The well-known symbol @@toPrimitive.

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[@@toPrimitive]() does). The language does not enforce alignment between the hint and the result type, although [@@toPrimitive]() must return a primitive, or a TypeError is thrown.

Objects without the @@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. @@toPrimitive allows full control over the primitive conversion process. For example, Date.prototype[@@toPrimitive] treats "default" as if it's "string" and calls toString() instead of valueOf(). Symbol.prototype[@@toPrimitive] ignores the hint and always returns a symbol, which means even in string contexts, Symbol.prototype.toString() won't be called, and Symbol objects must always be explicitly converted to strings through String().

Following example describes how Symbol.toPrimitive property can modify the primitive value converted from an object.

// An object without Symbol.toPrimitive property.
const obj1 = {};
console.log(+obj1); // NaN
console.log(`${obj1}`); // "[object Object]"
console.log(obj1 + ""); // "[object Object]"

// An object with Symbol.toPrimitive property.
const obj2 = {
  [Symbol.toPrimitive](hint) {
    if (hint === "number") {
      return 10;
    }
    if (hint === "string") {
      return "hello";
    }
    return true;
  },
};
console.log(+obj2); // 10        — hint is "number"
console.log(`${obj2}`); // "hello"   — hint is "string"
console.log(obj2 + ""); // "true"    — hint is "default"

• Polyfill of Symbol.toPrimitive in core-js
• Date.prototype[@@toPrimitive]()
• Symbol.prototype[@@toPrimitive]()
• Object.prototype.toString()
• Object.prototype.valueOf()