Why parseInt(0.0000005) returns 5 in JavaScript

An interesting occurrence showing the sheer weirdness of JavaScript:

Just when you thought you knew everything about parseInt:

The funny thing is it returns 0 for four and five zeroes after the decimal point.

But just add one more zero and it starts acting crazy?

The difference from Number is even more stark.

parseInt vs Number — four and five zeroes:

But:

parseInt vs Number — six zeroes:

But we figure it out eventually.

parseInt: The missing parts

By understanding how the parseInt function work:

It takes two arguments: the string and the base or radix.

JavaScript
parseInt(string) parseInt(string, radix)

Look what it does for 6 zeroes when it’s a string:

JavaScript
parseInt('0.5') // 0 parseInt('0.05') // 0 parseInt('0.005') // 0 parseInt('0.0005') // 0 parseInt('0.00005') // 0 parseInt('0.000005') // 0 // 6 parseInt('0.0000005') // 0 parseInt('015') // 15 parseInt('015', 8) // 13

But for Number:

JavaScript
Number('0.5') // 0.5 Number('0.05') // 0.05 Number('0.005') // 0.005 Number('0.0005') // 0.0005 Number('0.00005') // 0.00005 Number('0.000005') // 0.000005 // 6 ! Number('0.0000005') // 0.0000005 Number('015') // 15

But what happens when it’s a number?

A number like 0.0000005?

Step 1: Convert the number to a string

Number gets coerced to a string!

So look what parseInt does to 0.0000005:

JavaScript
String(0.5); // => '0.5' String(0.05); // => '0.05' String(0.005); // => '0.005' String(0.0005); // => '0.0005' String(0.00005); // => '0.00005' String(0.000005); // => '0.000005' String(0.0000005); // => '5e-7' 👈 this is important

Step 2: Do the actual rounding

So:

parseInt(0.0000005) parseInt('5e-7')

And now see what happens for '5e-7' for parseInt:

Why?

Because of how parseInt works: It interprets only the leading portion of the string as an integer value.

It doesn’t recognize e, so it ignores it and everything onwards.

This is same reason why:

  1. parseInt(999999999999999999999) parseInt(1e+21) 1
  2. parseInt(0.0000007) parseInt(7) 7

So Number is your best bet if you wanna avoid surprises when converting a string to a number

And if you need just the integer part, Math.floor() is here for you:

JavaScript
Math.floor(0.5); // 0 Math.floor(0.05); // 0 Math.floor(0.005); // 0 Math.floor(0.0005); // 0 Math.floor(0.00005); // 0 Math.floor(0.000005); // 0 Math.floor(0.0000005); // 0 ✅ perfect


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