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Maps

TypeScript (obviously) also has a built-in for maps, which are collections of key-value pairs. You can specify the types of the keys and values using type parameters <K, V>.

// A Map with string keys and number values
const podracerSpeeds = new Map<string, number>();

podracerSpeeds.set("Anakin Skywalker", 947);
podracerSpeeds.set("Sebulba", 941);

podracerSpeeds.set("R2-D2", true);
// Error: Argument of type 'boolean' is not assignable to parameter of type 'number'

podracerSpeeds.set(420, 69);
// Error: Argument of type 'number' is not assignable to parameter of type 'string'

A map is a "set-like" object, and as such uses the size property instead of length.

console.log(podracerSpeeds.size);
// 2

How to easily iterate over a map:

for (const [racer, speed] of podracerSpeeds) {
  console.log(`${racer} raced at ${speed} speed`);
}
// Anakin Skywalker raced at 947 speed
// Sebulba raced at 941 speed

Here's the most important methods of a map, get, delete, and has.

console.log(podracerSpeeds.get("Sebulba"));
// 941

console.log(podracerSpeeds.has("Sebulba"));
// true

podracerSpeeds.delete("Sebulba");
console.log(podracerSpeeds.get("Sebulba"));
// undefined

Assignment

Let's add file sharing to Support.ai's internal email system.

Complete the getFileLength function. It takes:

A Map<string, string> that represents filenames -> fileContents
A specific filename to get the length of

It returns the number of bytes in the file's contents encoded as UTF-8. If the filename is not present in the map, return 0.

The TextEncoder class and its encode method can encode the file contents (a string) into a UTF-8 Uint8Array. The array's length is the byte count.