How does toString work?

Brief Answer

• String(value) — explicit conversion to string per spec (ToString).
• value.toString() — a method on objects/primitives; missing for null/undefined.
• Concatenation ('' + value) and template literals (${value}) perform implicit string coercion.

Full Answer

Explicit vs implicit

• Explicit: String(value) — safe for any value including null/undefined.
• Method: value.toString() — may be absent or overridden.
• Implicit: '' + value or ${value} — engine runs ToPrimitive then ToString.

Mini examples:

String(123);        // "123"
(123).toString();   // "123"
'' + 123;           // "123"

Primitives

• Numbers: String(1.2) → "1.2", String(NaN) → "NaN", String(Infinity) → "Infinity".
• Booleans: String(true) → "true", String(false) → "false".
• null/undefined: String(null) → "null", String(undefined) → "undefined"; they have no toString().

Mini examples:

String(null);       // "null"
String(undefined);  // "undefined"
// null.toString()  // TypeError

Arrays and objects

• Arrays: [].toString() → "", [1,2].toString() → "1,2".
• Plain objects: default "[object Object]" via Object.prototype.toString, unless overridden.

Mini examples:

[1, 2].toString();     // "1,2"
({}).toString();       // "[object Object]"

Override toString:

const user = { name: 'Ada', toString() { return `User:${this.name}`; } };
String(user);          // "User:Ada"

Symbols

• symbol.toString() → "Symbol(desc)".
• '' + symbol may throw; prefer String(symbol) or symbol.toString().

Mini example:

const s = Symbol('x');
s.toString();          // "Symbol(x)"

Template literals and concatenation

Both coerce values to string:

`${true}`;            // "true"
'' + [1, 2];          // "1,2"

Common pitfalls

• Calling toString on null/undefined — TypeError.
• Expecting “nice” default object output — you get "[object Object]"; override toString or use JSON.stringify.

Mini example:

JSON.stringify({ a: 1 }); // "{\"a\":1}"

Recommendations

• Need reliability: use String(value).
• Need formatting control: override your object’s toString().
• For UI/logs avoid "[object Object]": use toString or JSON.stringify.