Explain Code Line by Line Prompt
Turn unfamiliar code into a line-by-line explanation of syntax, data flow, intent, side effects, edge cases, and safe ways to verify understanding.
Prompt Template
You are a patient senior developer and code tutor. Explain the supplied code accurately without pretending to know context that is missing. Code: [paste complete snippet] Language and version: [language, runtime version, or unknown] Framework or library: [name and version, or none] Where it runs: [browser, server, CLI, mobile app, database, embedded system] What I think it does: [your current understanding] My experience level: [beginner, intermediate, experienced in another language] Surrounding types, imports, schema, or caller: [paste relevant context] Inputs and example values: [values] Observed output or error: [details or none] Explanation depth: [plain English, line by line, language internals, architecture] Topics to emphasize: [syntax, control flow, async behavior, memory, types, security, performance] Constraints: [do not rewrite, keep terminology simple, compare with another language] Provide: 1. A two-sentence summary of the code's purpose, clearly labeled as confirmed or inferred. 2. A numbered line-by-line explanation that quotes only the relevant line or small block. 3. A data-flow trace using the supplied example input. 4. Definitions for unfamiliar syntax, APIs, types, and language features. 5. Side effects, mutations, I/O, exceptions, asynchronous boundaries, and state changes. 6. Assumptions and missing context that could change the explanation. 7. Edge cases, security concerns, and performance concerns actually supported by the code. 8. A compact pseudocode version. 9. Three comprehension questions with answers hidden below a divider. 10. Safe experiments or tests I can run to verify each important claim. Do not invent library behavior or surrounding application intent. If a function, type, or API is undefined, say what additional definition or documentation is needed.
Example Output
Summary
Confirmed: the function filters invalid numeric values and returns the sum of the remaining entries. Inferred: the caller probably expects an empty list to produce zero, but the surrounding contract is not shown.
Line by Line
1. `const valid = values.filter(Number.isFinite)` creates a new array containing only finite numbers; it does not mutate `values`.
2. `return valid.reduce((sum, value) => sum + value, 0)` starts the accumulator at zero and adds each valid value.
Data Flow
For `[4, NaN, 3]`, filtering produces `[4, 3]`; reducing then produces `7`.
Edge Cases
Numeric strings are removed because `Number.isFinite("4")` is false. An empty input returns zero. A non-array input throws before filtering.
Verify
Test an empty array, numeric strings, `Infinity`, and a frozen input array; confirm the result and that the input remains unchanged.
Tips for Best Results
- 💡Include imports, types, and the caller when library behavior or application intent matters.
- 💡Give one concrete input so the explanation can trace real values instead of paraphrasing syntax.
- 💡Ask the model to label confirmed facts and inferences; this makes missing context visible.
Frequently Asked Questions
What is the Explain Code Line by Line Prompt prompt?
Turn unfamiliar code into a line-by-line explanation of syntax, data flow, intent, side effects, edge cases, and safe ways to verify understanding. It's a free ChatGPT prompt template from our Coding collection — copy it, fill in the bracketed variables, and paste it into your AI tool.
Which AI tools work with this prompt?
It's written and tested for ChatGPT, Claude and Gemini. Any AI assistant that accepts free-form text prompts will handle it well.
How do I customize this ChatGPT prompt?
Replace the bracketed variables — such as [paste complete snippet], [language, runtime version, or unknown], [name and version, or none] — with your own details before running it. Include imports, types, and the caller when library behavior or application intent matters.
Is this prompt free to use?
Yes. Every prompt on PromptAtlas is free to copy, customize, and use — no signup required.
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