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JavaScript · 05 Functions and Pure Calculations

Series contents · Technology map

Stage: Values and calculations · Editable example · Storage ID: javascript-step-05

Goals and learning outcomes

Prerequisites

The work and experiment records from Unit 4, “Branches and Early Returns”

Key concepts and explanations

Pure Calculations

A pure calculation returns the same result for the same input and does not change external state. A separate function can handle DOM display.

Boundary Values

When the total count is 0, ordinary division cannot determine a ratio. Document the display policy for an empty list in the function contract.

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 score.js, prepare the cases 0/0, 1/4, and 4/4.
  2. Separate the completion ratio calculation from renderScore, and use console.assert to test only the calculation.
  3. Run with 0/0, 1/4, and 4/4. Results to check: Neither NaN nor Infinity is displayed for the three inputs. Record 3 calculation results obtained without the DOM. Record expected and actual results separately.

Variables to observe

VariableBaselineChangeExpected change / hypothesis
Total count40Hypothesis before running: The chosen empty-list policy will prevent NaN from being displayed. 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. How should 0 out of 0 be displayed?
  2. What becomes harder when calculation and display are combined?

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: Branches and Early Returns · Next: Scope and Closures

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