How to convert a string to a number

Short answer

The Number() function converts the whole string:

console.log(Number('42') + 1);

Output:

43

A string with extra characters

Number() returns NaN for such a string, while parseInt() and parseFloat() parse the digits at the start:

console.log(Number('42px'));
console.log(parseInt('42px', 10));
console.log(parseFloat('3.14em'));

Output:

NaN
42
3.14

Checking the result

NaN is not equal even to itself, so check for it with Number.isNaN():

const value = Number('abc');
 
console.log(Number.isNaN(value));

Output:

true

The unary plus

The + before a value works like Number(). It is short, but Number() is easier to read:

const input = '15';
 
console.log(+input + 5);

Output:

20

Common mistake

Values from forms are strings, and + with a string glues instead of adding. Convert before calculating:

const quantity = '2';
 
console.log(quantity + 1);
console.log(Number(quantity) + 1);

Output:

21
3