JavaScript: toString() vs valueOf()

The difference between toString() and valueOf() is simple: the former is used to convert an object to a string, and the latter to a number primitive value (thus a string or a number). The obvious ends there. When will JavaScript use one method and when the other?

Let’s assume we’re going to use the following variables:

var
    s = {
        toString: function() { return "abc"; }
    },
    v = {
        valueOf: function() { return "123"; }
    },
    sv = {
        toString: function() { return "zyx"; },
        valueOf: function() { return 987; }
    };

As you can see, we have a variable defining only toString(), a second defining only valueOf(), and a third defining both methods.

Operator + #

The + operator is used for concatenation and addition. JavaScript will prefer valueOf() whenever possible.

> s + v + sv
'abc123987'

So:

• when only the toString() method is defined, it is used,
• when only the valueOf() method is defined, it is used,
• when both methods are defined, valueOf() is used.

Array.prototype.join() #

The join() method is used to combine the elements of an array into one string. Only the toString() method will be used on elements.

> [s, v, sv].join()
'abc[object Object]zyx'

So:

• when only the toString() method is defined, it is used
• when only the valueOf() method is defined, the prototype toString() method is used
• when both methods are defined, toString() is used