JavaScript Puzzlers!

or: do you really know JavaScript?

(or: how insane do you think JavaScript actually is?)

Hi Internet!
This page wasn't ready yet, but a bunch of people are coming in, so sorry you caught us in underwear.
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This is a JavaScript quiz based on the behaviour as of ECMA 262 (5.1) in a browser environment.
Behaviour may be different if you try it in a node or jsc REPL, for example this and global are different at the top level of a firefox console, node repl or jsc shell. Sorry about that.

1. What is the result of this expression? (or multiple ones) ["1", "2", "3"].map(parseInt)
   ["1", "2", "3"] [1, 2, 3] [0, 1, 2] other

   what you actually get is [1, NaN, NaN] because parseInt takes two parameters (val, radix) and map passes 3 (element, index, array)

2. What is the result of this expression? (or multiple ones) [typeof null, null instanceof Object]
   ["object", false] [null, false] ["object", true] other

   typeof will always return "object" for native non callable objects.

3. What is the result of this expression? (or multiple ones) [ [3,2,1].reduce(Math.pow), [].reduce(Math.pow) ]
   an error [9, 0] [9, NaN] [9, undefined]

   Per spec: reduce on an empty array without an initial value throws TypeError

4. What is the result of this expression? (or multiple ones) var val = 'smtg'; console.log('Value is ' + (val === 'smtg') ? 'Something' : 'Nothing');
   Value is Something Value is Nothing NaN other

   it actually prints 'Something' the + operator has higher precedence than the ternary one.

5. What is the result of this expression? (or multiple ones) var name = 'World!'; (function () { if (typeof name === 'undefined') { var name = 'Jack'; console.log('Goodbye ' + name); } else { console.log('Hello ' + name); } })();
   Goodbye Jack Hello Jack Hello undefined Hello World

   The var declaration is hoisted to the function scope, but the initialization is not.

6. What is the result of this expression? (or multiple ones) var END = Math.pow(2, 53); var START = END - 100; var count = 0; for (var i = START; i <= END; i++) { count++; } console.log(count);
   0 100 101 other

   it goes into an infinite loop, 2^53 is the highest possible number in javascript, and 2^53+1 gives 2^53, so i can never become larger than that.

7. What is the result of this expression? (or multiple ones) var ary = [0,1,2]; ary[10] = 10; ary.filter(function(x) { return x === undefined;});
   [undefined × 7] [0, 1, 2, 10] [] [undefined]

   Array.prototype.filter is not invoked for the missing elements.

8. What is the result of this expression? (or multiple ones) var two = 0.2 var one = 0.1 var eight = 0.8 var six = 0.6 [two - one == one, eight - six == two]
   [true, true] [false, false] [true, false] other

   JavaScript does not have precision math, even though sometimes it works correctly.

9. What is the result of this expression? (or multiple ones) function showCase(value) { switch(value) { case 'A': console.log('Case A'); break; case 'B': console.log('Case B'); break; case undefined: console.log('undefined'); break; default: console.log('Do not know!'); } } showCase(new String('A'));
   Case A Case B Do not know! undefined

   switch uses === internally and new String(x) !== x

10. What is the result of this expression? (or multiple ones) function showCase2(value) { switch(value) { case 'A': console.log('Case A'); break; case 'B': console.log('Case B'); break; case undefined: console.log('undefined'); break; default: console.log('Do not know!'); } } showCase2(String('A'));
    Case A Case B Do not know! undefined

    String(x) does not create an object but does return a string, i.e. typeof String(1) === "string"

11. What is the result of this expression? (or multiple ones) function isOdd(num) { return num % 2 == 1; } function isEven(num) { return num % 2 == 0; } function isSane(num) { return isEven(num) || isOdd(num); } var values = [7, 4, '13', -9, Infinity]; values.map(isSane);
    [true, true, true, true, true] [true, true, true, true, false] [true, true, true, false, false] [true, true, false, false, false]

    Infinity % 2 gives NaN, -9 % 2 gives -1 (modulo operator keeps sign so it's result is only reliable compared to 0)

12. What is the result of this expression? (or multiple ones) parseInt(3, 8) parseInt(3, 2) parseInt(3, 0)
    3, 3, 3 3, 3, NaN 3, NaN, NaN other

    3 doesn't exist in base 2, so obviously that's a NaN, but what about 0? parseInt will consider a bogus radix and assume you meant 10, so it returns 3.

13. What is the result of this expression? (or multiple ones) Array.isArray( Array.prototype )
    true false error other

    Array.prototype is an Array. Go figure.

14. What is the result of this expression? (or multiple ones) var a = [0]; if ([0]) { console.log(a == true); } else { console.log("wut"); }
    true false "wut" other

    [0] as a boolean is considered true. Alas, when using it in the comparisons it gets converted in a different way and all goes to hell.

15. What is the result of this expression? (or multiple ones) []==[]
    true false error other

    == is the spawn of satan.

16. What is the result of this expression? (or multiple ones) '5' + 3 '5' - 3
    "53", 2 8, 2 error other

    Strings know about + and will use it, but they are ignorant of - so in that case the strings get converted to numbers.

17. What is the result of this expression? (or multiple ones) 1 + - + + + - + 1
    2 1 error other

    Great fun.

18. What is the result of this expression? (or multiple ones) var ary = Array(3); ary[0]=2 ary.map(function(elem) { return '1'; });
    [2, 1, 1] ["1", "1", "1"] [2, "1", "1"] other

    The result is ["1", undefined × 2], as map is only invoked for elements of the Array which have been initialized.

19. What is the result of this expression? (or multiple ones) function sidEffecting(ary) { ary[0] = ary[2]; } function bar(a,b,c) { c = 10 sidEffecting(arguments); return a + b + c; } bar(1,1,1)
    3 12 error other

    The result is 21, in javascript variables are tied to the arguments object so changing the variables changes arguments and changing arguments changes the local variables even when they are not in the same scope.

20. What is the result of this expression? (or multiple ones) var a = 111111111111111110000, b = 1111; a + b;
    111111111111111111111 111111111111111110000 NaN Infinity

    Lack of precision for numbers in JavaScript affects both small and big numbers.

21. What is the result of this expression? (or multiple ones) var x = [].reverse; x();
    [] undefined error window

    [].reverse will return this and when invoked without an explicit receiver object it will default to the default this AKA window

22. What is the result of this expression? (or multiple ones) Number.MIN_VALUE > 0
    false true error other

    Number.MIN_VALUE is the smallest value bigger than zero, -Number.MAX_VALUE gets you a reference to something like the most negative number.

23. What is the result of this expression? (or multiple ones) [1 < 2 < 3, 3 < 2 < 1]
    [true, true] [true, false] error other

    This is parsed as (1 > 2) > 3 and (3 > 2) > 1. Than it's implicit conversions at work: true gets intified and is 1, while false gets intified and becomes 0.

24. What is the result of this expression? (or multiple ones) // the most classic wtf 2 == [[[2]]]
    true false undefined other

    both objects get converted to strings and in both cases the resulting string is "2"

25. What is the result of this expression? (or multiple ones) 3.toString() 3..toString() 3...toString()
    "3", error, error "3", "3.0", error error, "3", error other

    3.x is a valid syntax to define "3" with a mantissa of x, toString is not a valid number, but the empty string is.

26. What is the result of this expression? (or multiple ones) (function(){ var x = y = 1; })(); console.log(y); console.log(x);
    1, 1 error, error 1, error other

    y is an automatic global, not a function local one.

27. What is the result of this expression? (or multiple ones) var a = /123/, b = /123/; a == b a === b
    true, true true, false false, false other

    Per spec Two regular expression literals in a program evaluate to regular expression objects that never compare as === to each other even if the two literals' contents are identical.

28. What is the result of this expression? (or multiple ones) var a = [1, 2, 3], b = [1, 2, 3], c = [1, 2, 4] a == b a === b a > c a < c
    false, false, false, true false, false, false, false true, true, false, true other

    Arrays are compared lexicographically with > and <, but not with == and ===

29. What is the result of this expression? (or multiple ones) var a = {}, b = Object.prototype; [a.prototype === b, Object.getPrototypeOf(a) === b]
    [false, true] [true, true] [false, false] other

    Functions have a prototype property but other objects don't so a.prototype is undefined.
    Every Object instead has an internal property accessible via Object.getPrototypeOf

30. What is the result of this expression? (or multiple ones) function f() {} var a = f.prototype, b = Object.getPrototypeOf(f); a === b
    true false null other

    f.prototype is the object that will become the parent of any objects created with new f while Object.getPrototypeOf returns the parent in the inheritance hierarchy.

31. What is the result of this expression? (or multiple ones) function foo() { } var oldName = foo.name; foo.name = "bar"; [oldName, foo.name]
    error ["", ""] ["foo", "foo"] ["foo", "bar"]

    name is a read only property. Why it doesn't throw an error when assigned, I do not know.

32. What is the result of this expression? (or multiple ones) "1 2 3".replace(/\d/g, parseInt)
    "1 2 3" "0 1 2" "NaN 2 3" "1 NaN 3"

    String.prototype.replace invokes the callback function with multiple arguments where the first is the match, then there is one argument for each capturing group, then there is the offset of the matched substring and finally the original string itself. so parseInt will be invoked with arguments [1, 0], [2, 2], [3, 4].

33. What is the result of this expression? (or multiple ones) function f() {} var parent = Object.getPrototypeOf(f); f.name // ? parent.name // ? typeof eval(f.name) // ? typeof eval(parent.name) // ?
    "f", "Empty", "function", "function" "f", undefined, "function", error "f", "Empty", "function", error other

    The function prototype object is defined somewhere, has a name, can be invoked, but it's not in the current scope.

34. What is the result of this expression? (or multiple ones) var lowerCaseOnly = /^[a-z]+$/; [lowerCaseOnly.test(null), lowerCaseOnly.test()]
    [true, false] error [true, true] [false, true]

    the argument is converted to a string with the abstract ToString operation, so it is "null" and "undefined".

35. What is the result of this expression? (or multiple ones) [,,,].join(", ")
    ", , , " "undefined, undefined, undefined, undefined" ", , " ""

    JavaScript allows a trailing comma when defining arrays, so that turns out to be an array of three undefined.

36. What is the result of this expression? (or multiple ones) var a = {class: "Animal", name: 'Fido'}; a.class
    "Animal" Object an error other

    The answer is: it depends on the browser. class is a reserved word, but it is accepted as a property name by Chrome, Firefox and Opera. It will fail in IE. On the other hand, everybody will accept most other reserved words (int, private, throws etc) as variable names too, while class is verboten.

37. What is the result of this expression? (or multiple ones) var a = new Date("epoch")
    Thu Jan 01 1970 01:00:00 GMT+0100 (CET) current time error other

    You get "Invalid Date", which is an actual Date object (a instanceof Date is true). But invalid. This is because the time is internally kept as a Number, which in this case it's NaN.

38. What is the result of this expression? (or multiple ones) var a = Function.length, b = new Function().length a === b
    true false error other

    It's false. Function.length is defined to be 1. On the other hand, the length property of the Function prototype object is defined to be 0.

39. What is the result of this expression? (or multiple ones) var a = Date(0); var b = new Date(0); var c = new Date(); [a === b, b === c, a === c]
    [true, true, true] [false, false, false] [false, true, false] [true, false, false]

    When Date is invoked as a constructor it returns an object relative to the epoch (Jan 01 1970). When the argument is missing it returns the current date. When it is invoked as a function, it returns a String representation of the current time.

40. What is the result of this expression? (or multiple ones) var min = Math.min(), max = Math.max() min < max
    true false error other

    Math.min returns +Infinity when supplied an empty argument list. Likewise, Math.max returns -Infinity.

41. What is the result of this expression? (or multiple ones) function captureOne(re, str) { var match = re.exec(str); return match && match[1]; } var numRe = /num=(\d+)/ig, wordRe = /word=(\w+)/i, a1 = captureOne(numRe, "num=1"), a2 = captureOne(wordRe, "word=1"), a3 = captureOne(numRe, "NUM=2"), a4 = captureOne(wordRe, "WORD=2"); [a1 === a2, a3 === a4]
    [true, true] [false, false] [true, false] [false, true]

    Regular expressions in JavaScript if defined using the /g flag will carry a state across matches, even if they are actually used on different strings (the lastIndex property). This means a3 will be null as the regular expression was applied starting from the index of the last matched string, even if it was a different one.

42. What is the result of this expression? (or multiple ones) var a = new Date("2014-03-19"), b = new Date(2014, 03, 19); [a.getDay() === b.getDay(), a.getMonth() === b.getMonth()]
    [true, true] [true, false] [false, true] [false, false]

    JavaScript inherits 40 years old design from C: days are 1-indexed in C's struct tm, but months are 0 indexed. In addition to that, getDay returns the 0-indexed day of the week, to get the 1-indexed day of the month you have to use getDate, which doesn't return a Date object.

43. What is the result of this expression? (or multiple ones) if ('http://giftwrapped.com/picture.jpg'.match('.gif')) { 'a gif file' } else { 'not a gif file' }
    'a gif file' 'not a gif file' error other

    String.prototype.match silently converts the string into a regular expression, without escaping it, thus the '.' becomes a metacharacter matching '/'.

44. What is the result of this expression? (or multiple ones) function foo(a) { var a; return a; } function bar(a) { var a = 'bye'; return a; } [foo('hello'), bar('hello')]
    ["hello", "hello"] ["hello", "bye"] ["bye", "bye"] other

    Variabled declarations are hoisted, but in this case since the variable exists already in the scope, they are removed altogether. In bar() the variable declaration is removed but the assignment remains, so it has effect.