Complete JavaScript Interview Guide
Questions
What is the difference between `==` and `===` in JavaScript?
The Scenario
You are a frontend engineer at a social media company. You are writing a new feature that needs to compare two values to see if they are equal.
You are not sure whether to use the == operator or the === operator.
The Challenge
Explain the difference between the == and === operators in JavaScript. What are the pros and cons of each approach, and which one would you choose for this use case?
A junior engineer might think that they are interchangeable. They might not be aware of the difference between type coercion and strict equality.
A senior engineer would be able to provide a detailed explanation of the differences between `==` and `===`. They would also be able to explain the trade-offs between each approach and would have a clear recommendation for which one to use in this use case.
Step 1: Understand the Key Differences
| Operator | Description |
|---|---|
== | The loose equality operator. It compares two values for equality, after converting both values to a common type. |
=== | The strict equality operator. It compares two values for equality, without any type conversion. |
Step 2: Choose the Right Tool for the Job
For our use case, we should use the === operator. This is because it is more predictable and less error-prone than the == operator.
Step 3: Code Examples
Here are some code examples that show the difference between the two approaches:
==:
console.log(1 == '1'); // true
console.log(true == 1); // true
console.log(null == undefined); // true===:
console.log(1 === '1'); // false
console.log(true === 1); // false
console.log(null === undefined); // falseWhen to use ==
You should almost never use the == operator. The only exception is when you want to check if a value is null or undefined.
if (myVar == null) {
// This will be true if myVar is null or undefined.
}Practice Question
What will `[] == ![]` return?