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JavaScript · 04 Branches and Early Returns

Series contents · Technology map

Stage: Values and calculations · Editable example · Storage ID: javascript-step-04

Goals and learning outcomes

Prerequisites

The work and experiment records from Unit 3, “Equality Comparisons and Defaults”

Key concepts and explanations

Handling Invalid Input

An early return handles invalid input at the start of a function. It reduces nesting in the normal path while keeping return values in a consistent shape.

Missing Branches

The default branch of a switch handles unexpected values. Treating an unknown value as a success hides the cause.

Practice setup

First check the execution scope guidance on the example screen. The following environment is for extending the assignment in a separate project. Implement it yourself in a separate local project and check it with browser developer tools. In a separate folder, connect your practice JS from index.html using script type="module" and open it through a local HTTP server. Use the Console, Network, and Elements panels.

The linked example is a starting point. It does not implement every task below. Complete work beyond its supported scope in a separate project.

Practice tasks

  1. In status.js, prepare the cases idle, running, done, null, and paused.
  2. Convert each status into a display message and return ok and message on every path.
  3. Run with idle, running, done, null, and paused. Results to check: Record outputs for 3 normal cases and 2 error cases. Every returned object has an ok field. Record expected and actual results separately.

Variables to observe

VariableBaselineChangeExpected change / hypothesis
StatusrunningpausedHypothesis before running: There will be an explicit error result. Keep the other conditions fixed and record the actual result separately after running.

Predictions in this table are not measured results. Change one condition at a time and record why the actual result differs.

Completion criteria

Self-check

  1. What does returning success from default hide?
  2. What code is skipped after an early return?

Official references

Linked example and execution scope

Example source · Runtime environments and limits

Compare the editable inputs with actual browser execution results.

Source code and records are not overwritten automatically. Resetting an existing example also preserves your notes.

Next learning

Previous: Equality Comparisons and Defaults · Next: Functions and Pure Calculations

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