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".

example

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

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

// An object with Symbol.toPrimitive property.
var 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"

memberof

NumberWrapper

toJSON() behavior

If an object being stringified has a property named toJSON whose value is a function, then the toJSON() method customizes JSON stringification behavior: instead of the object being serialized, the value returned by the toJSON() method when called will be serialized. JSON.stringify calls toJSON with one parameter:

• if this object is a property value, the property name
• if it is in an array, the index in the array, as a string
• an empty string if JSON.stringify was directly called on this object

example

var obj = {
  foo: 'foo',
  toJSON: function() {
    return 'bar';
  }
};
JSON.stringify(obj);        // '"bar"'
JSON.stringify({ x: obj }); // '{"x":"bar"}'

var obj2 = {
  foo: 'foo',
  toJSON: function(key) {
    if (key === '') {
      return 'bar only';
    } else {
      return 'bar in ' + key;
    }
  }
};

JSON.stringify(obj2);         // '"bar only"'
JSON.stringify({ x: obj2 });  // '{ "x":"bar in x"}'
JSON.stringify([obj2, obj2]); // '["bar in 0", "bar in 1"]'

memberof

NumberWrapper