Is it possible to use Date as a key in a Typescript Map?

Question

Is it possible to use Date as a key in a Typescript Map?

Within my application, I have a requirement for mapping objects according to specific dates. Given that typescript provides both the Map and Date objects, I initially assumed this task would be straightforward.

  let map: Map<Date, MyObject> = new Map<Date, MyObject>();

I thought I could simply utilize the set method to insert new key-value pairs. However, upon implementation, I discovered that retrieving values using date keys required the exact same instance of Date.

To verify this behavior, I wrote a unit test:

  it('Experiencing an Issue', () => {
      let d1: Date = new Date(2019, 11, 25);    // Christmas day
      let d2: Date = new Date(2019, 11, 25);    // The same Christmas day?

      let m: Map<Date, string> = new Map<Date, string>();
      m.set(d1, "X");

      expect(m.get(d1)).toBe("X");   // <- This works fine.
      expect(m.get(d2)).toBe("X");   // <- This test fails.
  });

This raises the question, why is it necessary to use the precise same instance in order to retrieve a value from the map?

angular typescript date dictionary

Answer 1

Answer №1

The reason it will always be falsy is because each of the date objects being compared are unique mutable javascript objects.

For example,

new Date(2020, 5, 10) === new Date(2020, 5, 10) ==> false

.

If you want to learn more about this topic, check out this helpful post.

Answer 2

The reason it will always be falsy is because each of the date objects being compared are unique mutable javascript objects.

For example,

new Date(2020, 5, 10) === new Date(2020, 5, 10) ==> false

.

If you want to learn more about this topic, check out this helpful post.

Answer 3

Answer №2

In the realm of programming, the core logic of a Map revolves around storing values in key-value pairs.

When it comes to comparing keys, they must always be of the same reference. In many programming languages, string literal references are considered equal, making strings a preferred choice as keys in maps.

This behavior extends beyond just date objects and applies to any other mutable object type.

For example:

let str1 = 'test';
let str2 = 'test';
str1 == str2; // true

let str1 = new String('test');
let str2 = new String('test');
str1 == str2; // false

When retrieving a value from a map, the actual data of the keys is not important. What matters is the unique identity of the key being searched for. Immutable objects may contain the same data, but if their references are different, they will be treated as separate keys.

To ensure consistency throughout your program, opt for types that maintain the same references, like string literals.

Consider one more example:

class Demo {
  field: string;
  constructor(f: string) {
    this.field = f;
  }
}

const d1 = new Demo('test');
const d2 = new Demo('test');

// Both objects appear identical in terms of data, but their references differ.
// Therefore, they will be treated as distinct keys.
const m: Map<any, string> = new Map<any, string>();
m.set(d1, 'value1');

console.log(m.get(d1)); // value1
console.log(m.get(d2)); // undefined

Answer 4

In the realm of programming, the core logic of a Map revolves around storing values in key-value pairs.

When it comes to comparing keys, they must always be of the same reference. In many programming languages, string literal references are considered equal, making strings a preferred choice as keys in maps.

This behavior extends beyond just date objects and applies to any other mutable object type.

For example:

let str1 = 'test';
let str2 = 'test';
str1 == str2; // true

let str1 = new String('test');
let str2 = new String('test');
str1 == str2; // false

When retrieving a value from a map, the actual data of the keys is not important. What matters is the unique identity of the key being searched for. Immutable objects may contain the same data, but if their references are different, they will be treated as separate keys.

To ensure consistency throughout your program, opt for types that maintain the same references, like string literals.

Consider one more example:

class Demo {
  field: string;
  constructor(f: string) {
    this.field = f;
  }
}

const d1 = new Demo('test');
const d2 = new Demo('test');

// Both objects appear identical in terms of data, but their references differ.
// Therefore, they will be treated as distinct keys.
const m: Map<any, string> = new Map<any, string>();
m.set(d1, 'value1');

console.log(m.get(d1)); // value1
console.log(m.get(d2)); // undefined

Answer 5

Answer №3

It's recommended to use primitive values like numbers or strings as keys in a Map:

let myMap: Map<string, string> = new Map<string, string>();

let date1: Date = new Date(2019, 11, 25);    // Christmas day
let date2: Date = new Date(2019, 11, 25);    // Same Christmas day?

myMap.set(date1.toDateString(), "X");

console.log(date1.toDateString())
console.log(date2.toDateString())
console.log(myMap.get(date2.toDateString()))

You can find more information about this behavior in the comments above.

Answer 6

It's recommended to use primitive values like numbers or strings as keys in a Map:

let myMap: Map<string, string> = new Map<string, string>();

let date1: Date = new Date(2019, 11, 25);    // Christmas day
let date2: Date = new Date(2019, 11, 25);    // Same Christmas day?

myMap.set(date1.toDateString(), "X");

console.log(date1.toDateString())
console.log(date2.toDateString())
console.log(myMap.get(date2.toDateString()))

You can find more information about this behavior in the comments above.

Is it possible to use Date as a key in a Typescript Map?

Answer №1

Answer №2

Answer №3

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