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Are You a JavaScript Guru? Try This Test

Friday, February 1, 2013

Think you know JavaScript? Really? Are you sure? Try the following quick quiz. Guess what each expression evaluates to. (Answers given at the end.)

1. ++Math.PI
2. (0.1 + 0.2) + 0.3 == 0.1 + (0.2 + 0.3)
3. typeof NaN
4. typeof typeof undefined
5. a = {null:null}; typeof a.null;
6. a = "5"; b = "2"; c = a * b;
7. a = "5"; b = 2; c = a+++b;
8. isNaN(1/null)
9. (16).toString(16)
10. 016 * 2
11. ~null
12. "ab c".match(/\b\w\b/)


1. 4.141592653589793
2. false
3. "number"
4. "string"
5. "object"
6. 10
7. 7
8. false
9. "10"
10. 28
11. -1
12. [ "c" ]

For people who work with JavaScript more than occasionally, score as follows:

(correct answers: score)

10 - 11: SAVANT
8 - 9: EXPERT
< 5: RUSTY


The answer to No. 2 is the same for JavaScript as for Java (or any other language that uses IEEE 754 floating point numbers), and it's one reason why you shouldn't use floating point arithmetic in any serious application involving monetary values. There's an interesting overview here.

No. 6: In an arithmetic expression involving multiplication, division, and/or subtraction, if the expression contains one or more strings, the interpreter will try to cast the strings to numbers first. If the arithmetic expression involves addition, however, all terms will be cast to strings.

No. 7: What you've got here is "a, post-incremented, plus b," not "a plus pre-incremented b."

No. 9: toString( ) takes a numeric argument (optionally, of course). An argument of "16" means base-16, hence the returned string is a hex representation of 16, which is "10." If you write .toString(2), you get a binary representation of the number, etc.

No. 10: 016 is octal notation for 14 decimal. Interestingly, though, the interpreter will treat "016" (in string form) as base-ten if you multiply it by one.

Don't feel bad if you didn't do well on this quiz, because almost every question was a trick question (obviously), and let's face it, trick questions suck ass. OTOH, if you did well on a test that sucks, you should take full credit. It means you're no fool.

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