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Cover image for Destructuring Tweets - Episode 4 - Summing Up

Destructuring Tweets - Episode 4 - Summing Up

Kai
FE Engineer for en expensive cars company. Archer at night. Ask me about company culture, side projects, performance and Vue.
・2 min read

Good Sunday, welcome to my series about destructuring one of those frequently shared JavaScript quizzes on Twitter. Welcome to the fourth episode!

Snippet of the Week

This week's snippet is from Nacho Iacovino:

console.log(0.1 + 0.2);
console.log(0.1 + 0.2 == 3);
console.log(0.1 + 0.2 === 3);
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What we have here seems to be relatively simple at first glance. The author creates three console logs. The first featuring an addition of 0.1 and 0.2, the second and third compare this addition to 0.3 with two different operators.
The first operator == is called "Equal", the second === "Strict Equal". Both return a boolean value, indicating whether the first and second statement is the same. So, a console.log either outputs true or false.

The Output

Well, the output is easy here, isn't it? It should be:

  • 0.3
  • true
  • true

Welp, surprisingly enough, none of these is correct!
It's actually:

  • 0.30000000000000004
  • false
  • false

The two falses are evident in that context. Since the first output is this odd (pun intended) number, the addition is indeed not equal to 0.3.
We end up with the fundamental question: why the hell 0.1 + 0.2 equals 0.30000000000000004?

Analysis

For that question to answer, we need to know that computers need to cut off decimal numbers at some point. Given two floating-point numbers, JavaScript tries to return the same.
Now, let's stay in the decimal (digits from 0 to 9). How'd you expect a computer to comprehend the number ⅓? It can't just think about it as a repeating decimal and note it down as one. What you need to do at one point is to cut it off. The last digit gets rounded, and that's it.
As you may already know, computers work in binary (digits from 0 to 1). Of course, the same problem exists there! Let's write down 0.1 in binary:
0.0001100110011001100110011...
Notice how "0011" repeats itself over and over again? That's exactly the issue here. JavaScript reserves 52 bits for the fraction, and after that, the number gets cut off and rounded. That's where it gets slightly higher than 0.1.
Mathematical evidence is beyond my article's scope, but I will add a link with detailed information on that.

Snippet Summary

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