Merge branch 'master' into feature/conditional-branching

Author: findiel

1-js/02-first-steps/12-nullish-coalescing-operator/article.md

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+# Nullish coalescing operator '??'
+
+[recent browser="new"]
+
+Here, in this article, we'll say that an expression is "defined" when it's neither `null` nor `undefined`.
+
+The nullish coalescing operator is written as two question marks `??`.
+
+The result of `a ?? b` is:
+- if `a` is defined, then `a`,
+- if `a` isn't defined, then `b`.
+
+
+In other words, `??` returns the first argument if it's not `null/undefined`. Otherwise, the second one.
+
+The nullish coalescing operator isn't anything completely new. It's just a nice syntax to get the first "defined" value of the two.
+
+We can rewrite `result = a ?? b` using the operators that we already know, like this:
+
+```js
+result = (a !== null && a !== undefined) ? a : b;
+```
+
+The common use case for `??` is to provide a default value for a potentially undefined variable.
+
+For example, here we show `Anonymous` if `user` isn't defined:
+
+```js run
+let user;
+
+alert(user ?? "Anonymous"); // Anonymous
+```
+
+Of course, if `user` had any value except `null/undefined`, then we would see it instead:
+
+```js run
+let user = "John";
+
+alert(user ?? "Anonymous"); // John
+```
+
+We can also use a sequence of `??` to select the first value from a list that isn't `null/undefined`.
+
+Let's say we have a user's data in variables `firstName`, `lastName` or `nickName`. All of them may be undefined, if the user decided not to enter a value.
+
+We'd like to display the user name using one of these variables, or show "Anonymous" if all of them are undefined.
+
+Let's use the `??` operator for that:
+
+```js run
+let firstName = null;
+let lastName = null;
+let nickName = "Supercoder";
+
+// shows the first defined value:
+*!*
+alert(firstName ?? lastName ?? nickName ?? "Anonymous"); // Supercoder
+*/!*
+```
+
+## Comparison with ||
+
+The OR `||` operator can be used in the same way as `??`, as it was described in the [previous chapter](info:logical-operators#or-finds-the-first-truthy-value).
+
+For example, in the code above we could replace `??` with `||` and still get the same result:
+
+```js run
+let firstName = null;
+let lastName = null;
+let nickName = "Supercoder";
+
+// shows the first truthy value:
+*!*
+alert(firstName || lastName || nickName || "Anonymous"); // Supercoder
+*/!*
+```
+
+The OR `||` operator exists since the beginning of JavaScript, so developers were using it for such purposes for a long time.
+
+On the other hand, the nullish coalescing operator `??` was added to JavaScript only recently, and the reason for that was that people weren't quite happy with `||`.
+
+The important difference between them is that:
+- `||` returns the first *truthy* value.
+- `??` returns the first *defined* value.
+
+In other words, `||` doesn't distinguish between `false`, `0`, an empty string `""` and `null/undefined`. They are all the same -- falsy values. If any of these is the first argument of `||`, then we'll get the second argument as the result.
+
+In practice though, we may want to use default value only when the variable is `null/undefined`. That is, when the value is really unknown/not set.
+
+For example, consider this:
+
+```js run
+let height = 0;
+
+alert(height || 100); // 100
+alert(height ?? 100); // 0
+```
+
+- The `height || 100` checks `height` for being a falsy value, and it really is.
+    - so the result is the second argument, `100`.
+- The `height ?? 100` checks `height` for being `null/undefined`, and it's not,
+    - so the result is `height` "as is", that is `0`.
+
+If the zero height is a valid value, that shouldn't be replaced with the default, then `??` does just the right thing.
+
+## Precedence
+
+The precedence of the `??` operator is rather low: `5` in the [MDN table](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Operator_Precedence#Table). So `??` is evaluated before `=` and `?`, but after most other operations, such as `+`, `*`.
+
+So if we'd like to choose a value with `??` in an expression with other operators, consider adding parentheses:
+
+```js run
+let height = null;
+let width = null;
+
+// important: use parentheses
+let area = (height ?? 100) * (width ?? 50);
+
+alert(area); // 5000
+```
+
+Otherwise, if we omit parentheses, then as `*` has the higher precedence than `??`, it would execute first, leading to incorrect results.
+
+```js
+// without parentheses
+let area = height ?? 100 * width ?? 50;
+
+// ...works the same as this (probably not what we want):
+let area = height ?? (100 * width) ?? 50;
+```
+
+### Using ?? with && or ||
+
+Due to safety reasons, JavaScript forbids using `??` together with `&&` and `||` operators, unless the precedence is explicitly specified with parentheses.
+
+The code below triggers a syntax error:
+
+```js run
+let x = 1 && 2 ?? 3; // Syntax error
+```
+
+The limitation is surely debatable, but it was added to the language specification with the purpose to avoid programming mistakes, when people start to switch to `??` from `||`.
+
+Use explicit parentheses to work around it:
+
+```js run
+*!*
+let x = (1 && 2) ?? 3; // Works
+*/!*
+
+alert(x); // 2
+```
+
+## Summary
+
+- The nullish coalescing operator `??` provides a short way to choose the first "defined" value from a list.
+
+    It's used to assign default values to variables:
+
+    ```js
+    // set height=100, if height is null or undefined
+    height = height ?? 100;
+    ```
+
+- The operator `??` has a very low precedence, only a bit higher than `?` and `=`, so consider adding parentheses when using it in an expression.
+- It's forbidden to use it with `||` or `&&` without explicit parentheses.