How do `parseInt()`

and `Number()`

behave differently when converting strings to numbers?

# What is the difference between parseInt() and Number()?

Well, they are *semantically different*, the `Number`

constructor called as a function performs *type conversion* and `parseInt`

performs *parsing*, e.g.:

```
// parsing:
parseInt("20px"); // 20
parseInt("10100", 2); // 20
parseInt("2e1"); // 2
// type conversion
Number("20px"); // NaN
Number("2e1"); // 20, exponential notation
```

Keep in mind that if `parseInt`

detects a leading zero on the string, it will parse the number in octal base, this has changed on ECMAScript 5, the new version of the standard, but it will take a long time to get in browser implementations (it's an incompatibility with ECMAScript 3), also `parseInt`

will ignore trailing characters that don't correspond with any digit of the currently used base.

The `Number`

constructor doesn't detect octals:

```
Number("010"); // 10
parseInt("010"); // 8, implicit octal
parseInt("010", 10); // 10, decimal radix used
```

But it can handle numbers in hexadecimal notation, just like `parseInt`

:

```
Number("0xF"); // 15
parseInt("0xF"); //15
```

In addition, a widely used construct to perform Numeric type conversion, is the Unary `+`

Operator (p. 72), it is equivalent to using the `Number`

constructor as a function:

```
+"2e1"; // 20
+"0xF"; // 15
+"010"; // 10
```

```
typeof parseInt("123") => number
typeof Number("123") => number
typeof new Number("123") => object (Number primitive wrapper object)
```

first two will give you better performance as it returns a primitive instead of an object.

If you are looking for performance then probably best results you'll get with bitwise right shift `"10">>0`

. Also multiply (`"10" * 1`

) or not not (`~~"10"`

). All of them are much faster of `Number`

and `parseInt`

.
They even have "feature" returning 0 for not number argument.
Here are Performance tests.

I found two links of performance compare among several ways of converting `string`

to `int`

.

```
parseInt(str,10)
parseFloat(str)
str << 0
+str
str*1
str-0
Number(str)
```

One minor difference is what they convert of `undefined`

or `null`

,

```
Number() Or Number(null) // returns 0
```

while

```
parseInt() Or parseInt(null) // returns NaN
```

I always use parseInt, but beware of leading zeroes that will force it into *octal* mode.

## Summary:

`parseInt()`

:

- Takes a string as a first argument, the radix (An integer which is the base of a numeral system e.g. decimal 10 or binary 2) as a second argument
- The function returns a integer number, if the first character cannot be converted to a number
`NaN`

will be returned. - If the
`parseInt()`

function encounters a non numerical value, it will cut off the rest of input string and only parse the part until the non numerical value. - If the radix is
`undefined`

or 0, JS will assume the following:- If the input string begins with "0x" or "0X", the radix is 16 (hexadecimal), the remainder of the string is parsed into a number.
- If the input value begins with a 0 the radix can be either 8 (octal) or 10 (decimal). Which radix is chosen is depending on JS engine implementation.
`ES5`

specifies that 10 should be used then. However, this is not supported by all browsers,**therefore always specify radix if your numbers can begin with a 0.** - If the input value begins with any number, the radix will be 10

`Number()`

:

- The
`Number()`

constructor can convert any argument input into a number. If the`Number()`

constructor cannot convert the input into a number,`NaN`

will be returned. - The
`Number()`

constructor can also handle hexadecimal number, they have to start with`0x`

.

## Example:

```
console.log(parseInt('0xF', 16)); // 15
// z is no number, it will only evaluate 0xF, therefore 15 is logged
console.log(parseInt('0xFz123', 16));
// because the radix is 10, A is considered a letter not a number (like in Hexadecimal)
// Therefore, A will be cut off the string and 10 is logged
console.log(parseInt('10A', 10)); // 10
// first character isnot a number, therefore parseInt will return NaN
console.log(parseInt('a1213', 10));
console.log('\n');
// start with 0X, therefore Number will interpret it as a hexadecimal value
console.log(Number('0x11'));
// Cannot be converted to a number, NaN will be returned, notice that
// the number constructor will not cut off a non number part like parseInt does
console.log(Number('123A'));
// scientific notation is allowed
console.log(Number('152e-1')); // 15.21
```

`parseInt()`

->Parses a number to specified redix.
`Number()`

->Converts the specified value to its numeric equivalent or NaN if it fails to do so.

Hence for converting some non-numeric value to number we should always use Number() function.

eg.

```
Number("")//0
parseInt("")//NaN
Number("123")//123
parseInt("123")//123
Number("123ac") //NaN,as it is a non numeric string
parsInt("123ac") //123,it parse decimal number outof string
Number(true)//1
parseInt(true) //NaN
```

there are various corner case to `parseInt()`

functions as it does redix conversion, hence we should avoid using parseInt() function for coersion purposes.
Now,to check weather the provided value is Numeric or not,we should use native`isNaN()`

function

parseInt converts to a integer number, that is, it strips decimals. Number does not convert to integer.

Its a good idea to stay away from parseInt and use Number and Math.round unless you need hex or octal. Both can use strings. Why stay away from it?

```
parseInt(0.001, 10)
0
parseInt(-0.0000000001, 10)
-1
parseInt(0.0000000001, 10)
1
parseInt(4000000000000000000000, 10)
4
```

It completly butchers really large or really small numbers. Oddly enough it works normally if these inputs are a string.

```
parseInt("-0.0000000001", 10)
0
parseInt("0.0000000001", 10)
0
parseInt("4000000000000000000000", 10)
4e+21
```

Instead of risking hard to find bugs with this and the other gotchas people mentioned, I would just avoid parseInt unless you need to parse something other than base 10. Number, Math.round, Math.foor, and .toFixed(0) can all do the same things parseInt can be used for without having these types of bugs.

If you really want or need to use parseInt for some of it's other qualities, never use it to convert floats to ints.