In Javascript, the nullish coalescing operator, or operator is used to return the right-hand side whenever the left-hand side is or . To understand a little bit better, let's look at a few examples: ?? null undefined // Is set to 0 let x = 0 ?? "hello"; // Is set to goodbye let y = undefined ?? "goodbye"; // Is set to hello let z = null ?? "hello"; // Is set to false let a = false ?? "goodbye"; The operator is useful in situations where something can be returned as either or , and helps us tighten up our code. For example, a function that returns undefined in some situations can be provided with a value: nullish coalescing null undefined default let myFunction = (a) => { if(a >= 5) { return "hello world"; } } // Will return "goodbye world", since `myFunction(4)` returns undefined. let runFunction = myFunction(4) ?? "goodbye world"; Differences between the logical OR operator In the past, we typically set values in Javascript using the logical ( ) operator. It has the same kind of functionality, in that it sets a value if the first value on the left doesn't meet certain criteria. However, the operator returns the right-hand value if the left-hand value is anything - and there are a lot of values, as shown in the list below. default OR || || falsy falsy Falsy Values false or or 0 -0 0n any empty string, i.e. "" null undefined NaN , using the operator would mean that if a function returned the value , and we really did want to use the value, we wouldn't be able to - since is false. With the nullish coalescing operator ( ), is a valid value since it only triggers if a value is or : As such || 0 0 0 ?? 0 null undefined // Is set to 0 let x = 0 ?? 5; // Is set to 5 let y = 0 || 5; Similarly, if a string is empty, the operator would default to the right-hand side - which is not always the desired behavior. The operator lets us avoid that: || ?? // Is set to "" let x = "" ?? "default text"; // Is set to "default text" let x = "" || "default text"; Chaining the nullish coalescing operator It is also possible to chain the nullish coalescing operator, as shown below: // Is set to "default text" let x = null ?? undefined ?? "default text"; But you cannot chain it with the logical operator, unless with parenthesis: || // Errors out: let x = 0 || undefined ?? "default text"; // Returns "default text"; let y = (0 || undefined) ?? "default text"; Also published . here
