How to define import variable type

I believe you mean something like…

import * as foo: IFoo from "foo"

or

import foo : IFoo from "foo"

Is that accurate?

This was discussed, but ultimately decided against.

Instead, it was recommended that you declare module 'bar' and give it your appropriate typing. Once that’s done you will be able to import * as foo from "bar" with proper typing.

See this issue for more details on the recommended approach

A potential example:

untyped.d.ts

declare module "bar" {
    const foo:IFoo;
    export = foo;
}

tsconfig.json

{
    "compilerOptions": {
        ...
    },
    "include": [
        "untyped.d.ts",
        "src/**/*.ts",
        "src/**/*.tsx"
    ]
}

the name “untyped.d.ts” has no real meaning here, I just personally use it as a catchall for the untyped modules in my personal projects. Feel free to name it whatever feels right to you.

p.s. you can also choose to use the files array property rather than include for this but I tend to not bother because files doesn’t respect the exclude property which is confusing to some folks. See the docs for details.