Not using runtime assertions

TypeScript safety comes from compile-time checks when writing code. You define a type for a value and then TypeScript verifies its fair usage. This process, however, covers only 50% of the safety in general. The other 50% comes from runtime safety. For example, let's take the following function in TypeScript:

RuntimeAssertions.ts

function divMod(x: number, y: number): [number, number] {
  return [Math.floor(x / y), x % y];
}

This gets compiled in JavaScript as follows:

"use strict";
function divMod(x, y) {
    return [Math.floor(x / y), x % y];
}

Looking at the aforementioned code, you could guess that there are many potential dangers here:

• The x and y parameters might not be numbers. If they are numbers encoded as strings, it will work but not if they are not a real number value:

  > divMod("1", 2)
  (2) [0, 1]
  > divMod("a", 2)

• Here in the second case, we passed...