[arrays] TypeScript Objects as Dictionary types as in C#

I have some JavaScript code that uses objects as dictionaries; for example a 'person' object will hold a some personal details keyed off the email address.

var people = {<email> : <'some personal data'>};

adding   > "people[<email>] = <data>;" 
getting  > "var data = people[<email>];" 
deleting > "delete people[<email>];"

Is it possible to describe this in Typescript? or do I have to use an Array?

You can use templated interfaces like this:

interface Map<T> {
    [K: string]: T;
}

let dict: Map<number> = {};
dict["one"] = 1;

Lodash has a simple Dictionary implementation and has good TypeScript support

Install Lodash:

npm install lodash @types/lodash --save

Import and usage:

import { Dictionary } from "lodash";
let properties : Dictionary<string> = {
    "key": "value"        
}
console.log(properties["key"])

You can also use the Record type in typescript :

export interface nameInterface { 
    propName : Record<string, otherComplexInterface> 
}

In addition to using an map-like object, there has been an actual Map object for some time now, which is available in TypeScript when compiling to ES6, or when using a polyfill with the ES6 type-definitions:

let people = new Map<string, Person>();

It supports the same functionality as Object, and more, with a slightly different syntax:

// Adding an item (a key-value pair):
people.set("John", { firstName: "John", lastName: "Doe" });

// Checking for the presence of a key:
people.has("John"); // true

// Retrieving a value by a key:
people.get("John").lastName; // "Doe"

// Deleting an item by a key:
people.delete("John");

This alone has several advantages over using a map-like object, such as:

• Support for non-string based keys, e.g. numbers or objects, neither of which are supported by Object (no, Object does not support numbers, it converts them to strings)
• Less room for errors when not using --noImplicitAny, as a Map always has a key type and a value type, whereas an object might not have an index-signature
• The functionality of adding/removing items (key-value pairs) is optimized for the task, unlike creating properties on an Object

Additionally, a Map object provides a more powerful and elegant API for common tasks, most of which are not available through simple Objects without hacking together helper functions (although some of these require a full ES6 iterator/iterable polyfill for ES5 targets or below):

// Iterate over Map entries:
people.forEach((person, key) => ...);

// Clear the Map:
people.clear();

// Get Map size:
people.size;

// Extract keys into array (in insertion order):
let keys = Array.from(people.keys());

// Extract values into array (in insertion order):
let values = Array.from(people.values());

You can use Record for this:

https://www.typescriptlang.org/docs/handbook/utility-types.html#recordkt

Example (A mapping between AppointmentStatus enum and some meta data):

  const iconMapping: Record<AppointmentStatus, Icon> = {
    [AppointmentStatus.Failed]: { Name: 'calendar times', Color: 'red' },
    [AppointmentStatus.Canceled]: { Name: 'calendar times outline', Color: 'red' },
    [AppointmentStatus.Confirmed]: { Name: 'calendar check outline', Color: 'green' },
    [AppointmentStatus.Requested]: { Name: 'calendar alternate outline', Color: 'orange' },
    [AppointmentStatus.None]: { Name: 'calendar outline', Color: 'blue' }
  }

Now with interface as value:

interface Icon { Name: string Color: string }

Usage:

const icon: SemanticIcon = iconMapping[appointment.Status]