2012-03-29

Converting a value to string in JavaScript

In JavaScript, there are three main ways in which any value can be converted to a string. This blog post explains each way, along with its advantages and disadvantages.

Three approaches for converting to string

The three approaches for converting to string are:
  1. value.toString()
  2. "" + value
  3. String(value)
The problem with approach #1 is that it doesn’t work if the value is null or undefined. That leaves us with approaches #2 and #3, which are basically equivalent.
  • ""+value: The plus operator is fine for converting a value when it is surrounded by non-empty strings. As a way for converting a value to string, I find it less descriptive of one’s intentions. But that is a matter of taste, some people prefer this approach to String(value).
  • String(value): This approach is nicely explicit: Apply the function String() to value. The only problem is that this function call will confuse some people, especially those coming from Java, because String is also a constructor. However, function and constructor produce completely different results:
        > String("abc") === new String("abc")
        false
    
        > typeof String("abc")
        'string'
        > String("abc") instanceof String
        false
    
        > typeof new String("abc")
        'object'
        > new String("abc") instanceof String
        true
    
    The function produces, as promised, a string (a primitive [1]). The constructor produces an instance of the type String (an object). The latter is hardly ever useful in JavaScript, which is why you can usually forget about String as a constructor and concentrate on its role as converting to string.

A minor difference between ""+value and String(value)

Until now you have heard that + and String() convert their “argument” to string. But how do they actually do that? It turns out that they do it in slightly different ways, but usually arrive at the same result.

Converting a primitives to string

Both approaches use the internal ToString() operation to convert primitives to string. “Internal” means: a function specified by the ECMAScript 5.1 (§9.8) that isn’t accessible to the language itself. The following table explains how ToString() operates on primitives.

ArgumentResult
undefined"undefined"
null"null"
boolean valueeither "true" or "false"
number valuethe number as a string, e.g. "1.765"
string valueno conversion necessary

Converting objects to string

Both approaches first convert an object to a primitive, before converting that primitive to string. However, + uses the internal ToPrimitive(Number) operation (except for dates [2]), while String() uses ToPrimitive(String).
  • ToPrimitive(Number): To convert an object obj to a primitive, invoke obj.valueOf(). If the result is primitive, return that result. Otherwise, invoke obj.toString(). If the result is primitive, return that result. Otherwise, throw a TypeError.
  • ToPrimitive(String): Works the same, but invokes obj.toString() before obj.valueOf().
With the following object, you can observe the difference:
    var obj = {
        valueOf: function () {
            console.log("valueOf");
            return {}; // not a primitive, keep going
        },
        toString: function () {
            console.log("toString");
            return {}; // not a primitive, keep going
        }
    };  
Interaction:
    > "" + obj
    valueOf
    toString
    TypeError: Cannot convert object to primitive value

    > String(obj)
    toString
    valueOf
    TypeError: Cannot convert object to primitive value

The results are usually the same

The above described difference rarely becomes apparent in practice. Here is why: Most objects use the default implementation of valueOf() which returns this for objects.
    > var x = {}
    > x.valueOf() === x
    true
Hence, ToPrimitive(Number) will skip that method and return the result of toString(), just like ToPrimitive(String). If, however, an object is an instance of Boolean, Number or String, then valueOf() will return a primitive (the one wrapped by the object). That means that the two operations now compute their results as follows:
  • ToPrimitive(Number) returns the result of applying ToString() to the result of valueOf() (the wrapped primitive).
  • ToPrimitive(String) returns the result of toString() (the result of applying ToString() to the wrapped primitive).
Thus, they both still arrive at the same destination, but by a different route.

Conclusion

Which of the three approaches for converting to string should you choose? value.toString() can be OK, if you are sure that value will never be null or undefined. Otherwise, ""+value and String(value) are mostly equivalent. Which one people prefer is a matter of taste. I find String(value) more explicit.

Related posts

  1. JavaScript values: not everything is an object [primitives versus objects]
  2. What is {} + {} in JavaScript? [explains how the + operator works]
  3. String concatenation in JavaScript [how to best concatenate many strings]

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