Lets say I have an array of key value objects:

const data = [
    {key: "object1", value: "data1"},
    {key: "object2", value: "data2"},
    {key: "object3", value: "data3"},
]

const mappedData = data.map(x => [x.key, x.value]);

const ES6Map = new Map<string, string>(mappedData.values())

How do I convert it to ES 6 map? It works in JavaScript but TypeScript will complain. Here I got the error of Argument of type 'IterableIterator<string[]>' is not assignable to parameter of type 'ReadonlyArray<[string, string]>'. Property 'length' is missing in type 'IterableIterator<string[]>'.

Lets say I have an array of key value objects:

const data = [
    {key: "object1", value: "data1"},
    {key: "object2", value: "data2"},
    {key: "object3", value: "data3"},
]

const mappedData = data.map(x => [x.key, x.value]);

const ES6Map = new Map<string, string>(mappedData.values())

How do I convert it to ES 6 map? It works in JavaScript but TypeScript will complain. Here I got the error of Argument of type 'IterableIterator<string[]>' is not assignable to parameter of type 'ReadonlyArray<[string, string]>'. Property 'length' is missing in type 'IterableIterator<string[]>'.

• javascript
• typescript
• dictionary
• ecmascript-6

• 1 Just remove the .values() call – dotconnor Commented Nov 27, 2018 at 4:07
• then it gets Argument of type 'string[][]' is not assignable to parameter of type 'ReadonlyArray<[string, string]>'. Type 'string[]' is not assignable to type '[string, string]'. Property '0' is missing in type 'string[]'. – Cerlancism Commented Nov 27, 2018 at 4:08
• The problem is in your type declarations. I don't know typescript, but if type declaration is optional, why not just do const ES6Map = new Map(mappedData)? – ba_ul Commented Nov 27, 2018 at 4:13

1 Answer 1

You need to do type assertion and tell typescript that your mappedData is of type Array<[string,string]> instead of string[][] which is a sub type for Array<[any,any]> as needed by Map constructor.

Do

const mappedData = data.map(x => [x.key, x.value] as [string, string]);

instead of

const mappedData = data.map(x => [x.key, x.value]);

and also

drop the values() call as pointed out in comments.