Table of Contents
What happens when objects are added obj1 + obj2
, subtracted obj1 - obj2
or printed using alert(obj)
?
JavaScript doesn’t exactly allow 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 an 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, as the result of obj1 + obj2
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 do much here, there’s no maths with objects in real projects. When it happens, it’s usually 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:
- It will allow us to understand what’s going on in case of coding mistakes, when such an operation happened accidentally.
- 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.
1. 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.
- All objects are
true
in a boolean context. There are only numeric and string conversions. - 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 ofdate1 - date2
is the time difference between two dates. - As for the string conversion – it usually happens when we output an object like
alert(obj)
and in similar contexts.
We can fine-tune string and numeric conversion, using special object methods.
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;
"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 both strings and numbers would do. 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, we don’t need to remember these peculiar details, because 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 can do the same.
No "boolean"
hint
Please note – there are only three hints. It’s that simple.
There is no “boolean” hint (all objects are true
in boolean context) or anything else. And if we treat "default"
and "number"
the same, like most built-ins do, then there are only two conversions.
To do the conversion, JavaScript tries to find and call three object methods:
- Call
obj[Symbol.toPrimitive](hint)
– the method with the symbolic keySymbol.toPrimitive
(system symbol), if such method exists, - Otherwise if hint is
"string"
- try
obj.toString()
andobj.valueOf()
, whatever exists.
- try
- Otherwise if hint is
"number"
or"default"
- try
obj.valueOf()
andobj.toString()
, whatever exists.
- try
2. 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.
3. toString/valueOf
If there’s no Symbol.toPrimitive
then JavaScript tries to find methods toString
and valueOf
:
- For the “string” hint:
toString
, and if it doesn’t exist, thenvalueOf
(sotoString
has the priority for string conversions). - For other hints:
valueOf
, and if it doesn’t exist, thentoString
(sovalueOf
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 valueOf
, toString
will handle all primitive conversions.
4. 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 a 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
must return a primitive, otherwise there will be an error.
5. 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:
- The object is converted to a primitive (using the rules described above).
- If the resulting primitive isn’t of the right type, it’s 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
- The multiplication
obj * 2
first converts the object to primitive (that’s a string"2"
). - Then
"2" * 2
becomes2 * 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
6. 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"
(foralert
and other operations that need a string)"number"
(for maths)"default"
(few operators)
The specification describes explicitly which operator uses which hint. There are very few operators that “don’t know what to expect” and use the "default"
hint. Usually for built-in objects "default"
hint is handled the same way as "number"
, so in practice the last two are often merged together.
The conversion algorithm is:
- Call
obj[Symbol.toPrimitive](hint)
if the method exists, - Otherwise if hint is
"string"
- try
obj.toString()
andobj.valueOf()
, whatever exists.
- try
- Otherwise if hint is
"number"
or"default"
- try
obj.valueOf()
andobj.toString()
, whatever exists.
- try
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.
As for math operations, JavaScript doesn’t provide a way to “override” them using methods, so real life projects rarely use them on objects.