Quick Answer

Calling new on non-constructable values

Understanding the Issue

TypeScript prevents using new with values that aren't constructors. This error occurs when the target lacks a construct signature.

The Problem

This code demonstrates the issue:

Typescript Error 
// Problem: Invalid constructor
interface Foo { bar: string; }
const foo: Foo = { bar: "test" };
new foo(); // Error

The Solution

Here's the corrected code:

Typescript Fixed 
// Solution 1: Use actual constructor
class Bar {
    constructor() { ... }
}
new Bar();

// Solution 2: Check for construct signature
interface Constructable {
    new (): any;
}
function createInstance(c: Constructable) {
    return new c();
}

Key Takeaways

Only use new with values that are known to be constructors.