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JavaScript: What's the output
For each given JavaScript code snippet, determine the correct output to the console.
Warning: these questions largely test for quirks of syntax which you have hopefully never had to encounter before.
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Last updated: February 3, 2022
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| First submitted | February 3, 2022 |
| Times taken | 399 |
| Average score | 26.7% | Report this quiz | Report |
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1. let x = 1;
console.log(x++);
console.log(x++);
x++ returns x before incrementing.
0
1
2
Uncaught SyntaxError
2. console.log(0 || 5);
The || operator returns the first argument if it is truthy, otherwise it returns the second argument.
false
true
0
5
3. console.log(-1 / 0);
1/0 returns Infinity, so its negative returns -Infinity.
NaN
Infinity
-Infinity
Uncaught FloatingPointError
4. let x = 3, y = 5;
console.log(x ^ y);
console.log(x ^ y);
^ is the bitwise XOR operator. 11 XOR 101 = 110.
6
15
243
2
5. let s = "";
for (const i in [0,0,0])
s += 1 + i;
console.log(s);
for (const i in [0,0,0])
s += 1 + i;
console.log(s);
A for (.. in ..) loop loops over the keys of the object as strings, so this loops over the strings "0", "1" and "2".
101112
111
101010
123
6. console.log(! 5 < 6);
Operator precedence: !5 gets evaluated first, which becomes false. false < 6 is true.
true
5
Uncaught SyntaxError
false
7. console.log("" + Array(10));
Adding an array to a string joins the array by commas. All elements are empty, so this just becomes a string of 9 commas.
[,,,,,,,,,]
[empty × 10]
,,,,,,,,,
Array(10)
8. let s = "";
for (let i = 5; i--, i > 0; )
s += i;
console.log(s);
for (let i = 5; i--, i > 0; )
s += i;
console.log(s);
The conditional of the loop contains two separate expressions. The conditional gets evaluated as first step of the loop after the initialization. Both expressions get evaluated, but only the latter actually functions as a condition to terminate the loop.
54321
43210
543210
4321
9. const x = 1n;
console.log(x + 1);
console.log(x + 1);
1n is a BigInt. Adding a BigInt to a Number is not allowed, so this gives an Uncaught TypeError.
2n
Uncaught TypeError
2
1n1
10. console.log(![]);
For some reason arrays are always truthy, even when they are empty.
[]
false
true
1
11. console.log(011);
Prefixing a number with a 0 makes it octal.
10
12
9
13
12. const x = {
a: 1,
a: 2
};
console.log( JSON.stringify(x) );
a: 1,
a: 2
};
console.log( JSON.stringify(x) );
JSON format requires the key to be between quotation marks. "a":1 gets overwritten by "a":2.
{a:2}
{"a":1,"a":2}
{"a":2}
{"a":1}
13. const x = "x" - 1;
const y = "y" - 1;
console.log(x == y);
const y = "y" - 1;
console.log(x == y);
Subtracting a number from a string that cannot be parsed to a number returns NaN. NaN == NaN is false.... somehow.
true
NaN
Uncaught SyntaxError
false
14. console.log(5 - 3 + "e2" - 1);
This gets evaluated from left to right. 5 - 3 == 2. 2 + "e2" == "2e2". 2e2 is exponential notation, which can be parsed from a string to a number, and becomes 200. Therefore "2e2" - 1 == 200 - 1, and 200 - 1 == 199.
Uncaught SyntaxError
199
2e
NaN
15. // WARNING: hard
let s = "";
for (let i = 5, j = 0, r = 1; j += r *= j < i || -1;)
s += j;
console.log(s);
let s = "";
for (let i = 5, j = 0, r = 1; j += r *= j < i || -1;)
s += j;
console.log(s);
So long as r remains equal to 1, j gets incremented by 1 over every iteration. When j == 5, j < i will be false. At this point r will be multiplied by -1. From that point on, j will decrement until it reaches 0 and the loop exits.
12345
Infinite Loop
Uncaught SyntaxError
"a"
0123
123454321
[0]
25
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1 Comments
garcogait
+1
Level 67
Mar 3, 2024
Fun lol i. suck
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