Your Test Suite Is Lying to You. 90% Coverage and Zero Confidence.

We had 92% code coverage. We had 1,400 tests. They all passed. We deployed on a Thursday. By Friday morning, 3 critical bugs were in production: a payment double-charge, a data leak between users, and a crash when users had emoji in their names. None of our 1,400 tests caught any of them. 92% coverage. Zero confidence. Here's why, and how I rebuilt our testing philosophy. Why Coverage Lies Coverage measures "lines of code that were executed during tests." It does not measure "lines of code that were verified to be correct." // This function has a critical bug: function calculateDiscount ( price : number , userTier : string ): number { if ( userTier === ' premium ' ) return price * 0.8 ; if ( userTier === ' basic ' ) return price * 0.95 ; return price ; // Bug: no validation. Negative prices? NaN? Infinity? } // This test gives 100% coverage: test ( ' calculates premium discount ' , () => { const result = calculateDiscount ( 100 , ' premium ' ); expect ( result ). toBe ( 80 ); // ✅ Line