ts-mocha and chai - error TS2695: Left side of comma operator is unused and has no side effects

Question

Tool/Lib Versions:

"ts-mocha": "^8.0.0",
"ts-node": "^10.3.0",
"chai": "^4.3.4",

Code:

expect(wrapper.find(MyListItem)).to.have.length(3);

Command:

ts-mocha tests/**/*.tsx  -r unitTestSetup.ts

My typescript compiler options:

{
"compilerOptions": {
  "target": "es5",
  "module": "commonjs",
  "declaration": true,
  "outDir": "./lib",
  "strict": true,
  "esModuleInterop": true,
},

Error:

tests/index.spec.tsx:41:10 - error TS2695: Left side of comma operator is unused and has no side effects.
41         (0, chai_1.expect)(wrapper.find(MyListItem)).to.have.length(3); 

This seems to be caused by if an optional dependency for ts-mocha is not installed. ( tsconfig-paths )

Is there a better way to fix this than doing a npm install --force on the ts-mocha package to force it to install tsconfig-paths? (the answer I provided)

Answer 1

I fixed this by creating a 'fix-ts-mocha' script in my package.json file:

"fix-ts-mocha": "cd node_modules && cd ts-mocha && npm install --force"

I run this once after npm install (or each time ts-mocha is updated).

Answer 2

I ran into the same issue with plain mocha, using ts-node. The reason was code infection by an ES module package. See also here .

Some people try to switch to ES modules like here but this didn't work for me as I still had mostly commonjs modules.

My solution was to get rid of the npm package using the ES module (ie having "type": "module" in package.json ).