CODEWITHSUNDEEP
javascript
Bhojendra Rauniyar
Bhojendra Rauniyar

Reputation: 85573

How does the following multiplication work?

I could not understand the following:

3 * { valueOf: function(){ return 5} }
//outputs 15

When I try to use like this:

3 * { 5 }
//syntax-error

3 * {valueOf: 5}
//outputs NaN

So, how exactly is this working with {} ?

Upvotes: 1

Views: 43

Answers (2)

ssube
ssube

Reputation: 48317

The difference is between valueOf being a function that returns a number, or simply being a number. The valueOf method must be a function on the object, so the numeric version isn't used.

The spec talks about how valueOf is supposed to work in section 8.12.8, stating that it must be callable to be considered while attempting to convert the object:

  1. Let valueOf be the result of calling the [[Get]] internal method of object O with argument "valueOf".
  2. If IsCallable(valueOf) is true then,
    1. Let val be the result of calling the [[Call]] internal method of valueOf, with O as the this value and an empty argument list.
    2. If val is a primitive value, return val.

This does not seem to have any case where x.valueOf is not callable, which is why your {valueOf: 5} example doesn't work.

Your first case (3 * { 5 }) is a syntax error, but even if you had a valid object on the right side, wouldn't work as section 11.5 of the spec attempts to convert the right operand to a number, unless you define a valueOf method which returns something that can be converted.

Note that 3 * { valueOf: function(){ return "5"} } works just fine, as "5" can be run through toNumber just fine (following normal coercion rules).

Upvotes: 5

Brennan
Brennan

Reputation: 5742

valueOf is a special attribute on objects. When an object has a valueOf attribute that is a function, that function is called to get the literal value of the object.

In your example, you are making valueOf a literal of 5, which is not a function that can be called by javascript

Upvotes: 0

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