27 JS Questions

Begin
Original URL: http://javascript-puzzlers.herokuapp.com/
This article is the topic from the above URL, parsing is written by oneself

1. ["1", "2", "3"].map (parseint)
A ["1", "2", "3"] B [0,1,3] C [+] D Other
Select D ohter that is [1,nan,nan]:p arseint ("string", Redix), Redix represents the cardinality of the number to parse. The value is between 2 and 36, which is the binary number, and the map function passes in 3 parameters (Item,index,array), so [parseint ("1", 0), parseint ("1", 1), parseint ("1", 2)] When Redix is 0 o'clock, The default decimal, if it starts with "0x" or "0X", will be base 16.

2. [typeof null, NULL instanceof Object]
A ["Object", False] B [Null,false] C ["Object", True] D other
Select a null type is an object, typeof is a type that shows NULL, but NULL is not a data type with object Nature null Instanceof object is False
3. [[3,2,1].reduce (Math.pow), [].reduce (MATH.POW)]
A error b[9,0] C[9,nan] d[9,undefined]
Select A [9, error] [2,3].reduce (MATH.POW) =2^3=8

4. var val = ' Smtg ';
Console.log (' Value is ' + (val = = = ' Smtg ')? ' Something ': ' Nothing ');
Answer: Something
Parse: ' Value is ' + (val = = = ' Smtg ') is string true, so print something

5.
var name = ' world! ';
(function () {
if (typeof name = = = ' undefined ') {
var name = ' Jack ';
Console.log (' Goodbye ' + name);
} else {
Console.log (' Hello ' + name);
}
})();
Answer: Goodbye Jack parsing: If there are also variable declarations in advance

6.
var END = Math.pow (2, 53);
var START = END-100;
var count = 0;
for (var i = START; I <= END; i++) {
count++;
}
Console.log (count);
A: Count will enter an infinite loop 2^53 too large, start no matter how much plus one equals end

7.
var ary = [0,1,2];
ARY[10] = 10;
Ary.filter (function (x) {return x = = = undefined;});
Answer: [] Parse: Filter will jump directly to the number of non-existent parts of the value, so the value meets the requirements

8.
var = 0.2
var one = 0.1
var eight = 0.8
var six = 0.6
What are the following results?
[Two-one = = one, Eight-six = = one]
Answer: [True,false] two-one=0.1 0.8-0.6=0.20000000000000007 0.1+0.2=0.30000000000000004

9.
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 '));
A: ' does not know! ' switch uses = = =, new String (' A ') is the object
10.
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 '));
Answer: There's nothing to say case a
11.
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, ' + ',-9, Infinity];
Values.map (Issane);//? Return array yes?
Answer [True,,true,true,false,false] -9%2 is-1, ' 13 '%2 will automatically transition to number,infinity%2 to Nan
12.
parseint (3, 8)
parseint (3, 2)
parseint (3, 0)
Answer: [3,nan,3] said above

Array.isarray (Array.prototype)
Answer: True array prototypes are also arrays

14.
var a = [0];
if ([0]) {
Console.log (A = = true);
} else {
Console.log ("Wut");
}
A: false parsing: in if ([0]) as a Boolean value of True,[0]==true as 0 is false

15. []==[]
A: False parsing is not good to know: https://www.zhihu.com/question/29615998

16.
' 5 ' + 3
' 5 '-3
A: ' 53 ', 2 ' + ' to make 3 into a string, but '-' to convert the string to a number exp:-' 223 ' for-223

17.1 +-+ + +-+ 1
A: 2 seems to be no mystery of 1 +-+ + +-+-1 for 0 see the final symbol is added or minus

18.
var ary = Array (3);
ary[0]=2
Ary.map (function (elem) {return ' 1 ';}); /? The returned array is?
Answer: [1,,] parse: The map function skips the part with no value

19.
function sideffecting (ary) {
Ary[0] = ary[2];
}
function Bar (a,b,c) {
c = 10
sideffecting (arguments);
Return a + B + C;
}
Bar (1,1,1)
Answer: 21 Analysis: In sideffecting (arguments); input console.log (arguments); you will find that the print out {' 0 ': 10, ' 1 ': 1, ' 2 ': 10},
The arguments object is a collection of parameters, like an array but not an array, in the bar function can initially be considered to change 3 times, the arguments input is {' 0 ': 1, ' 1 ': 1, ' 2 ': 10} through the sideffecting function {' 0 ': 10, ' 1 ': 1, ' 2 ': 10} so a=10,b=1,c=10.

var a = 111111111111111110000,
b=1111;
A + b;
Answer 111111111111111110000: The accuracy problem in JS affects both decimal and large numbers, but if B = 111111111111111111111, then the result a+b is 222222222222222230000

var x = [].reverse;
X ();
Answer: Window search analysis: []. Reverse will return this, and if no explicit sink object is called, it defaults to this aka window

Number.min_value > 0
Answer: True the minimum value for resolving number is greater than 0

[1 < 2 < 3, 3 < 2 < 1]
Answer: [True,true] True is 1,false is 0

24.2 = = [[[2]]]
Answer: True parsing: Both sides are converted to strings

25.
3.toString ()
3..toString ()
3...toString ()
Answer: Error, "3", error

26.
(function () {
var x = y = 1;
})();
Console.log (y);
Console.log (x);

A: 1,error y is the global one to note

27.
var a =/123/,
b =/123/;
A = = B
A = = = B
Answer: False,false parsing: Matching objects are not equal

27 JS Questions