Imports must be top level

That is, they must be in the main sequence of instructions that is performed when a module is executed. There is no way to conditionally import something.

if (1 < 2) {
    import { readFile } from 'fs'
}

This also applies to functions and loops.

function doSomething() {
    import path from 'path'
    console.log(path.join('foo', 'bar'))
}

If you need to load something dynamically, use the import function, which returns a Promise.

Why?

The TypeScript compiler, along with bundlers like Webpack or ESBuild, rely on static analysis - the process of analyzing a program's code to determine dependencies and optimizations.