Using typescript transpiler for only .ts and .tsx files.

[sainthkh]

EDIT: I learned that the case below can be solved by adding downlevelIteration option. So, close this.

• Operating System: N/A
• Cypress Version: 4.4.1
• Browser Version: N/A

Is this a Feature or Bug?

Bug

Current behavior:

In cypress-io/cypress#7098, test failed because the code below generates wrong ES5 code:

[...Array(100).keys()].map(x => x + "");

Array(100).keys().slice().map(function (x) { return x + ""; });

slice() doesn't exist in iterator. It's a failure. It happened because Cypress 4.4.1 transpiled the code with typescript.

It was a famous bug in microsoft/TypeScript#8856, and fixed in TypeScript 3.6.x. Unfortunately, it's still not working in typescript 3.8.3 (current).

I was naive. I thought everything that works fine in babel would work fine in typescript. They're different programs. It's a nonsense.

Desired behavior:

Currently, when typescript exists in the user's project folder, Cypress uses it to transpile every test file.

To solve this problem, we need to use typescript for .ts and .tsx files and babel for .js, .jsx.

I was worrying about the breaking change. But we can avoid it by adding typescriptOnly option. If it's true, every file will be transpiled with typescript when typescript exists. If not, only ts, tsx files will be transpiled with typescript.

If you agree with this idea, I'll go implement this.