## DEV Community

Alessio Michelini

Posted on

# You should stop using `parseInt()`

This is something I see all the times, you have some code where at some point you have a variable that contains a number, maybe it comes from a form, or from the results of some API calls, or whatever, and you want convert it into an integer.

Something like this for example:

``````const myNumber = '1';

if (parseInt(myNumber, 10) === 1) {
// do something
}
``````

While the code above works, it’s an extremely inefficient way to parse it.
What you should use instead is the `Number()` function, and convert the code above to this:

``````const myNumber = '1';

if (Number(myNumber) === 1) {
// do something
}
``````

From the changes above you gain two things:

• a much more performing way to transform a string to an integer

## But what’s the difference between `Number` and `parseInt`?

The `Number(string)` function evaluate the full string and converts it to a string, and if the string is not a number it will just return `NaN`.
While `parseInt(string, [radix])` will try to find the first number in the string passed, and convert it to the radix passed, which is `10` by default, and it will return `NaN` only if it doesn’t find any number.

This means that if you pass a string like `5e2` , it `parseInt` will stop when it sees the `e` and it will just return `5`, while `Number` will evaluate the whole string and return the correct value `500`.

Here you can see some cases compared between the two functions:

``````console.log(Number('a')); // NaN
console.log(Number('1')); // 1
console.log(Number('5e2')); // 500
console.log(Number('16px')); // NaN
console.log(Number('3.2')); // 3.2

console.log(parseInt('a')); // NaN
console.log(parseInt('1')); // 1
console.log(parseInt('5e2')); // 5
console.log(parseInt('16px')); // 16
console.log(parseInt('3.2')); // 3
``````

## It’s also a matter of execution time

Maybe you are still undecided, and you think that “I just need to convert a simple number to an integer, why I should use Number instead?”.
Well, because of performances.

For example, let’s do a simple function, that loops for 100m times, and that accepts a callback, and we call it twice using `Number` in the first case and `parseInt` in the second.

``````function runBench(cb) {
const start = new Date();
for (let i = 0; i < 100000000; i++) {
cb();
}
const end = new Date();
console.log(`It took \${end - start} ms`);
}

const process1 = () => Number('3.2');
const process2 = () => parseInt('3.2', 10);

runBench(process1); // It took 140 ms
runBench(process2); // It took 4546 ms
``````

Sure, you are not going to run a loop of 100 millions, but it’s to make evident the performance difference between the two functions, and also when you use `parseInt` in multiple places on the same function, things might just sum up at the end.

## So should I just avoid `parseInt` all the times?

No, not always, there are of course use cases where it’s beneficial to use it, for example if you want to extrapolate an integer out of a floating number, which is a good 50% faster than `Math.round()`.
For example ifyou want to convert a string with pixels on it to just a number, like `32px` to `32`, then you should use `parseInt`, but most of the times you better stick with `Number` instead.
Or even if you want to convert a number from a decimal system to something else.

## Conclusions

Unless some specific cases, where `parseInt` returns what you need and `Number` doesn’t, for 99% of the cases you should better start to use the latter one.

## Update: a few more benchmarks

Just to give a broader picture as there are more ways to convert a string to a number, I also added tests using `parseFloat` and the Unary operator, here there results:

``````function runBench(cb) {
const start = new Date();
for (let i = 0; i < 100000000; i++) {
cb();
}
const end = new Date();
console.log(`It took \${end - start} ms`);
}

const process1 = () => Number('1');
const process2 = () => parseInt('1', 10);
const process3 = () => parseFloat('1');
const process4 = () => +'1';

runBench(process1); // It took 70 ms
runBench(process2); // It took 4552 ms
runBench(process3); // It took 5082 ms
runBench(process4); // It took 412 ms
``````

As you can see above, using the `Number()` is still the fastest way to do the conversion.

Kyle Stephens • Edited

Unless you're doing this in the order of hundreds of thousands, performance is neglible (I've rarely come across use cases like this in prod systems)

While it's nice to think about these things and how you write code, I will always push back on people looking to enforce issues or 'standards' like this in PRs because they're rather subjective POV on style or rely on contrived performance benefits. If this seems like a harsh response, I apologise, but I'm wary of articles with titles like you 'should' be doing this, or you 'must not' do this, etc.

Max Ziebell

This is so true. Instead of pushing back just add a math.round to the Number() and the readability suffers and probably the performance too. I agree, the suggested „optimization“ is not worth implementing and worth thinking about in most real world use cases.

there are of course use cases where it’s beneficial to use it, for example if you want to extrapolate an integer out of a floating number, which is a good 50% faster than `Math.round()`.

Two things here -

1. `parseInt` doesn't do any rounding, only truncation. The equivalent `Math` function would be `Math.trunc`:

``````Math.round('0.999') // 1
Math.trunc('0.999') // 0
parseInt('0.999')   // 0

Math.round('-0.999') // -1
Math.trunc('-0.999') // -0
parseInt('-0.999')   // -0
``````
2. Running benchmarks in Chrome, the performance benefit of `parseInt` over `Math.round` is reversed if you explicitly convert to a number first:

``````const bench = (desc, cb) => {
const start = new Date()

for (let i = 0; i < 1e7; ++i) {
cb()
}

const end = new Date()

console.log(desc, `\${end - start} ms`)
}

bench('parseInt',       () => parseInt('3.2', 10))       // 374 ms
bench('round coerced',  () => Math.round('3.2'))         // 738 ms
bench('round explicit', () => Math.round(Number('3.2'))) //  70 ms
bench('trunc coerced',  () => Math.trunc('3.2'))         // 671 ms
bench('trunc explicit', () => Math.trunc(Number('3.2'))) //  62 ms
``````

Kudos!

Alessio Michelini

Good to know, thanks!

Andrei Dascalu

Ok, but you did sum up the issue yourself in the examples. ParseInt will order to an integer whereas Number will get the proper numeric representation. Which means the outcome ma not be an int, which is what ParseInt guarantees .
The use of ParseInt can be replaced with Number if and only if it's accompanied by Math.round. This is not an edge case or an odd use case, it's the missing piece to have the intended outcome: an integer. The odd case is requiring the use of radix.

Alessio Michelini

True, in fact if you want to take a string that could contain a float, and you want an integer, in that case you want to use that, but I'm not talking about that case, I'm talking about having the actual number correctly translated from a string to a number, which is probably the most common case.

Andrei Dascalu

My point is mostly:

• the performance comparison is moot because the functions discussed don't do the same thing
• the correct way to formulate the use case for Number is: you should use Number when you want to extract the correct numeric value in full from a string AND you don't care about the resulting type.
• use case for ParseInt: you want to ensure conversion to an integer OR the partial extraction of an integer (similar for float)

Andrei Dascalu

The most common case for ParseInt is to get the correct number from a string with the even expectation of getting a float .... from a function that has int in the name? I hardly believe that.
I have never seen in 20 years a case of parseInt used with the expectation of getting anything except an int.

Alessio Michelini

Unfortunately I see it using it for what Number is supposed to do, many, many times.
Btw, if you just need to get the integer (that is a positive number) from a string containing a float, then you should use double tilde operator, which does the same as `Math.floor`, but just 10 times faster (in Node at least), for example:

``````console.log(~~'3.2'); // returns 3
console.log(parseInt('3.2')); // returns 3
``````

Mark Goho

double tilde...🤯

lczw

Today is finally the day I could use the information from this article. I came across a case where I had to cast a string to number. Worked flawlessly and `Number()` provides a nice interface to work with 👍

Taufik Nurrohman • Edited
``````const myNumber = '1';
console.log(+myNumber);
``````
``````const myNumber = '1.5';
console.log(+myNumber);
``````

Shorter.

By the way, you will need `parseInt` anyway, if you want to deal with custom base number other than `10`. Example is to convert HEX color code into RGB color code:

``````console.log([
parseInt('ff', 16),
parseInt('a5', 16),
parseInt('00', 16)
]);
``````

Andreas Riedmüller

Keep in mind that both return different results when when passing nullish or boolean values:

``````parseInt(); // NaN
parseInt(null); // NaN
parseInt(''); // NaN
parseInt(true); // NaN

Number(); // 0
Number(null); // 0
Number(''); // 0
Number(true); // 1
``````

coolprofessor

What about `eval()`?

blackr1234 • Edited

It will be an overkill. There are so many working ways of parsing integers. Why bother using such a dangerous way?

hidden_dude

eval is dangerous..

coolprofessor • Edited

I get your point, but you can already run JS commands in the console. Also, you can use `str.includes("()")`.

blackr1234

However, end users being able to use console to execute any code doesn't necessarily mean that they will want to do it proactively. If you use `eval` and if the input is harmful, the end user may be passively affected.

coolprofessor

Potentially, but can't you check the string for functions using 'str.includes("()")'?

blackr1234 • Edited

If you are referring to checking if the string contains function call by searching for `"()"`, no it won't work because there are way too many scenarios. Consider a case when there are spaces in between the parenthesis, e.g. `foo( )` and your code will then allow it to run. It will be better if you only allow whitelisted characters. However, it will still take unnecessary effort and still potentially cause the program to hang (if you are going to search/parse the whole string which can be very long). So just use the built-in functions that work just fine and don't reinvent the wheel, which is something stupid.

hidden_dude

Don't do it.

eval() should never be used on user input.

Often parsing strings to Int is done for security reasons. Using eval() would just lead you to code injection and XSS problems.

Don't do it!

Jon Randy 🎖️

Or use +

Alessio Michelini

But thanks for pointing it out, I've updated the article adding the test results using the unary operator and `parseFloat`

Alessio Michelini

with the unary operator is still 5/6 times slower than `Number`

Jon Randy 🎖️

Interesting - I tested on Firefox and using the unary `+` was almost twice as fast as `Number` - jsbench.me/v2kurpao3r/1

Jon Randy 🎖️ • Edited

The same bench on Chrome showed the two methods almost exactly the same speed - sometimes one would be faster than the other, sometimes not

Alessio Michelini

This is interesting, I run the tests using Node (v14.17.6), in "theory" it should give you similar results to Chrome as they both use V8 as the engine, but it's clearly different

Mike Becker

This post is already a bit old, but the topic is still relevant. And the advice in this post is very much incomplete. Watch this:

``````Number('0x4') // 4
Number('-0x4') // NaN
Number(null) // 0
Number(undefined) // NaN
Number('') // 0
Number('  ') // 0
``````

The Number constructor is highly unreliable when it comes to interpreting user input from a form.

Unfortunately all oneliners in Javascript are broken. Nothing works. Neither `parseInt` nor `Number` nor any other implicit or explicit attempt to convert the value.

You always have to use a combination of different functions plus some manual checking for special cases like empty string, null, or undefined... otherwise you will always experience inconsistent behavior.

Domagoj Vidovic

This is cool, thanks for sharing.

Btw. for measuring the performance use `performance.now()` instead of `Date`, it's more precise.

Alessio Michelini

Pacharapol Withayasakpunt

Why not compare to parseFloat?

Anyway, parseInt forgivingness is so bad.

Alessio Michelini • Edited

parseFloat is even worst than parseInt if you just want to convert a string to a number

Athena Ozanich

Valid, and in the case of any kind of game development this could have a massive impact on the performance of said game.

jonosellier

FYI, `Math.round()` will round the number, not chop off the decimal. Use `floor` or better yet `|0` to drop the decimal

Jesse Phillips

5e2[...] Number will evaluate the whole string and return the correct value 500.

Are you saying it will return an http code of 500?

Alessio Michelini

ehm, no? :D

Jesse Phillips

Strange, it looked like something had gone horribly wrong if you got 500 out of 1506.

I can at least see how an algorithm can utilize 5, but 500 is mind-boggling wrong.

Clément

One thing to note that caugth me by surprise is that `Number` will parse any falsy value as `0`, so `undefined`, `null`, `false` all parse fine and return `0`, can be unexpected.