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How to convert number start with 0 to string equivalent of the value?

I want to convert a number start with 0 to string equivalent of the value.
If I run
var num = 12;
var int = num.toString();
console.log(int);
it logs 12 as expected but if I apply the toString() to a number start with 0 like,
var num = 012;
var int = num.toString();
console.log(int);
it logs 10, why?

Number starting with 0 is interpreted as octal (base-8).

In sloppy mode (the default) numbers starting with 0 are interpreted as being written in octal (base 8) instead of decimal (base 10). If has been like that from the first released version of Javascript, and has this syntax in common with other programming languages. It is confusing, and have lead to many hard to detect buggs.
You can enable strict mode by adding "use strict" as the first non-comment in your script or function. It removes some of the quirks. It is still possible to write octal numbers in strict mode, but you have to use the same scheme as with hexadecimal and binary: 0o20 is the octal representation of 16 decimal.
The same problem can be found with the function paseInt, that takes up to two parameters, where the second is the radix. If not specified, numbers starting with 0 will be treated as octal up to ECMAScript 5, where it was changed to decimal. So if you use parseInt, specify the radix to be sure that you get what you expected.
"use strict";
// Diffrent ways to write the same number:
const values = [
0b10000, // binary
0o20, // octal
16, // decimal,
0x10 // hexadecimal
];
console.log("As binary:", values.map( value => value.toString(2)).join());
console.log("As decimal:", values.join());
console.log("As ocal", values.map( value => value.toString(8)).join());
console.log("As hexadecimal:", values.map( value => value.toString(16)).join());
console.log("As base36:", values.map( value => value.toString(36)).join());

All you have to do is add String to the front of the number that is
var num = 12;
var int = String(num);
console.log(int);
And if you want it to look like this 0012 all you have to do is
var num = 12;
var int = String(num).padStart(4, '0');
console.log(int);

Syntax of "return +(test) && recursion" [duplicate]

I was wondering what the = +_ operator means in JavaScript. It looks like it does assignments.
Example:
hexbin.radius = function(_) {
if (!arguments.length)
return r;
r = +_;
dx = r * 2 * Math.sin(Math.PI / 3);
dy = r * 1.5;
return hexbin;
};

r = +_;
+ tries to cast whatever _ is to a number.
_ is only a variable name (not an operator), it could be a, foo etc.
Example:
+"1"
cast "1" to pure number 1.
var _ = "1";
var r = +_;
r is now 1, not "1".
Moreover, according to the MDN page on Arithmetic Operators:
The unary plus operator precedes its operand and evaluates to its
operand but attempts to converts it into a number, if it isn't
already. [...] It can convert string representations of integers and
floats, as well as the non-string values true, false, and null.
Integers in both decimal and hexadecimal ("0x"-prefixed) formats are
supported. Negative numbers are supported (though not for hex). If it
cannot parse a particular value, it will evaluate to NaN.
It is also noted that
unary plus is the fastest and preferred way of converting something into a number

It is not an assignment operator.
_ is just a parameter passed to the function.
hexbin.radius = function(_) {
// ^ It is passed here
// ...
};
On the next line r = +_; + infront casts that variable (_) to a number or integer value and assigns it to variable r
DO NOT CONFUSE IT WITH += operator

=+ are actually two operators = is assignment and + and _ is variable name.
like:
i = + 5;
or
j = + i;
or
i = + _;
My following codes will help you to show use of =+ to convert a string into int.
example:
y = +'5'
x = y +5
alert(x);
outputs 10
use: So here y is int 5 because of =+
otherwise:
y = '5'
x = y +5
alert(x);
outputs 55
Where as _ is a variable.
_ = + '5'
x = _ + 5
alert(x)
outputs 10
Additionally,
It would be interesting to know you could also achieve same thing with ~ (if string is int string (float will be round of to int))
y = ~~'5' // notice used two time ~
x = y + 5
alert(x);
also outputs 10
~ is bitwise NOT : Inverts the bits of its operand. I did twice for no change in magnitude.

It's not =+. In JavaScript, + means change it into number.
+'32' returns 32.
+'a' returns NaN.
So you may use isNaN() to check if it can be changed into number.

It's a sneaky one.
The important thing to understand is that the underscore character here is actually a variable name, not an operator.
The plus sign in front of that is getting the positive numerical value of underscore -- ie effectively casting the underscore variable to be an int. You could achieve the same effect with parseInt(), but the plus sign casting is likely used here because it's more concise.
And that just leaves the equals sign as just a standard variable assignment.
It's probably not deliberately written to confuse, as an experienced Javascript programmer will generally recognise underscore as a variable. But if you don't know that it is definitely very confusing. I certainly wouldn't write it like that; I'm not a fan of short meaningless variable names at the best of times -- If you want short variable names in JS code to save space, use a minifier; don't write it with short variables to start with.

= +_ will cast _ into a number.
So
var _ = "1",
r = +_;
console.log(typeof r)
would output number.

I suppose you mean r = +_;? In that case, it's conversion of the parameter to a Number. Say _ is '12.3', then +'12.3' returns 12.3. So in the quoted statement +_ is assigned to r.

_ is just a a variable name, passed as a parameter of function hexbin.radius , and + cast it into number
Let me make a exmple same like your function .
var hexbin = {},r ;
hexbin.radius = function(_) {
if (!arguments.length)
return r;
console.log( _ , typeof _ )
r = +_;
console.log( r , typeof r , isNaN(r) );
}
and run this example function .. which outputs
hexbin.radius( "1");
1 string
1 number false
hexbin.radius( 1 );
1 number
1 number false
hexbin.radius( [] );
[] object
0 number false
hexbin.radius( 'a' );
a string
NaN number true
hexbin.radius( {} );
Object {} object
NaN number true
hexbin.radius( true );
true boolean
1 number false

It Will assign new value to left side variable a number.
var a=10;
var b="asg";
var c=+a;//return 10
var d=-a;//return -10
var f="10";
var e=+b;
var g=-f;
console.log(e);//NAN
console.log(g);//-10

Simply put, +_ is equivalent to using the Number() constructor.
In fact, it even works on dates:
var d = new Date('03/27/2014');
console.log(Number(d)) // returns 1395903600000
console.log(+d) // returns 1395903600000
DEMO:
http://jsfiddle.net/dirtyd77/GCLjd/
More information can also be found on MDN - Unary plus (+) section:
The unary plus operator precedes its operand and evaluates to its operand but attempts to converts it into a number, if it isn't already. Although unary negation (-) also can convert non-numbers, unary plus is the fastest and preferred way of converting something into a number, because it does not perform any other operations on the number. It can convert string representations of integers and floats, as well as the non-string values true, false, and null. Integers in both decimal and hexadecimal ("0x"-prefixed) formats are supported. Negative numbers are supported (though not for hex). If it cannot parse a particular value, it will evaluate to NaN.

+_ is almost equivalent of parseFloat(_) . Observe that parseInt will stop at non numeric character such as dot, whereas parshFloat will not.
EXP:
parseFloat(2.4) = 2.4
vs
parseInt(2.4) = 2
vs
+"2.4" = 2.4
Exp:
var _ = "3";
_ = +_;
console.log(_); // will show an integer 3
Very few differences:
Empty string "" evaluates to a 0, while parseInt() evaluates to NaN
For more info look here: parseInt vs unary plus - when to use which

In this expression:
r = +_;
'+' acts here as an unary operator that tries to convert the value of the right operand. It doesn't convert the operand but the evaluated value. So _ will stay "1" if it was so originally but the r will become pure number.
Consider these cases whether one wants to apply the + for numeric conversion
+"-0" // 0, not -0
+"1" //1
+"-1" // -1
+"" // 0, in JS "" is converted to 0
+null // 0, in JS null is converted to 0
+undefined // NaN
+"yack!" // NaN
+"NaN" //NaN
+"3.14" // 3.14
var _ = "1"; +_;_ // "1"
var _ = "1"; +_;!!_ //true
var _ = "0"; +_;!!_ //true
var _ = null; +_;!!_ //false
Though, it's the fastest numeric converter I'd hardly recommend one to overuse it if make use of at all. parseInt/parseFloat are good more readable alternatives.

parseInt not converting decimal to binary?

From my understanding the binary number system uses as set of two numbers, 0's and 1's to perform calculations.
Why does:
console.log(parseInt("11", 2)); return 3 and not 00001011?
http://www.binaryhexconverter.com/decimal-to-binary-converter

Use toString() instead of parseInt:
11..toString(2)
var str = "11";
var bin = (+str).toString(2);
console.log(bin)
According JavaScript's Documentation:
The following examples all return NaN:
parseInt("546", 2); // Digits are not valid for binary representations

parseInt(number, base) returns decimal value of a number presented by number parameter in base base.
And 11 is binary equivalent of 3 in decimal number system.
var a = {};
window.addEventListener('input', function(e){
a[e.target.name] = e.target.value;
console.clear();
console.log( parseInt(a.number, a.base) );
}, false);
<input name='number' placeholder='number' value='1010'>
<input name='base' placeholder='base' size=3 value='2'>

As stated in the documentation for parseInt: The parseInt() function parses a string argument and returns an integer of the specified radix (the base in mathematical numeral systems).
So, it is doing exactly what it should do: converting a binary value of 11 to an integer value of 3.
If you are trying to convert an integer value of 11 to a binary value than you need to use the Number.toString method:
console.log(11..toString(2)); // 1011

.toString(2) works when applied to a Number type.
255.toString(2) // syntax error
"255".toString(2); // 255
var n=255;
n.toString(2); // 11111111
// or in short
Number(255).toString(2) // 11111111
// or use two dots so that the compiler does
// mistake with the decimal place as in 250.x
255..toString(2) // 11111111

The parseInt() function parses a string argument and returns an integer of the specified radix (the base in mathematical numeral systems).
So you are telling the system you want to convert 11 as binary to an decimal.
Specifically to the website you are referring, if you look closer it is actually using JS to issue a HTTP GET to convert it on web server side. Something like following:
http://www.binaryhexconverter.com/hesapla.php?fonksiyon=dec2bin&deger=11&pad=false

The shortes method I've found for converting a decimal string into a binary is:
const input = "54654";
const output = (input*1).toString(2);
print(output);

I think you should understand the math behind decimal to binary conversion. Here is the simple implementation in javascript.
main();
function main() {
let input = 12;
let result = decimalToBinary(input);
console.log(result);
}
function decimalToBinary(input) {
let base = 2;
let inputNumber = input;
let quotient = 0;
let remainderArray = [];
let resultArray = [];
if (inputNumber) {
while (inputNumber) {
quotient = parseInt(inputNumber / base);
remainderArray.push(inputNumber % base);
inputNumber = quotient;
}
for (let i = remainderArray.length - 1; i >= 0; i--) {
resultArray.push(remainderArray[i]);
}
return parseInt(resultArray.join(''));
} else {
return `${input} is not a valid input`;
}
}

This is an old question, however I have another solution that might contribute a little bit. I usually use this function to convert a decimal number into a binary:
function dec2bin(dec) {
return (dec >>> 0).toString(2);
}
The dec >>> 0 converts the number into a byte and then toString(radix) function is called to return a binary string. It is simple and clean.
Note: a radix is used for representing a numeric value. Must be an integer between 2 and 36. For example:
2 - The number will show as a binary value
8 - The number will show as an octal value
16 - The number will show as an hexadecimal value

function num(n){
return Number(n.toString(2));
}
console.log(num(5));

This worked for me: parseInt(Number, original_base).toString(final_base)
Eg: parseInt(32, 10).toString(2) for decimal to binary conversion.
Source: https://www.w3resource.com/javascript-exercises/javascript-math-exercise-3.php

Here is a concise recursive version of a manual decimal to binary algorithm:
Divide decimal number in half and aggregate remainder per operation until value==0 and print concatenated binary string
Example using 25: 25/2 = 12(r1)/2 = 6(r0)/2 = 3(r0)/2 = 1(r1)/2 = 0(r1) => 10011 => reverse => 11001
function convertDecToBin(input){
return Array.from(recursiveImpl(input)).reverse().join(""); //convert string to array to use prototype reverse method as bits read right to left
function recursiveImpl(quotient){
const nextQuotient = Math.floor(quotient / 2); //divide subsequent quotient by 2 and take lower limit integer (if fractional)
const remainder = ""+quotient % 2; //use modulus for remainder and convert to string
return nextQuotient===0?remainder:remainder + recursiveImpl(nextQuotient); //if next quotient is evaluated to 0 then return the base case remainder else the remainder concatenated to value of next recursive call
}
}

To get better understanding, I think you should try to do the math of that conversion by yourself.
(1) 11 / 2 = 5
(1) 5 / 2 = 2
(0) 2 / 2 = 1
(1) 1 / 2 = 0
I made a function based on that logic
function decimalToBinary(inputNum) {
let binary = [];
while (inputNum > 0) {
if (inputNum % 2 === 1) {
binary.splice(0,0,1);
inputNum = (inputNum - 1) / 2;
} else {
binary.splice(0,0,0);
inputNum /= 2;
}
}
binary = binary.join('');
console.log(binary);
}

This is what I did to get the solution:
function addBinary(a,b) {
// function that converts decimal to binary
function dec2bin(dec) {
return (dec >>> 0).toString(2);
}
var sum = a+b; // add the two numbers together
return sum.toString(2); //converts sum to binary
}
addBinary(2, 3);
I first converted the decimal number to binary like it said, and I got the function from w3schools under the JavaScript Bitwise lesson. Then to make it easier on myself, I created the variable "sum" which does the addition and finally, I made the addBinary function return the sum as a binary code, then called it. It passed in CodeWars. I hope this makes sense and it helps you.

Just use Number(x).toString(base). Where base needs to be equals 2.
var num1=13;
Number(num1).toString(2)
result: "1101"
Number(11).toString(2)
result: "1011"

It seems like the conversion with the string radix (dec >>> 0).toString(2) is returning the binary number formatted in the wrong direction. I have validated this solution in Chrome. In case anyone wants to manually calculate binary for validation, from left to right you add the numbers together that correspond to a 1 position in your binary number mapping to [1][2][4][8][16][32][64][128] ....
For example:
10 in binary is 0101 OR 0 + 2 + 0 + 8.
13 in binary is 1011 OR 1 + 0 + 4 + 8.
255 in binary is 11111111 OR 1 + 2 + 4 + 8 + 16 + 32 + 64 + 128
function dec2bin(dec){
return (dec >>> 0).toString(2).split('').reverse().join('');
}

This will give the decimal to binary:
let num = "1234"
console.log(num.toString(2));
This will give binary to decimal:
let num = "10011010010";
console.log(parseInt(num, 2));

How to format numbers in javascript to two decimal digits?

I need to format numbers to two decimal digits in javascript. In order to do this I am using toFixed method which is working properly.
But in cases, where numbers don't have any decimal digits, it should not show decimal point
e.g. 10.00 should be 10 only and not 10.00.

.toFixed() converts the result to String,
so you need to convert it back to Number:
parseFloat( num.toFixed(2) )
or by simply using the Unary +
+num.toFixed(2)
both will give the following:
// 15.00 ---> 15
// 15.20 ---> 15.2
If you only want to get rid of the .00 case, than you can go for String manipulation using .replace()
num.toFixed(2).replace('.00', '');
Note: the above will convert your Number to String.

As an alternative to make this change global(if you need, of course), try this:
var num1 = 10.1;
var num2 = 10;
var tofixed = Number.prototype.toFixed;
Number.prototype.toFixed = function(precision)
{
var num = this.valueOf();
if (num % 1 === 0)
{
num = Number(num + ".0");
}
return tofixed.call(num, precision);
}
console.log(num1.toFixed(2));
console.log(num2.toFixed(2));
Fiddle. This is a mix of this and this post.

Why does parseInt() return an unexpected result when I pass a string with a leading zero?

parseInt('1') = 1
parseInt('01') = 1
parseInt('5') = 5
parseInt('05') = 5
parseInt('8') = 8
But why:
parseInt('08') = 0
parseInt('09') = 0

Always use the overload that takes the radix as the second parameter:
parseInt('011',10) = 11

here is a hint:
parseInt('011') = 9
If it's starting with 0 it's parsed as an octal number.

Numbers start with 0(not 0x) are octal numbers. Therefore 8 and 9 are not a valid octal numbers.

When converting a string into decimal number in JavaScript, you may use the plus operator instead parseInt or parseFloat, like in this example:
x = +("12"); // to x is assigned a number 12, after its conversion from a string "12"
x = +("1" + "2"); // to x is assigned a number 12, after its conversion from a string "12"

Others have answered why part.
To remove, you can use parseFloat() or parseInt(value, base)
Fiddle