What is the difference between == and === in JavaScript?

In JavaScript, == (double equals) performs loose equality comparison with type coercion, while === (triple equals) performs strict equality comparison without type conversion.

Double Equals (==) - Loose Equality

The == operator converts operands to the same type before comparing them. This can lead to unexpected results:

console.log(4 == 4);        // true
console.log('4' == 4);      // true - string converted to number
console.log(4 == '4');      // true - number converted to string
console.log(0 == false);    // true - false converted to 0
console.log('' == false);   // true - empty string converted to false
console.log(null == undefined); // true - special case

true
true
true
true
true
true

Triple Equals (===) - Strict Equality

The === operator compares both value and type without any conversion. It returns false if types differ:

console.log(4 === 4);       // true - same type and value
console.log(4 === '4');     // false - different types
console.log(0 === false);   // false - different types
console.log('' === false);  // false - different types
console.log(null === undefined); // false - different types

let v1 = {'value': 'key'};
let v2 = {'value': 'key'};
console.log(v1 === v2);     // false - different object references

true
false
false
false
false
false

Comparison Table

Best Practice Example

// Problematic with ==
function checkValue(input) {
    if (input == 0) {
        return "Zero or falsy value";
    }
    return "Non-zero value";
}

console.log(checkValue(0));       // "Zero or falsy value"
console.log(checkValue(''));      // "Zero or falsy value" - unexpected!
console.log(checkValue(false));   // "Zero or falsy value" - unexpected!

// Better with ===
function checkValueStrict(input) {
    if (input === 0) {
        return "Exactly zero";
    }
    return "Not zero";
}

console.log(checkValueStrict(0));     // "Exactly zero"
console.log(checkValueStrict(''));    // "Not zero"
console.log(checkValueStrict(false)); // "Not zero"

Zero or falsy value
Zero or falsy value
Zero or falsy value
Exactly zero
Not zero
Not zero

Conclusion

Always use === for equality comparisons to avoid unexpected type coercion bugs. Use == only when you specifically need type conversion, which is rare in modern JavaScript.