JavaScript idiosyncrasies

This is a collection of things in JavaScript that may not be well recognized, espcially to beginners.

Disclaimer: Some of these snippets are simply to demonstrate the quirky parts of JavaScript and by no means encourage best practices and should never be seen in production code.

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In no particular order:

Q. What's the result?

(function() {
    var foo = new Object();
    var bar = new Object();
    var map = new Object();

    map[foo] = 'foo';
    map[bar] = 'bar';

    return map[foo];
})();

A.

"bar"

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Q. What's the result?

function f() {
    return 'foo';
}
(function() {
    if (1 === 0) {
        function f() {
            return 'bar';
        }
    }
    return f();
})();

A.

"bar"

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Q. What's the result?

(function() {
    return NaN === NaN;
})();

A.

false

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Q. What's the result?

(function() {
    function foo() {
        return 'a';
    }

    return foo();

    function foo() {
        return 'b';
    }
})();

A.

"b"

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Q. What's the result?

(function(limit) {
  for (var i = 0; i < limit; i++) {
    setTimeout(function() {
      console.log(i);
    }, 0);
  }
})(3);

A.

3
3
3

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Q. What's the result?

(function(a, b) {
  arguments[1] = 3;
  return b;
})(1, 2);

A.

3

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Q. What's the result?

(function(a, b, c) {
  delete arguments[0];

  return arguments.length;
})(1, 2, 3);

A.

3

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Q. What's the result?

(function() {
    return (function (a, b) {}).length;
})();

A.

2

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Q. What's the result?

(function(a, b) {
   var foo, bar;

   foo = (bar = a, b);

   return foo;
})(1, 2);

A.

2

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Q. What's the result?

(function(undefined) {
  var foo;
  return foo === undefined;
})(true);

A.

false

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Q. What's the result?

(function(n) {
    return ~(n);
})(-3);

A.

2

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Q. What's the result?

(function(n) {
    return ~~n;
})(-1.5);

A.

-1

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Q. What's the result?

(function(x) {
    return !!x;
})('a');

A.

true

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Q. What's the result?

(function() {
    return typeof null === 'null';
})();

A.

false

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Q. What's the result?

(function() {
    return +(new Date())
})();

A.

1393812837139

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Q. What's the result?

(function() {
    return (new Date()).valueOf();
})();

A.

1393812845834

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Q. What's the result?

(function() {
    return ''+(new Date());
})();

A.

"Sun Mar 02 2014 18:14:01 GMT-0800 (PST)"

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Q. What's the result?

(function() {
    var foo = 'a';
    (function(foo) {
        foo = 'b';
    })(foo);
    return foo;
})();

A.

"a"

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Q. What's the result?

(function() {
    return arguments.toString();
})();

A.

"[object Arguments]"

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Q. What's the result?

(function() {
    return (function(){}) === (function(){});
})();

A.

false

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Q. What's the result?

(function(n) {
    return n === new Number(n);
})(10);

A.

false

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Q. What's the result?

(function(x) {
    return new String(x) === x;
})('a');

A.

false

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Q. What's the result?

(function() {
    return [1+1] === [2];
})()

A.

false

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Q. What's the result?

(function() {
    return {foo: 'bar'} === {foo: 'bar'};
})();

A.

false

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Q. What's the result?

(function() {
    for(;;);
    return 1;
})();

A.

*Infinite loop*

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Q. What's the result?

(function() {
    return ['10','10','10','10'].map(parseInt);
})();

A.

[10, NaN, 2, 3]

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Q. What's the result?

(function() {
    var o = {
        toString: function() {
            return 'a';
        },
        valueOf: function () {
            return 1;
        }
    };

    return o + o;
})();

A.

2

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Q. What's the result?

(function() {

  var f = function g() {
    return 1;
  };

  return g();

})();

A.

ReferenceError: g is not defined

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Q. What's the result?

(function() {
  return void (1+1);
})();

A.

undefined

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Q. What's the result?

(function() {

  var a = [1,2];
  var b = a;

  a = [1,2];

  return a === b;

})();

A.

false

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Q. What's the result?

(function() {
    var x = 1;

    return (function () {
        return x;

        var x = 2;
    })();
})();

A.

undefined

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Q. What's the result?

(function(n) {
    var even = function (num) {
        return (num === 0) || !(even(num - 1))
    };

    var _even = even;

    even = void 0;

    return _even(n);
})(2);

A.

TypeError: undefined is not a function

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Q. What's the result?

(function() {
    var n;

    function name() {
        return this.name
    };

    n = name.bind({name: 'foo'});
    n = n.bind({name: 'bar'})

    return n();
})();

A.

"foo"

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Q. What's the result?

(function() {
    return ('3' > '12') === ('03' > '12');
})();

A.

false

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Q. What's the result?

(function() {
    return Math.pow(2,53) === (Math.pow(2,53) + 1);
}();

A.

true

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Q. What's the result?

(function() {
    return Math.pow(2,1024) === Infinity;
})();

A.

true

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Q. What's the result?

(function() {
    return (Infinity - 100) === Infinity;
}();

A.

true

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Q. What's the result?

(function() {
    return (0.1 + 0.2 === 0.3);
})();

A.

false

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Q. What's the result?

(function() {
    return (0.1).toFixed(20);
})();

A.

"0.10000000000000000555"

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Q. What's the result?

(function() {
    return parseFloat('3.3.4');
})();

A.

3.3

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Q. What's the result?

(function() {
    return 010;
})();

A.

8

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Q. What's the result?

(function() {
    return (parseInt('10000000000000000', 10) ===
            parseInt('10000000000000001', 10)
    );
})();

A.

true

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Q. What's the result?

(function(n) {
    return (Function)('var n = n || 2; return n;')(n);
})(1);

A.

2

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Q. What's the result? (assuming window scope)

var a = 1;
b = 1;

(function() {
  return (delete window.a) === (delete window.b);
})();

A.

false

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(function(x) {
  var isMatch,
      regex = /[\w]/gi;

  return (regex.test(x) === regex.test(x));
})('a');

A.

false

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Q. What's the result?

(function() {
    return ![];
})();

A.

false

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Q. What's the result?

(function() {
    return +[];
})();

A.

0

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Q. What's the result?

(function() {
    return [][[]];
})();

A.

undefined

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Q. What's the result?

(function() {
    return +!+[];
})();

A.

1

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Q. What's the result?

(function() {
    return []+[];
})();

A.

""

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Q. What's the result?

(function() {
    return true + 1;
})();

A.

2

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Q. What's the result?

(function() {
    return 1 / '';
})();

A.

Infinity

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Q. What's the result?

(function() {
    return 1 * null;
})();

A.

0

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Q. What's the result?

(function() {
    return new Array() == false;
})();

A.

true

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Q. What's the result?

(function() {
  if ([]) {
    return [] == false;
  }
})();

A.

true

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Q. What's the result?

(function() {
  return ''.concat(null);
})();

A.

"null"

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Q. What's the result?

(function(a, b) {
  return a + + b;
})('a','b');

A.

"aNaN"

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Q. What's the result?

(function() {
  Object.prototype.foo = 'foo';

  return foo;
})();

A.

"foo"

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Q. What's the result?

(function() {
  return 1000 === 1e3;
})();

A.

true

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Q. What's the result?

(function() {
  return 9999999999999999;
})();

A.

10000000000000000

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Q. What's the result?

(function() {
  (function() {
    var a = b = 1;
  })();

  return typeof a === typeof b;
})();

A.

false

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Q. What's the result?

(function() {
  return Array(3).map(function(o) { return 'a'; });
})();

A.

[undefined, undefined, undefined]

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Q. What's the result?

(function() {
  var foo = [0,1,2];
  foo[10] = 10;
  return foo.filter(function(n) { return n === undefined; });
})();

A.

[]

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Q. What's the result?

(function(x) {
  var ret = null;

  switch(x) {
    case 'A':
        ret = 'A';
        break;
    default:
        ret = 'unknown';
        break;
  }

  return ret;
})(new String('A'));

A.

"unknown"

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Q. What's the result?

(function() {
  var x = {};

  return x.prototype === Object.getPrototypeOf(x);
})();

A.

false

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Q. What's the result?

(function() {
  function foo() {}
  foo.name = 'bar';

  return foo.name;
})();

A.

"foo"

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Q. What's the result?

(function() {
  return Array(5).join(',').length;
})();

A.

4

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Q. What's the result?

(function(x) {
  return x++ + ++x;
})(2);

A.

6

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Q. What's the result?

(function() {
  'use strict';

  let type = typeof foo;
  let foo = 1;

  return type;
})();

A.

ReferenceError: foo is not defined

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License

MIT