Normalize input types
Convert external input before arithmetic or comparison.
JS Puzzles · Const
Type coercion
Type coercion is JavaScript's automatic or implicit conversion from one data type to another. It explains why operations such as addition may produce a string or number depending on the operands.
How it works
JavaScript may automatically convert a value when an operation expects another type. Primitive conversion uses the value's conversion rules, while object conversion consults Symbol.toPrimitive first and then valueOf() and toString().
Key points
- Type coercion changes a value's type automatically or implicitly.
- Many operations accept a value of one type and convert it to the type they require.
- Object-to-primitive conversion tries Symbol.toPrimitive, then valueOf(), then toString() for the default hint.
- JavaScript has separate Number and BigInt numeric types.
- Primitive coercion uses the default hint and follows the listed method order.
Common mistakes
- Operations with mismatched types can silently trigger implicit conversion instead of throwing a type error.
- A custom Symbol.toPrimitive method must return a primitive, not another object.
Example: Implicit versus explicit conversion
const value = "5";
const implicit = value + 1;
const explicit = Number(value) + 1;
console.log(implicit, typeof implicit); // "51" string
console.log(explicit, typeof explicit); // 6 numberThe plus operator chooses string concatenation when either operand becomes a string; Number() makes the conversion explicit.
When to use it
Customize object conversion
Define how a domain object behaves in numeric or string contexts.
Replace implicit conversion
Make conversion intent visible in application code.
Interview questions
QWhat is type coercion?
Type coercion is JavaScript's automatic or implicit conversion between data types. It commonly appears when an operation receives values whose types do not match the operation's expected type.
Follow-up: How can implicit coercion be avoided? Use explicit conversions such as Number(value) or String(value), and choose operators whose behavior is clear.
QHow are objects converted to primitive values?
For object-to-primitive conversion, JavaScript first checks Symbol.toPrimitive. If that does not provide the result, it tries valueOf() and then toString(), using an order determined by the conversion hint.
Follow-up: What must Symbol.toPrimitive return? A primitive value; returning an object causes a TypeError.
QWhat does Number(value) do for common inputs?
Number(value) performs numeric conversion: null becomes 0, booleans become 1 or 0, undefined becomes NaN, and strings are parsed as numeric literals. A failed string parse produces NaN.
Follow-up: What special issue applies to BigInt conversion? Converting BigInt to Number can lose precision, and Number and BigInt cannot be mixed directly in operations.
Related
Number()String()Boolean()Symbol.toPrimitivevalueOf()toString()BigInt()Number