What is the difference between `==` and `===` operators?

Q: What is the difference between `==` and `===` operators?

`==` is the equality operator that performs type coercion, meaning it converts the operands to the same type before making the comparison. `===` is the strict equality operator that compares both value and type without performing type console.log("" == 0); // true (Wait, what?) console.log("\n \t" == 0); // true (Even weirder!) // This checks for BOTH null and undefined if (value == null) { // do something } // Equivalent to: if (value === null || value === undefined) { ... }

Difficulty: 2/10

type coercion, strict vs loose equality, implicit conversion

1
== is the equality operator that performs type coercion, meaning it converts the operands to the same type before making the comparison.
2
=== is the strict equality operator that compares both value and type without performing type

In 99% of cases, you should always use ===. Using == can lead to strange, silent bugs that are hard to track down because JavaScript is making assumptions behind your back.

The Only Common Exception: Some developers use == specifically to check if a value is Nullish (either null or undefined) in one go

Scenario Questions

0-2 years experience

1You need to check if a variable `userId` equals the string `'123'`. Which operator would you use and why?
2If you write `if (value == null) { ... }`, what values will cause the block to run? Explain the behavior.
3Given `let a = '0'; let b = 0;`, what will `a == b` and `a === b` evaluate to? Explain.

Follow-up Questions

Can you show a real bug that arose from using `==`?
How would you enforce the use of `===` in a codebase?
Are there any performance differences between the two operators?