When transpiling TS to JS compatible with NodeJS v14, using the following config:
{
"compilerOptions": {
"lib": ["es2020"],
"rootDir": "src",
"outDir": "build",
"module": "commonjs",
"moduleResolution": "node",
"target": "es2020",
"experimentalDecorators": true,
"emitDecoratorMetadata": true,
"skipLibCheck": true,
}
}
It was expected that the usage of at(index) would be converted to a compatible JS code, but in fact, when I ran the built code it generates (...).at is not a function error.
[1,2,3].at(-1)
The transpiled code still makes use of .at(index), but I was expecting it would get transpiled to something compatible with the target set on tsconfig.js
target: "es2020"
What am I not getting correctly here?
The TypeScript compiler considers this method as syntactically correct, which is why it does not downlevel it. You can read it better explained in this great answer to a similar question.
However I did not mark your question as duplicate because there is one information that's missing: why don't you have a compiler error?!
If you go to the TS playground, you can see that there is an immediate error if you try to write:
[1, 2, 3].at(0);
Property 'at' does not exist on type 'number[]'. Do you need to change your target library? Try changing the 'lib' compiler option to 'es2022' or later.
My guess is that you don't have an error because you are using a wrong version of the package @types/node (most probably the latest one). Indeed, as indicated on node.green, Array.prototype.at() is supported since NodeJS 16.8.0. As such, the latest typings for NodeJS reflect that fact and they provide a signature for that method, which gets automatically included by TSC.
In conclusion, install the same version of NodeJS types as your NodeJS runtime and you should get the TSC error:
npm install @types/node@14