Symbol.toPrimitive

The Symbol.toPrimitive well-known symbol specifies a method that accepts a preferred type and returns a primitive representation of an object. It is called in priority by all type coercion algorithms.

Value

The well-known symbol Symbol.toPrimitive.

Description

With the help of the Symbol.toPrimitive property (used as a function value), an object can be converted to a primitive value. The function is called with a string argument hint, which specifies the preferred type of the result primitive value. The hint argument can be one of "number", "string", and "default".

The "number" hint is used by numeric coercion algorithms. The "string" hint is used by the string coercion algorithm. The "default" hint is used by the primitive coercion algorithm. The hint only acts as a weak signal of preference, and the implementation is free to ignore it (as Symbol.prototype[Symbol.toPrimitive]() does). The language does not enforce alignment between the hint and the result type, although [Symbol.toPrimitive]() must return a primitive, or a TypeError is thrown.

Objects without the Symbol.toPrimitive property are converted to primitives by calling the valueOf() and toString() methods in different orders, which is explained in more detail in the type coercion section. Symbol.toPrimitive allows full control over the primitive conversion process. For example, Symbol.prototype.toString() won't be called, and Symbol objects must always be explicitly converted to strings through String().