Debugging TypeScript Programs

Debugging observes actual runtime values and control flow to find why behavior differs from expectations. Breakpoints, step-in/step-over actions, watch expressions, and source maps help navigate emitted JavaScript back to the TypeScript source. Logging is useful, but avoid logging secrets and personal data.

A breakpoint after reduce can reveal total=18 and values.length=3, explaining the output 6. The empty-input guard avoids a division by zero scenario.

TypeScript static checking does not discover every logical error, such as choosing the wrong denominator.

Save as debugging-programs.ts. The following program passed strict TypeScript compilation, and the displayed output was captured by executing its emitted JavaScript with Node.js 22.16.0.

TypeScript source

function average(values: number[]): number {
  const total = values.reduce((a, b) => a + b, 0);
  return values.length === 0 ? 0 : total / values.length;
}
console.log(average([3, 6, 9]));

Verified output

6

Compile: npx tsc --strict --target es2022 --module commonjs debugging-programs.ts; run: node debugging-programs.js. For Node.js API examples, install @types/node first. For browser projects, use the browser-specific instructions below.

The example demonstrates the code-level behavior; the related configuration or test commands are shown separately when needed.

A breakpoint after reduce can reveal total=18 and values.length=3, explaining the output 6. The empty-input guard avoids a division by zero scenario.

Predict what changes if you alter one input. Then run the code to compare your reasoning with the real output.

TypeScript static checking does not discover every logical error, such as choosing the wrong denominator.

Identify whether the failure is a type-checking issue, a configuration issue, or a runtime behavior issue before choosing a fix.

Debug a deliberately wrong average function that divides by values.length + 1. Watch the values at the return line.

Write down your expected result first. For configuration examples, compare compiler output and diagnostics; for test examples, make the assertion fail deliberately to verify that the tool reports it.

Debugging observes actual runtime values and control flow to find why behavior differs from expectations.

Run the primary example, then complete the practice task or environment-specific lab.