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.

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parseInt(string)

parseInt(string, radix)

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

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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:

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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:

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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:

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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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