Playwright Interview Questions and Answers (My Personal Experience)

Interview Series — JavaScript Fundamentals for Test Automation

Q1. What is the difference between slice() and substring() in JavaScript?

Both slice() and substring() are used to extract part of a string. At first, they look like they work the same — but they behave differently when negative values or reversed start–end values are involved.

Let’s use one example string:

let str = "Playwright";

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Basic Extraction

console.log(str.slice(0, 4));     // "Play"
console.log(str.substring(0, 4)); // "Play"

In normal cases, both give the same output.

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Key Difference 1: Handling Negative Indices

console.log(str.slice(-5));       // "right"
console.log(str.substring(-5));   // "Playwright"

Behavior	Result
slice(-5) → Counts from end	✅ Returns "right"
substring(-5) → Converts -5 to 0	❌ Returns entire string

So:

• slice() understands negative indexing
• substring() ignores negative indexing

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Key Difference 2: If Start > End

console.log(str.slice(5, 2));      // "" (empty string)
console.log(str.substring(5, 2));  // "ayw"

Method	Behavior
slice() returns empty when start > end	Keeps order exactly
substring() automatically swaps the values	Makes it work anyway

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Comparison Table

Feature	slice(start, end)	substring(start, end)
Negative index support	✅ Yes (counts from end)	❌ No (converted to 0)
If start > end	❌ Returns empty	✅ Automatically swaps values
Works on	Strings + Arrays	Strings only
Best Use	Flexible slicing	Safe string extraction when values are uncertain

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Internal Explanation (Simple Words)

• slice() adjusts negative index by adding it to string length.
• substring() treats any negative value as 0, and auto-reorders parameters.

This is why:

str.slice(-5) → last 5 chars  
str.substring(-5) → treats -5 as 0 → returns whole string

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How to Answer in an Interview (Simple Version)

"slice() supports negative indexing and keeps start/end order.
substring() does not support negative indexing and swaps start and end if needed.
So slice() is more flexible, while substring() is safer for simple extractions."

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Real QA Usage (Why Testers Should Know This)

When parsing API responses, log messages, or dynamic text:

• Use slice() when taking values from end of string
• Use substring() when taking values from beginning range safely

Example: Extract last 4 digits of an ID:

id.slice(-4);

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Related Interview Follow-Up Questions (Highly Asked)

1. What is the difference between slice() and splice()?
2. How is substr() different from slice()? Why is substr() deprecated?
3. How do you reverse a string without using built-in reverse() method?
4. How do you extract numbers from a string?
5. What happens if slice() receives floating number arguments?

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Quick Summary (Easy to Remember)

Ask Yourself	Which One to Use
Need to slice from the end?	✅ Use slice()
Index values might be reversed?	✅ Use substring()
Working with arrays?	✅ Use slice()
Simple safe string cut?	✅ Use substring()

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