JavaScript How to Convert String to Integer Easily

You can convert a string to an integer in JavaScript using parseInt(string) or Number(string), for example: parseInt('123') returns 123.

Examples

Input'123'

Output123

Input'42abc'

Output42

Input'abc123'

OutputNaN

How to Think About It

To convert a string to an integer, think about extracting the number part from the string. Use parseInt to read the number from the start of the string until it hits a non-digit. If the string starts with digits, it returns those as a number; otherwise, it returns NaN. Alternatively, Number tries to convert the whole string strictly to a number.

Algorithm

1

Take the input string.

2

Use a function that reads the string and extracts the integer value.

3

If the string starts with digits, convert those digits to an integer.

4

If the string does not start with digits, return NaN.

5

Return the integer value.

Code

javascript

const str1 = '123';
const str2 = '42abc';
const str3 = 'abc123';

console.log(parseInt(str1)); // 123
console.log(parseInt(str2)); // 42
console.log(parseInt(str3)); // NaN

Output

123
42
NaN

Dry Run

Let's trace '42abc' through the code

1

Input string

'42abc'

2

Apply parseInt

Reads '42' as number, stops at 'a'

3

Return result

42 as integer

Step	String Read	Result
1	'42abc'	N/A
2	'42'	42
3	N/A	42

Why This Works

Step 1: parseInt reads from the start

The parseInt function reads the string from the beginning and converts digits until it finds a non-digit.

Step 2: Stops at first non-digit

When parseInt encounters a non-digit character, it stops reading and returns the number parsed so far.

Step 3: Returns NaN if no digits at start

If the string does not start with digits, parseInt returns NaN to indicate conversion failure.

Alternative Approaches

Number() function

javascript

const str = '123';
console.log(Number(str)); // 123

Converts the whole string strictly; returns NaN if any non-digit characters exist.

Unary plus operator

javascript

const str = '123';
console.log(+str); // 123

A shorthand to convert string to number; behaves like Number() and returns NaN if invalid.

Complexity: O(n) time, O(1) space

Time Complexity

Conversion functions scan the string once, so time grows linearly with string length.

Space Complexity

No extra memory is needed besides a few variables, so space is constant.

Which Approach is Fastest?

parseInt, Number, and unary plus all run in similar time; choose based on input format.

Approach	Time	Space	Best For
parseInt	O(n)	O(1)	Strings with numbers followed by text
Number	O(n)	O(1)	Strict numeric strings only
Unary plus	O(n)	O(1)	Quick conversion of clean numeric strings