What is the explanation for these bizarre JavaScript behaviours mentioned in the 'Wat' talk for CodeMash 2012?

Question

The 'Wat' talk for CodeMash 2012 basically points out a few bizarre quirks with Ruby and JavaScript.

I have made a JSFiddle of the results at http://jsfiddle.net/fe479/9/.

The behaviours specific to JavaScript (as I don't know Ruby) are listed below.

I found in the JSFiddle that some of my results didn't correspond with those in the video, and I am not sure why. I am, however, curious to know how JavaScript is handling working behind the scenes in each case.

Empty Array + Empty Array
[] + []
result:
<Empty String>

I am quite curious about the + operator when used with arrays in JavaScript. This matches the video's result.

Empty Array + Object
[] + {}
result:
[Object]

This matches the video's result. What's going on here? Why is this an object. What does the + operator do?

Object + Empty Array
{} + []
result:
[Object]

This doesn't match the video. The video suggests that the result is 0, whereas I get [Object].

Object + Object
{} + {}
result:
[Object][Object]

This doesn't match the video either, and how does outputting a variable result in two objects? Maybe my JSFiddle is wrong.

Array(16).join("wat" - 1)
result:
NaNNaNNaNNaNNaNNaNNaNNaNNaNNaNNaNNaNNaNNaNNaNNaN

Doing wat + 1 results in wat1wat1wat1wat1...

I suspect this is just straightforward behaviour that trying to subtract a number from a string results in NaN.

Rogers

The {} + [] is basicaly the only tricky and implementation dependent one, as I explain here, because it depends on being parsed as an statement or as an expression. What environment are you testing in (I got the expected 0 in Firefow and Chrome but got the "[object Object]" in NodeJs)? – Pressure

I'm running Firefox 9.0.1 on windows 7, and JSFiddle evaluates it to [Object] – Rogers

@missingno I get 0 in the NodeJS REPL – Hindustani

Javascript ran from an REPL is different than Javascript executed directly from code – Anandrous

Array(16).join("wat" - 1) + " Batman!" – Incapacitate

@missingno Posted the question here, but for {} + {}. – Stokehole

Answer 1

Here's a list of explanations for the results you're seeing (and supposed to be seeing). The references I'm using are from the ECMA-262 standard.

1. [] + []

   When using the addition operator, both the left and right operands are converted to primitives first (§11.6.1). As per §9.1, converting an object (in this case an array) to a primitive returns its default value, which for objects with a valid toString() method is the result of calling object.toString() (§8.12.8). For arrays this is the same as calling array.join() (§15.4.4.2). Joining an empty array results in an empty string, so step #7 of the addition operator returns the concatenation of two empty strings, which is the empty string.

2. [] + {}

   Similar to [] + [], both operands are converted to primitives first. For "Object objects" (§15.2), this is again the result of calling object.toString(), which for non-null, non-undefined objects is "[object Object]" (§15.2.4.2).

3. {} + []

   The {} here is not parsed as an object, but instead as an empty block (§12.1, at least as long as you're not forcing that statement to be an expression, but more about that later). The return value of empty blocks is empty, so the result of that statement is the same as +[]. The unary + operator (§11.4.6) returns ToNumber(ToPrimitive(operand)). As we already know, ToPrimitive([]) is the empty string, and according to §9.3.1, ToNumber("") is 0.

4. {} + {}

   Similar to the previous case, the first {} is parsed as a block with empty return value. Again, +{} is the same as ToNumber(ToPrimitive({})), and ToPrimitive({}) is "[object Object]" (see [] + {}). So to get the result of +{}, we have to apply ToNumber on the string "[object Object]". When following the steps from §9.3.1, we get NaN as a result:

   If the grammar cannot interpret the String as an expansion of StringNumericLiteral, then the result of ToNumber is NaN.

5. Array(16).join("wat" - 1)

   As per §15.4.1.1 and §15.4.2.2, Array(16) creates a new array with length 16. To get the value of the argument to join, §11.6.2 steps #5 and #6 show that we have to convert both operands to a number using ToNumber. ToNumber(1) is simply 1 (§9.3), whereas ToNumber("wat") again is NaN as per §9.3.1. Following step 7 of §11.6.2, §11.6.3 dictates that

   If either operand is NaN, the result is NaN.

   So the argument to Array(16).join is NaN. Following §15.4.4.5 (Array.prototype.join), we have to call ToString on the argument, which is "NaN" (§9.8.1):

   If m is NaN, return the String "NaN".

   Following step 10 of §15.4.4.5, we get 15 repetitions of the concatenation of "NaN" and the empty string, which equals the result you're seeing. When using "wat" + 1 instead of "wat" - 1 as argument, the addition operator converts 1 to a string instead of converting "wat" to a number, so it effectively calls Array(16).join("wat1").

As to why you're seeing different results for the {} + [] case: When using it as a function argument, you're forcing the statement to be an ExpressionStatement, which makes it impossible to parse {} as empty block, so it's instead parsed as an empty object literal.

Shumpert

So why is []+1 => "1" and []-1 => -1 ? – Phelps

@RobElsner []+1 pretty much follows the same logic as []+[], just with 1.toString() as rhs operand. For []-1 see the explanation of "wat"-1 in point 5. Remember that ToNumber(ToPrimitive([])) is 0 (point 3). – Shumpert

This explanation is missing/omits a lot details. E.g. "converting an object (in this case an array) to a primitive returns its default value, which for objects with a valid toString() method is the result of calling object.toString() " is completely missing that valueOf of [] is called first, but because return value not a primitive (it is an array), the toString of [] is used instead. I would recommend to look this instead for real indepth explanation 2ality.com/2012/01/object-plus-object.html – Regality

Answer 2

This is more of a comment than an answer, but for some reason I can't comment on your question. I wanted to correct your JSFiddle code. However, I posted this on Hacker News and someone suggested that I repost it here.

The problem in the JSFiddle code is that ({}) (opening braces inside of parentheses) is not the same as {} (opening braces as the start of a line of code). So when you type out({} + []) you are forcing the {} to be something which it is not when you type {} + []. This is part of the overall 'wat'-ness of Javascript.

The basic idea was simple JavaScript wanted to allow both of these forms:

if (u)
    v;

if (x) {
    y;
    z;
}

To do so, two interpretations were made of the opening brace: 1. it is not required and 2. it can appear anywhere.

This was a wrong move. Real code doesn't have an opening brace appearing in the middle of nowhere, and real code also tends to be more fragile when it uses the first form rather than the second. (About once every other month at my last job, I'd get called to a coworker's desk when their modifications to my code weren't working, and the problem was that they'd added a line to the "if" without adding curly braces. I eventually just adopted the habit that the curly braces are always required, even when you're only writing one line.)

Fortunately in many cases eval() will replicate the full wat-ness of JavaScript. The JSFiddle code should read:

function out(code) {
    function format(x) {
        return typeof x === "string" ?
            JSON.stringify(x) : x;
    }   
    document.writeln('>>> ' + code);
    document.writeln(format(eval(code)));
}
document.writeln("<pre>");
out('[] + []');
out('[] + {}');
out('{} + []');
out('{} + {}');
out('Array(16).join("wat" + 1)');
out('Array(16).join("wat - 1")');
out('Array(16).join("wat" - 1) + " Batman!"');
document.writeln("</pre>");

[Also that is the first time I have written document.writeln in many many many years, and I feel a little dirty writing anything involving both document.writeln() and eval().]

Smtih

This was a wrong move. Real code doesn't have an opening brace appearing in the middle of nowhere - I disagree (sort of): I have often in the past used blocks like this to scope variables in C. This habit was picked up for a while back when doing embedded C where variables on the stack take up space, so if they're no longer needed, we want the space freed at the end of the block. However, ECMAScript only scopes within function(){} blocks. So, while I disagree that the concept is wrong, I agree that the implementation in JS is (possibly) wrong. – Carlisle

@JessTelford In ES6, you can use let to declare block-scoped variables. – Amoeba

Answer 3

I second @Ventero’s solution. If you want to, you can go into more detail as to how + converts its operands.

First step (§9.1): convert both operands to primitives (primitive values are undefined, null, booleans, numbers, strings; all other values are objects, including arrays and functions). If an operand is already primitive, you are done. If not, it is an object obj and the following steps are performed:

1. Call obj.valueOf(). If it returns a primitive, you are done. Direct instances of Object and arrays return themselves, so you are not done yet.
2. Call obj.toString(). If it returns a primitive, you are done. {} and [] both return a string, so you are done.
3. Otherwise, throw a TypeError.

For dates, step 1 and 2 are swapped. You can observe the conversion behavior as follows:

var obj = {
    valueOf: function () {
        console.log("valueOf");
        return {}; // not a primitive
    },
    toString: function () {
        console.log("toString");
        return {}; // not a primitive
    }
}

Interaction (Number() first converts to primitive then to number):

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

Second step (§11.6.1): If one of the operands is a string, the other operand is also converted to string and the result is produced by concatenating two strings. Otherwise, both operands are converted to numbers and the result is produced by adding them.

More detailed explanation of the conversion process: “What is {} + {} in JavaScript?”

Chub

Answer 4

We may refer to the specification and that's great and most accurate, but most of the cases can also be explained in a more comprehensible way with the following statements:

• + and - operators work only with primitive values. More specifically +(addition) works with either strings or numbers, and +(unary) and -(subtraction and unary) works only with numbers.
• All native functions or operators that expect primitive value as argument, will first convert that argument to desired primitive type. It is done with valueOf or toString, which are available on any object. That's the reason why such functions or operators don't throw errors when invoked on objects.

So we may say that:

• [] + [] is same as String([]) + String([]) which is same as '' + ''. I mentioned above that +(addition) is also valid for numbers, but there is no valid number representation of an array in JavaScript, so addition of strings is used instead.
• [] + {} is same as String([]) + String({}) which is same as '' + '[object Object]'
• {} + []. This one deserves more explanation (see Ventero answer). In that case, curly braces are treated not as an object but as an empty block, so it turns out to be same as +[]. Unary + works only with numbers, so the implementation tries to get a number out of []. First it tries valueOf which in the case of arrays returns the same object, so then it tries the last resort: conversion of a toString result to a number. We may write it as +Number(String([])) which is same as +Number('') which is same as +0.
• Array(16).join("wat" - 1) subtraction - works only with numbers, so it's the same as: Array(16).join(Number("wat") - 1), as "wat" can't be converted to a valid number. We receive NaN, and any arithmetic operation on NaN results with NaN, so we have: Array(16).join(NaN).

Uranology

Answer 5

To buttress what has been shared earlier.

The underlying cause of this behaviour is partly due to the weakly-typed nature of JavaScript. For example, the expression 1 + “2” is ambiguous since there are two possible interpretations based on the operand types (int, string) and (int int):

• User intends to concatenate two strings, result: “12”
• User intends to add two numbers, result: 3

Thus with varying input types,the output possibilities increase.

The addition algorithm

1. Coerce operands to primitive values

The JavaScript primitives are string, number, null, undefined and boolean (Symbol is coming soon in ES6). Any other value is an object (e.g. arrays, functions and objects). The coercion process for converting objects into primitive values is described thus:

• If a primitive value is returned when object.valueOf() is invoked, then return this value, otherwise continue

• If a primitive value is returned when object.toString() is invoked, then return this value, otherwise continue

• Throw a TypeError

Note: For date values, the order is to invoke toString before valueOf.

2. If any operand value is a string, then do a string concatenation

3. Otherwise, convert both operands to their numeric value and then add these values

Knowing the various coercion values of types in JavaScript does help to make the confusing outputs clearer. See the coercion table below

+-----------------+-------------------+---------------+
| Primitive Value |   String value    | Numeric value |
+-----------------+-------------------+---------------+
| null            | “null”            | 0             |
| undefined       | “undefined”       | NaN           |
| true            | “true”            | 1             |
| false           | “false”           | 0             |
| 123             | “123”             | 123           |
| []              | “”                | 0             |
| {}              | “[object Object]” | NaN           |
+-----------------+-------------------+---------------+

It is also good to know that JavaScript's + operator is left-associative as this determines what the output will be cases involving more than one + operation.

Leveraging the Thus 1 + "2" will give "12" because any addition involving a string will always default to string concatenation.

You can read more examples in this blog post (disclaimer I wrote it).

Latrena