName: string, index?: number, propertyNames?: string[]
        ) => Function,
	    // get_setterFunction takes the property name as first argument and 
	    // returns the setter function.  The setter function must take one 
	    // parameter and return void.
	    
        get_getterFunction?: (
             propertyName: string, index?: number, propertyNames?: string[]
        ) => Function
	    // get_getterFunction takes the property name as first argument and 
	    // returns the getter function.  The getter function must return something.
    }
    *********************/ 
	   
	   
protected   _returnThis_after(voidExpression: any) : this
    // voidExpression is executed, then function returns this.
    // Even if voidExpression returns something, the returned data isn't used.


protected   _errorIfPropertyHasNoValue(
                property: string, // can contain dot-notation, i.e., 'property.subproperty'
                propertyNameInError? = ''
            ) : void
    // If value of this[property] is undefined or null, it triggers fatal error:
    // `The property "${propertyNameInError}" has no value.`

// It might be a good idea to name each class instance after its key.
// After instantiation, you wouldn't modify its `key` property.

let user1 = new ObjectInSessionStorage(
    'user1',
    {username: 'papasmurf', password: 'i_love_smurfette'}
);

let user2 = new ObjectInSessionStorage(
    'user2',
    {username: 'smurfette', password: 'i_love_papa'}
);

// Or, you could create a singleton instance to handle all stored objects,
// and change its `key` when you want to change what specific object to handle.

let objInSessionStorage = new ObjectInSessionStorage();
objInSessionStorage.key = 'user1';
objInSessionStorage.set({username: 'papasmurf', password: 'i_love_smurfette'});

objInSessionStorage.key = 'user2';
objInSessionStorage.set({username: 'smurfette', password: 'i_love_papa'});