🔁 Type Coercion (Auto-Conversion) in JS

Sachin Suman — Wed, 10 Sep 2025 21:40:01 +0000

Basically, type coercion is a concept in which JavaScript auto-converts types in some operations.
Example:
"5" + 1 // "51" → number converted to string
"5" - 1 // 4 → string converted to number
true + 1 // 2
null + 1 // 1
undefined + 1 // NaN
📝 Two Main Types of Coercion

Implicit Coercion Happens automatically when JavaScript converts one data type to another during operations.

Example:
console.log(1 + '2'); // "12" (number 1 converted to string)
console.log('5' * 2); // 10 (string '5' converted to number)
console.log(5 == '5'); // true (string '5' converted to number for comparison)

2.Explicit Coercion
Happens when you intentionally convert a value using built-in functions or operators.

Example:
Number('123'); // 123
String(123); // "123"
Boolean(0); // false

⚙️ Common Conversion Rules

String + Number → String (concatenation)
String * Number / String - Number → Number (if string is numeric)
== (loose equality) allows type coercion
=== (strict equality) does not allow type coercion (compares value & type)

📝 Common Type Coercion Table
Expression What Happens Internally Result / Type
1 + '2' Number 1 → String "1" then "1" + "2" "12" (string)
'5' * 2 String '5' → Number 5 then 5 * 2 10 (number)

'10' - 3 String '10' → Number 10 then 10 - 3 7 (number)
true + 1 Boolean true → Number 1 then 1 + 1 2 (number)
false + 10 Boolean false → Number 0 then 0 + 10 10 (number)
null + 1 null → Number 0 then 0 + 1 1 (number)
undefined + 1 undefined → NaN NaN (number)
[] + [] Both arrays → Strings "" + "" "" (string)

💡 Best Practices

Prefer explicit coercion to avoid unexpected results.

Use === instead of == for comparisons unless you intentionally want coercion.

DEV Community: Sachin Suman

🔁 Type Coercion (Auto-Conversion) in JS