Equality

While writing a program we frequently need to determine the equality of variables in relation to other variables. This is done using an equality operator. The most basic equality operator is the == operator. This operator does everything it can to determine if two variables are equal, even if they are not of the same type.

For example, assume:

let foo = 42;
let bar = 42;
let baz = "42";
let qux = "life";

foo == bar will evaluate to true and baz == qux will evaluate to false, as one would expect. However, foo == baz will also evaluate to true despite foo and baz being different types. Behind the scenes the == equality operator attempts to force its operands to the same type before determining their equality. This is in contrast to the === equality operator.

The === equality operator determines that two variables are equal if they are of the same type and have the same value. With the same assumptions as before, this means that foo === bar will still evaluate to true, but foo === baz will now evaluate to false. baz === qux will still evaluate to false.

let str1 = "hello";
let str2 = "HELLO";
let bool1 = true;
let bool2 = 1;
// compare using ==
let stringResult1 = str1 == str2 // false
let boolResult1 =  bool1 == bool2 // true

// compare using ===
let stringResult2 = str1 === str2 // false
let boolResult2 = bool1 === bool2 // false

assert(stringResult1 === false && stringResult2 === false && boolResult1 ==true &&  boolResult2 === false );