Which equals operator (== vs ===) should be used in JavaScript comparisons?

In JavaScript, === (strict equality) is generally preferred over == (loose equality) because it checks both value and type without implicit type coercion. Below are detailed examples illustrating the differences:

1. Comparing Primitives

Example	== Result	=== Result	Explanation
"5" == 5	true	false	"5" (string) is coerced to 5 (number).
"5" === 5	false	false	Different types.
0 == false	true	false	false is coerced to 0 (number).
0 === false	false	false	Different types (number vs boolean).
"0" == false	true	false	Both coerced to 0 (string "0" → 0, false → 0).
null == undefined	true	false	Special case: null and undefined are only loosely equal.
NaN == NaN	false	false	NaN is not equal to itself in JavaScript. Use Object.is(NaN, NaN) instead.

2. Comparing Objects/Arrays

Example	== Result	=== Result	Explanation
[] == ""	true	false	Empty array [] is coerced to "" (string).
[] === ""	false	false	Different types (object vs string).
{} == "[object Object]"	true	false	Object coerced to "[object Object]" (string).
{} === {}	false	false	Different object instances (reference comparison).

3. Edge Cases

Example	== Result	=== Result	Explanation
"true" == true	false	false	"true" → NaN (number), true → 1 (number). NaN != 1.
"1" == true	true	false	"1" → 1, true → 1.
"\n" == 0	true	false	"\n" (string) → 0 (number).
" " == 0	true	false	Empty/whitespace strings coerce to 0.

When to Use == (Rare Cases)

1. Check for null or undefined:

   if (value == null) { ... } // true if value is `null` or `undefined`

2. Intentional Type Coercion (e.g., user input as strings vs numbers):

   const userInput = "10";
   if (userInput == 10) { ... } // true

Best Practices

• Default to === for predictable and safe comparisons.
• Avoid == unless you explicitly want type coercion and understand the rules.
• Use Object.is for edge cases like NaN checks:

  Object.is(NaN, NaN); // true

Summary

Scenario	Use
Most comparisons	===
Checking null/undefined	value == null
Intentional type coercion	== (cautiously)

By using ===, you avoid subtle bugs caused by unexpected type coercion.