5

As you can see in this javascript equality table, javascript equality transitivity is weird.

I understand most of the results of the table, but I have a question regarding the following comparison:

[] == true // false
[] == false // true
!![] == true // true
!![] == false // false

But also:

if ([]) {
    // this code will run
}

if (!![]) {
    // and of course this too
}

Why? Is this another extremly bad part? Have this a reasonable explanation?

Improve this question
8
  • 2
    @Tomalak: I dare you to run [] == false in your console :-P Commented Jan 2, 2015 at 20:17
  • 2
    When using ==, it needs to convert both operands to the same type. So what it does is converts [] to a string ('') and then to a number (0). It then converts false to a number (0), then compares them. I'm still trying to figure out why !![] is true when [] == false is also true. Commented Jan 2, 2015 at 20:19
  • 1
    Look into Truthy and Falsy comparisons sitepoint.com/javascript-truthy-falsy Commented Jan 2, 2015 at 20:19
  • You're forgetting that JavaScript is dynamically typed : !![] == [] is false. Please use === or length, typeof foo === "undefined". Commented Jan 2, 2015 at 20:20
  • The fun part of this question (and JavaScript's stupid truth table) is that (!![]) === ([] == false) is true :) Commented Jan 2, 2015 at 20:20
Related questions
12232
How can I remove a specific item from an array in JavaScript?
8540
What does "use strict" do in JavaScript, and what is the reasoning behind it?
5801
Loop (for each) over an array in JavaScript
Load 7 more related questions Show fewer related questions

0

Reset to default

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.