Type coercion in JavaScript (implicit vs explicit)

What is Type Coercion?

Type coercion is the automatic or manual conversion of a value from one data type to another. JavaScript is a weakly typed language, so it frequently performs type conversions behind the scenes.

Explicit Coercion (Type Casting)

When you intentionally convert a type using built-in functions:

To String

javascript

String(123);        // "123"
String(true);       // "true"
String(null);       // "null"
String(undefined);  // "undefined"
(123).toString();   // "123"

To Number

javascript

Number("123");      // 123
Number("abc");      // NaN
Number(true);       // 1
Number(false);      // 0
Number(null);       // 0
Number(undefined);  // NaN
Number("");         // 0
parseInt("42px");   // 42
parseFloat("3.14"); // 3.14
+"42";              // 42 (unary plus)

To Boolean

javascript

Boolean(0);         // false
Boolean("");        // false
Boolean(null);      // false
Boolean(undefined); // false
Boolean(NaN);       // false
Boolean(1);         // true
Boolean("hello");   // true
Boolean({});        // true
Boolean([]);        // true
!!value;            // double NOT trick

Falsy Values in JavaScript

There are exactly 8 falsy values:

Value	Type
`false`	Boolean
`0`	Number
`-0`	Number
`0n`	BigInt
`""`	String (empty)
`null`	null
`undefined`	undefined
`NaN`	Number

Everything else is truthy, including [], {}, "0", "false".

Implicit Coercion

When JavaScript automatically converts types during operations:

String Concatenation (`+`)

javascript

"5" + 3;        // "53" (number → string)
"5" + true;     // "5true"
"5" + null;     // "5null"
"5" + {};       // "5[object Object]"

Numeric Context (`-`, `*`, `/`, `%`)

javascript

"5" - 3;        // 2 (string → number)
"5" * 2;        // 10
"5" / "2";      // 2.5
true + 1;       // 2
false + 1;      // 1
null + 5;       // 5 (null → 0)

Comparison Coercion

javascript

"5" == 5;       // true (string → number)
null == undefined; // true (special rule)
null == 0;      // false (null only equals undefined)
"" == 0;        // true
"0" == false;   // true
[] == false;    // true

Common Interview Traps

javascript

[] + [];          // "" (empty string)
[] + {};          // "[object Object]"
{} + [];          // 0 (block + unary plus)
true + true;      // 2
"2" + "2" - "2";  // 20 ("22" - "2" = 20)
null == undefined; // true
null === undefined; // false
NaN === NaN;      // false

Important:

Always use strict equality (===) to avoid unexpected coercion. When you need type conversion, do it explicitly with Number(), String(), or Boolean() for clarity.

Short Answer

Interview ready

Premium

A concise answer to help you respond confidently on this topic during an interview.

Finished reading?

Practice Problems