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";

Basic Extraction

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

In normal cases, both give the same output.

Key Difference 1: Handling Negative Indices

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

So:

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

Key Difference 2: If Start > End

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

Comparison Table

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

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.”

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);

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?

Quick Summary (Easy to Remember)