What is type coercion in JavaScript?

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Type coercion is JavaScript's automatic conversion of a value from one type to another. It happens whenever an operation demands a specific type and the engine silently converts operands to satisfy it. Coercion can be explicit (requested by the developer) or implicit (triggered by operators). Explicit coercion Called directly using conversion functions or constructors. Number("42") // 42 String(42) // "42" Boolean(0) // false parseInt("12px") // 12 Implicit coercion Triggered by operators like + , - , == , and template literals. "5" + 1 // "51" (number coerced to string) "5" - 1 // 4 (string coerced to number) !!"hi" // true (string to boolean via !) [] + {} // "[object Object]" The three coercion paths ToPrimitive — objects call valueOf() , then toString() (or the other way around when a string hint is expected). ToNumber — strings parse as numbers ( "42" → 42 , "abc" → NaN ), booleans become 0 or 1 , null becomes 0 , undefined becomes NaN . ToString — most values call their toString() method; arrays use a comma-joined form; objects usually become "[object Object]" . Falsy values When coerced to boolean, these become false : false , 0 , -0 , 0n , "" , null , undefined , and NaN . Everything else, including "0" , "false" , [] , and {} , is truthy. The + operator is special If either operand is a string, + performs concatenation; otherwise it performs numeric addition. All other arithmetic operators ( - , * , / , % ) force numeric coercion. Coercion pitfalls [] == false // true [0] == false // true null == 0 // false null == undefined // true "" == 0 // true How to tame coercion Prefer === to avoid implicit equality coercion. Use explicit conversions ( Number(x) , String(x) , Boolean(x) ) instead of relying on operators. Treat + carefully when mixing types; use template literals or String() for intentional concatenation. Memorize the 6 falsy values. Know that [] and {} are truthy. The + operator behavior (string concatenation vs addition) is the most commonly tested. '5' + 3 = '53' but '5' - 3 = 2 is the classic question.