Invert a Map object

user_1097618 0
Viewed 8971

I was wondering, what is the most convenient way to invert keys and values in a Map. Is there any builtin method or should it be done by iterating over keys and values?

const map: Map<string, number> = new Map()

const inverse: Map<number, string>
typescript javascript
3 Answers

You could pass the inverse tuples to the constructor, using Array.from and Array#reverse:

new Map(Array.from(origMap, a => a.reverse()))

See it run on an example:

const origMap = new Map([[1, "a"],[2, "b"]]);
console.log(...origMap);

// Reverse:
const inv = new Map(Array.from(origMap, a => a.reverse()));
console.log(...inv);

user_54598390

Given the assumption that there are no duplicate values, you can do an inverse like this:

const map: Map<string, number> = new Map();
map.set('3', 3);
map.set('4', 4);

const inverse: Map<number, string> = new Map();
map.forEach((value, key) => inverse.set(value, key));
user_37948120

You can use the spread operator on your original map to obtain key-value array pairs which you can .map to swap each pair. Lastly, dump the result into the Map constructor for your copy. Any duplicate keys will be lost.

const orig = new Map([["a",1],["b",2]]);
const cpy = new Map([...orig].map(e => e.reverse()));

console.log([...orig]);
console.log([...cpy]);

By the way, if your original values are indeed numbers and aren't sparse, consider using a plain old array for your "flipped" map. This might improve performance and semantics, entirely dependent on your use case.

user_62433520
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