Instrumenting with nyc produces invalid Typescript

Link to bug demonstration repository

https://github.com/jedateach/nyc-typescript/runs/515835523

Expected Behavior

Instrument command produces valid typescript that has no errors

Observed Behavior

When instrumenting this file: https://github.com/jedateach/nyc-typescript/blob/master/src/constants.ts

We get this output: https://github.com/jedateach/nyc-typescript/blob/master/instrumented/constants.ts#L49

Then trying to run a typescript compiler using tsc instrumented/index.ts, gives:

instrumented/comparitor.ts:98:3 - error TS2539: Cannot assign to 'cov_1qahgqgynf' because it is not a variable.

98   cov_1qahgqgynf = function () {
     ~~~~~~~~~~~~~~

instrumented/constants.ts:49:3 - error TS2539: Cannot assign to 'cov_1im0zfgwer' because it is not a variable.

49   cov_1im0zfgwer = function () {

Troubleshooting steps

• still occurring when I put cache: false in my nyc config

Environment Information

npx: installed 1 in 1.263s

  System:
    OS: macOS 10.15.3
    CPU: (16) x64 Intel(R) Core(TM) i9-9980HK CPU @ 2.40GHz
    Memory: 1.26 GB / 32.00 GB
  Binaries:
    Node: 8.16.2 - ~/.nvm/versions/node/v8.16.2/bin/node
    Yarn: 1.3.2 - /usr/local/bin/yarn
    npm: 6.4.1 - ~/.nvm/versions/node/v8.16.2/bin/npm
  npmPackages:
    nyc: ^15.0.0 => 15.0.0 
    source-map-support: ^0.5.16 => 0.5.16 
    ts-node: ^8.6.2 => 8.6.2 
    typescript: ^3.8.3 => 3.8.3 

[coreyfarrell]

@codefoster you are using ts-node/register/transpile-only, I’m guessing that ts-node/register would experience this issue until the ts-ignore I’m added gets released.

[coreyfarrell]

https://github.com/istanbuljs/istanbuljs/blob/master/packages/istanbul-lib-instrument/src/visitor.js#L550 is the line which generates code that offends TypeScript. The generated code is (clipped for briefness):

function cov_XXXXXX() {
 // Clipped initialization of actualCoverage
 cov_XXXXXX = function () {
   return actualCoverage;
 }
 return actualCoverage;
}

So TypeScript does not allow cov_XXXXXX to be re-assigned, JavaScript does. If TypeScript has a configuration option to allow this I don’t think it’s appropriate for istanbuljs to require that users set it when we can just inject a ts-ignore hint into the generated code.