--- title: "\"JavaScript\" != \"Javascript\"" url: "https://www.bekk.christmas/post/2020/22/%22javascript%22-!-%22javascript%22" authors: ["Charlie Midtlyng"] published: "2020-12-22" updated: "2024-11-07T13:05:28Z" language: "en-US" type: "article" tags: ["JavaScript"] --- # "JavaScript" != "Javascript" When I studied computer science for five years there was a lot of mathematics. Equality was probably the simplest part. After ten years of working, I’ve done a lot of JavaScript. Equality is not the simplest part… Why? ```javascript false == 0 // true ['1', '2', '3'] == '1,2,3' // true undefined == null // true "wtf" == "WTF" // false ``` ## In short JavaScript equality has two modes: `===` compares values without coercion, while `==` applies a long set of type-conversion rules across strings, numbers, booleans, objects, `null`, `undefined`, and `BigInt`. Understanding those rules matters when refactoring or working in codebases that already use `==`, but `===` is the safer default. - **For:** JavaScript developers - **Good to know beforehand:** Basic JavaScript knowledge _AI-generated_ First of all, JavaScript has two operators for equality: `==` and `===`. In order to understand equality, we have to understand the difference between these two options. As we know equality from other programming languages and mathematics, `===` behaves the way we are used to. So, if you’re able to keep `==` away from the JavaScript-codebase you’ll probably save yourself from unnecessary heart disease. Unfortunately, sooner or later we have to make changes in a codebase where `==` is commonly used - then it’s important to understand the consequences of making changes! ## x == y There are a lot of rules to be aware of when using `==`. Let’s take a walkthrough of these rules and try to understand them by some examples. #### 1 If `x` and `y` is the same data type the behavior of `==` is the same as `===` ```javascript 1==1 => 1===1 "abc" == "ABC => "abc" === "ABC" [1,2,3] == [1,2,3] => [1,2,3] === [1,2,3] ``` This is a rule that can make refactoring `==` to `===` a bit easier. If you expect both values to be the same data type - add another equal sign and breathe! #### 2 Truthy if `x` and `y` is `undefined` and `null` ```javascript null == undefined // true undefined == null // true null === undefined // false ``` `null` and `undefined` are special cases and we have to accept that they are equal when using double-quotes. But I’m not sure if I like it... #### 3 If `x` and `y` are the data types number and string - try to convert the string into number and perform `==` ```javascript '2' == 2 => Number('2') == 2 // true '2.5' == 2.5 => Number('2.5') == 2.5 // true '' == 0 => Number('') == 0 // true 'evil' == 666 => Number('evil') == 666 // false BigInt(2) == "2" -> BigInt(2) == BigInt("2") // true ``` If one value is a string type and the other is numeric, JavaScript compares the numeric value of the string. Remember, converting an empty string to a number results in `0`. #### 4 If `x` or `y` is a boolean type - convert it to a number and perform `==` ```javascript false == 0 => 0 == 0 // true false == '0' => 0 == '0' => 0 == 0 // true true == 1 => 1 == 1 // true false == '' => 0 == '' => 0 == 0 // true ``` Boolean values converted to a number type results in 0 (`false`) or 1 (`true`). Be aware of the second line, after converting `false` to 0 the comparison is now a numeric and string data type. Fortunately, you have just learned how this works! #### 5 If `x` and `y` is respectively an Object and either String, Number, Bigint or Symbol - convert the Object to primitive and perform `==` ```javascript ['1', '2', '3'] == '1,2,3' => ['1', '2', '3'].toString() === '1,2,3' // true [1,2,3] == '1,2,3' => true const me = { toString() { return "lazy"; }, valueOf() { return "1337" } } const you = { toString() { return "lazy"; }, valueOf() { return { key: "1337" } } } me == "lazy" // false me == "1337" // true me == [1,3,3,7] // false you == "lazy" // true you == {key: "1337"} // false me == BigInt(1337) // true me == 1337 // true ``` Converting the data type object into primitive is usually done by returning the value of `valueOf()`-function. If this is not implemented or does not return a primitive value, then it returns the value of `toString()`-function. #### 6 If `x` and `y` is BigInt and Number then return `true` if the mathematical value is the same ```javascript 123 == BigInt(123) // true 999 == BigInt(123) // false ``` #### 7 If none of the above applies, return `false`! ```javascript [1,2,3] == { valueOf() { return "1,2,3" } } // false since both are objects 0 == null // false 0 == NaN // false 0 == undefined // false null == false // false NaN == false // false undefined == false // false ``` As mentioned above, avoid using `==` in your JavaScript code. Unless all of the rules above fits in your, and your teammates, head - please use `===`! ## Relevant resources recommended by the author - [JavaScript for impatient programmers](https://exploringjs.com/impatient-js/downloads/impatient-js-preview-book.pdf) - [Abstract equality comparison](https://www.ecma-international.org/ecma-262/#sec-abstract-equality-comparison) - [Object-to-primitive conversions](http://www.adequatelygood.com/Object-to-Primitive-Conversions-in-JavaScript.html)