Type coersion in javascript

Question

What will be the output of the of following code:

console.log({}.valueOf()+15);//Output:[object Object]15

Will the return value from the .valueOf() method be string in the above code or is simply an object since .toString() will result in same but with this method returned value will be string anyhow.I just wanted to know what will be the type of the value the .valueOf() returns in above code.If return value is a string ,for what other object will valueOf() method returns string value except for [new String("abc")]

• Maizere Pathak.Nepal: Who wants to close this post????

Answer 1

The .valueOf() of a plain object will be the same object. (This is NOT the same as its .toString() method.)

var o = {};
typeof o.valueOf(); // "object"
o === o.valueOf(); // true

The + operator sees that you're not using a number for the first operand, so instead of addition, it does string concatenation and calls the .toString() value of both of its operands.

That's why you get [object Object]15.

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You can test this by changing the toString() output.

var o = {};
o.toString = function() { return "foobar" }
o.toString(); // "foobar"
o + 15; // "foobar15"
o.valueOf() + 15; // "foobar15"

• Maizere Pathak.Nepal: the valueOf() method returns plain object which is a string
• scram: @Maizere: No, the .valueOf() object returns a plain object which is not a string. It's just the same object. The + operator returns a string for reasons that I explained in my answer below the first code example.
• Maizere Pathak.Nepal: plz see the comment in the Klink post above
• scram: @Maizere: Kolink's answer and comment is wrong. The .valueOf() is not the same as the .toString(), and does not return a string.
• c-smile: @scram This o === o.valueOf(); may not always be true. Implementation specific according to ECMA-262.

Answer 2

valueOf returns a primitive value. Exactly what type of primitive value depends on the object.

To demonstrate:

typeof Number(0).valueOf(); // number
typeof Boolean(0).valueOf(); // boolean

The point is that, whatever it returns, it is a primitive value. toString, on the other hand, always returns a string.

In the case of {}, there is no valueOf method other than the root Object.prototype.valueOf, which is identical to Object.prototype.toString, returning a string of the form [object Constructor]. Which being a string results in concatenation with 15.

• Maizere Pathak.Nepal: but i m adding with a number
• Maizere Pathak.Nepal: will number also gets coerced to String
• Niet the Dark Absol: Yes, because the first operand to + is a string in this case. If it were a number, you would be adding it.
• Maizere Pathak.Nepal: Now i have started to know about type casting in javascript
• Maizere Pathak.Nepal: last question for what other object will valueOf() method returns string

Answer 3

To get the type of an object in JS, you can use the typeof function:

typeof({}) // outputs 'object'
typeof({}.valueOf()) // outputs 'object'
typeof({}.valueOf()+15) // outputs 'string'

This indicates that the value coercion is taking place when trying to "add" 15, which instead is interpreted as string concatenation.