The JavaScript parseInt() function is used to get an integer value that can be extracted from other value types.
One of the most common uses of the parseInt() function is to get an integer from a string value as shown below:
let parsed = parseInt("50");
console.log(parsed); // 50
console.log(typeof parsed); // "number"
As you can see, the type of parsed variable is a number instead of a string.
The parseInt() method will return a NaN value when the given argument doesn’t have a valid integer representation.
The example below shows the parseInt() result of a boolean value and an empty string:
let parseA = parseInt(true);
let parseB = parseInt("");
console.log(parseA); // NaN
console.log(parseB); // NaN
When you pass an array of numbers, the parseInt() function will return the first element only.
This is because the function immediately returns the first integer value it can found from your argument:
let parseA = parseInt([28, 37, 61]);
console.log(parseA); // 28
let parseB = parseInt("32 years 8 months old");
console.log(parseB); // 32
The parseInt() function accepts a second number argument between 2 and 36 representing the base (or radix) numeral system used by the first argument.
For example, when you want to convert a binary number string to an integer, you can pass 2 as the function’s second argument:
let parseA = parseInt("1101", 2);
console.log(parseA); // 13
let parseB = parseInt("010", 2);
console.log(parseB); // 2
The binary numbers 1101 and 010 are converted to their base 10 representation with the parseInt() function.
When you omit the base number argument, the parseInt() function will consider the strings to be base 10 numbers.
You can also make the function think the argument is a base 16 number by adding 0X before the string as shown below:
let parseA = parseInt("FE8");
console.log(parseA); // NaN
let parseB = parseInt("0XFE8");
console.log(parseB); // 4072
The hexadecimal number FE8 represents the number 4072 in the base 10 system.
Without 0X in front of the values, parseInt() will consider the string to be a base 10 number. And the result is a NaN value.
Finally, the parseInt() function always return a whole number from any value you specify as its argument:
let parseA = parseInt("5.962");
console.log(parseA); // 5
let parseB = parseInt("32.07");
console.log(parseB); // 32
If you want a number with decimal points, you need to use the parseFloat() function instead of parseInt().
Consider the example below:
let parseA = parseFloat("5.962");
console.log(parseA); // 5.962
let parseB = parseFloat("21");
console.log(parseB); // 21
Now you’ve learned how to use the JavaScript parseInt() function. Great job! 😉