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Object to primitive conversion

What happens when objects are added obj1 + obj2, subtracted obj1 - obj2 or printed using alert(obj)?

JavaScript doesn’t allow you to customize how operators work on objects. Unlike some other programming languages, such as Ruby or C++, we can’t implement a special object method to handle addition (or other operators).

In case of such operations, objects are auto-converted to primitives, and then the operation is carried out over these primitives and results in a primitive value.

That’s an important limitation: the result of obj1 + obj2 (or another math operation) can’t be another object!

E.g. we can’t make objects representing vectors or matrices (or achievements or whatever), add them and expect a “summed” object as the result. Such architectural feats are automatically “off the board”.

So, because we can’t technically do much here, there’s no maths with objects in real projects. When it happens, with rare exceptions, it’s because of a coding mistake.

In this chapter we’ll cover how an object converts to primitive and how to customize it.

We have two purposes:

  1. It will allow us to understand what’s going on in case of coding mistakes, when such an operation happened accidentally.
  2. There are exceptions, where such operations are possible and look good. E.g. subtracting or comparing dates (Date objects). We’ll come across them later.

Conversion rules

In the chapter Type Conversions we’ve seen the rules for numeric, string and boolean conversions of primitives. But we left a gap for objects. Now, as we know about methods and symbols it becomes possible to fill it.

  1. There’s no conversion to boolean. All objects are true in a boolean context, as simple as that. There exist only numeric and string conversions.
  2. The numeric conversion happens when we subtract objects or apply mathematical functions. For instance, Date objects (to be covered in the chapter Date and time) can be subtracted, and the result of date1 - date2 is the time difference between two dates.
  3. As for the string conversion – it usually happens when we output an object with alert(obj) and in similar contexts.

We can implement string and numeric conversion by ourselves, using special object methods.

Now let’s get into technical details, because it’s the only way to cover the topic in-depth.

Hints

How does JavaScript decide which conversion to apply?

There are three variants of type conversion, that happen in various situations. They’re called “hints”, as described in the specification:"string"

For an object-to-string conversion, when we’re doing an operation on an object that expects a string, like alert:

// output
alert(obj);

// using object as a property key
anotherObj[obj] = 123;

"number"

For an object-to-number conversion, like when we’re doing maths:

// explicit conversion
let num = Number(obj);

// maths (except binary plus)
let n = +obj; // unary plus
let delta = date1 - date2;

// less/greater comparison
let greater = user1 > user2;

Most built-in mathematical functions also include such conversion."default"

Occurs in rare cases when the operator is “not sure” what type to expect.

For instance, binary plus + can work both with strings (concatenates them) and numbers (adds them). So if a binary plus gets an object as an argument, it uses the "default" hint to convert it.

Also, if an object is compared using == with a string, number or a symbol, it’s also unclear which conversion should be done, so the "default" hint is used.

// binary plus uses the "default" hint
let total = obj1 + obj2;

// obj == number uses the "default" hint
if (user == 1) { ... };

The greater and less comparison operators, such as < >, can work with both strings and numbers too. Still, they use the "number" hint, not "default". That’s for historical reasons.

In practice though, things are a bit simpler.

All built-in objects except for one case (Date object, we’ll learn it later) implement "default" conversion the same way as "number". And we probably should do the same.

Still, it’s important to know about all 3 hints, soon we’ll see why.

To do the conversion, JavaScript tries to find and call three object methods:

  1. Call obj[Symbol.toPrimitive](hint) – the method with the symbolic key Symbol.toPrimitive (system symbol), if such method exists,
  2. Otherwise if hint is "string"
    • try calling obj.toString() or obj.valueOf(), whatever exists.
  3. Otherwise if hint is "number" or "default"
    • try calling obj.valueOf() or obj.toString(), whatever exists.

Symbol.toPrimitive

Let’s start from the first method. There’s a built-in symbol named Symbol.toPrimitive that should be used to name the conversion method, like this:

obj[Symbol.toPrimitive] = function(hint) {
  // here goes the code to convert this object to a primitive
  // it must return a primitive value
  // hint = one of "string", "number", "default"
};

If the method Symbol.toPrimitive exists, it’s used for all hints, and no more methods are needed.

For instance, here user object implements it:

let user = {
  name: "John",
  money: 1000,

  [Symbol.toPrimitive](hint) {
    alert(`hint: ${hint}`);
    return hint == "string" ? `{name: "${this.name}"}` : this.money;
  }
};

// conversions demo:
alert(user); // hint: string -> {name: "John"}
alert(+user); // hint: number -> 1000
alert(user + 500); // hint: default -> 1500

As we can see from the code, user becomes a self-descriptive string or a money amount, depending on the conversion. The single method user[Symbol.toPrimitive] handles all conversion cases.

toString/valueOf

If there’s no Symbol.toPrimitive then JavaScript tries to find methods toString and valueOf:

  • For the "string" hint: call toString method, and if it doesn’t exist or if it returns an object instead of a primitive value, then call valueOf (so toString has the priority for string conversions).
  • For other hints: call valueOf, and if it doesn’t exist or if it returns an object instead of a primitive value, then call toString (so valueOf has the priority for maths).

Methods toString and valueOf come from ancient times. They are not symbols (symbols did not exist that long ago), but rather “regular” string-named methods. They provide an alternative “old-style” way to implement the conversion.

These methods must return a primitive value. If toString or valueOf returns an object, then it’s ignored (same as if there were no method).

By default, a plain object has following toString and valueOf methods:

  • The toString method returns a string "[object Object]".
  • The valueOf method returns the object itself.

Here’s the demo:

let user = {name: "John"};

alert(user); // [object Object]
alert(user.valueOf() === user); // true

So if we try to use an object as a string, like in an alert or so, then by default we see [object Object].

The default valueOf is mentioned here only for the sake of completeness, to avoid any confusion. As you can see, it returns the object itself, and so is ignored. Don’t ask me why, that’s for historical reasons. So we can assume it doesn’t exist.

Let’s implement these methods to customize the conversion.

For instance, here user does the same as above using a combination of toString and valueOf instead of Symbol.toPrimitive:

let user = {
  name: "John",
  money: 1000,

  // for hint="string"
  toString() {
    return `{name: "${this.name}"}`;
  },

  // for hint="number" or "default"
  valueOf() {
    return this.money;
  }

};

alert(user); // toString -> {name: "John"}
alert(+user); // valueOf -> 1000
alert(user + 500); // valueOf -> 1500

As we can see, the behavior is the same as the previous example with Symbol.toPrimitive.

Often we want a single “catch-all” place to handle all primitive conversions. In this case, we can implement toString only, like this:

let user = {
  name: "John",

  toString() {
    return this.name;
  }
};

alert(user); // toString -> John
alert(user + 500); // toString -> John500

In the absence of Symbol.toPrimitive and valueOftoString will handle all primitive conversions.

A conversion can return any primitive type

The important thing to know about all primitive-conversion methods is that they do not necessarily return the “hinted” primitive.

There is no control whether toString returns exactly a string, or whether Symbol.toPrimitive method returns a number for the hint "number".

The only mandatory thing: these methods must return a primitive, not an object.

Historical notes

For historical reasons, if toString or valueOf returns an object, there’s no error, but such value is ignored (like if the method didn’t exist). That’s because in ancient times there was no good “error” concept in JavaScript.

In contrast, Symbol.toPrimitive is stricter, it must return a primitive, otherwise there will be an error.

Further conversions

As we know already, many operators and functions perform type conversions, e.g. multiplication * converts operands to numbers.

If we pass an object as an argument, then there are two stages of calculations:

  1. The object is converted to a primitive (using the rules described above).
  2. If necessary for further calculations, the resulting primitive is also converted.

For instance:

let obj = {
  // toString handles all conversions in the absence of other methods
  toString() {
    return "2";
  }
};

alert(obj * 2); // 4, object converted to primitive "2", then multiplication made it a number
  1. The multiplication obj * 2 first converts the object to primitive (that’s a string "2").
  2. Then "2" * 2 becomes 2 * 2 (the string is converted to number).

Binary plus will concatenate strings in the same situation, as it gladly accepts a string:

let obj = {
  toString() {
    return "2";
  }
};

alert(obj + 2); // 22 ("2" + 2), conversion to primitive returned a string => concatenation

Summary

The object-to-primitive conversion is called automatically by many built-in functions and operators that expect a primitive as a value.

There are 3 types (hints) of it:

  • "string" (for alert and other operations that need a string)
  • "number" (for maths)
  • "default" (few operators, usually objects implement it the same way as "number")

The specification describes explicitly which operator uses which hint.

The conversion algorithm is:

  1. Call obj[Symbol.toPrimitive](hint) if the method exists,
  2. Otherwise if hint is "string"
    • try calling obj.toString() or obj.valueOf(), whatever exists.
  3. Otherwise if hint is "number" or "default"
    • try calling obj.valueOf() or obj.toString(), whatever exists.

All these methods must return a primitive to work (if defined).

In practice, it’s often enough to implement only obj.toString() as a “catch-all” method for string conversions that should return a “human-readable” representation of an object, for logging or debugging purposes.