Number() vs parseInt()

Both Number() and parseInt() are often used to convert a string to number.

Differences

1. Number() converts the type whereas parseInt parses the value of input.

   // Parsing
   parseInt('32px'); // 32
   parseInt('5e1'); // 5
   
   // Convert type
   Number('32px'); // NaN
   Number('5e1'); // 50

   As you see, parseInt will parse up to the first non-digit character. On the other hand, Number will try to convert the entire string.

2. parseInt accepts two parameters. The second parameter is used to indicate the radix number.

   parseInt('0101'); // 101
   parseInt('0101', 10); // 101
   parseInt('0101', 2); // 5
   
   Number('0101'); // 101

3. They return different results when we passing special values such as undefined or null:

   parseInt(); // NaN
   parseInt(null); // NaN
   parseInt(true); // NaN
   parseInt(''); // NaN
   
   Number(); // 0
   Number(null); // 0
   Number(true); // 1
   Number(''); // 0

Good practices

1. Always pass the radix to parseInt.

   The parseInt method takes two parameters:

   parseInt(value, radix);

   The second parameter specifies the current numeral system. In the case it's not specified, then it will be set automatically based on the value.

   • If the value starts with 0x or 0X, then the radix is 16 (hexadecimal)
   • In other cases, the radix is 10 (decimal).

   In the older versions of JavaScript, if the string starts with 0 then the radix is set as 8 (octal).

   parseInt('0xF'); // 15
   parseInt('0XF'); // 15
   parseInt('0xF', 16); // 15
   
   parseInt('0xF', 10); // 0

   Since the method could be implemented differently in different versions of JavaScript and browsers, it's recommended to pass the radix number.

2. Trim the spaces before parsing the number.

   Both Number() and parseInt accept the spaces in input. But be aware that you could get different result when passing a value with spaces as following:

   parseInt('   5   '); // 5
   parseInt('12 345'); // 12, not 12345

   To avoid the similar situations, you should remove all spaces before parsing:

   parseInt(value.replace(/\s+/g, ''), 10);

3. Don't use new Number() to compare the numbers.

   Number('2') == 2; // true
   Number('2') === 2; // true
   
   new Number('2') == 2; // true
   new Number('2') === 2; // false
   
   const a = new Number('2');
   const b = new Number('2');
   
   a == b; // false
   a === b; // false

Tip

Instead of using the Number() constructor to convert a string to number, you can use the + operator:

+'010'; // 10
+'2e1'; // 20
+'0xF'; // 15