TypeScript

TypeScript is JavaScript with a type system on top, checked before the code runs rather than discovered when it fails in front of a user. It has become the default in commercial work, which makes it a filter you have to clear more often than an advantage you can lead with.

What the work consists of

Describing the shape of your data accurately, then letting the compiler hold you to it. The payoff arrives on the day you change one function and the editor immediately shows you the eleven places that now disagree — a class of bug that, in plain JavaScript, reaches production and gets found by a customer.

What employers screen for

Whether your types describe the problem or merely silence the compiler. A codebase scattered with `any` carries all of TypeScript’s cost and none of its benefit, and a reviewer sees that in the first file they open. The question behind the question is whether you reach for the escape hatch when modelling gets difficult.

• Modelling a state that cannot be invalid, rather than checking for invalid states everywhere downstream
• Knowing when a type is genuinely hard to express and when you are overcomplicating it
• Reading a long compiler error without flinching, which is a learnable skill in itself

The honest limit

It is one to two months on top of working JavaScript, not a parallel subject. Learning both at once generally produces someone who cannot write either without the editor telling them what comes next, and that dependency is visible the moment the editor is taken away. Get comfortable in JavaScript first; the types will take a fortnight after that.

It also does nothing for correctness beyond shape. A function can be perfectly typed and still calculate the wrong answer, which is why typed codebases still need tests.