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The nullish coalescing operator is written as two question marks `??`.
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As it treats `null` and `undefined` similarly, we'll use a special term here, in this article. We'll say that an expression is "defined" when it's neither `null` nor `undefined`.
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As it treats `null` and `undefined` similarly, we'll use a special term here, in this article. For brevity, we'll say that a value is "defined" when it's neither `null` nor `undefined`.
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The result of `a ?? b` is:
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- if `a` is defined, then `a`,
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Now it should be absolutely clear what `??` does. Let's see where it helps.
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The common use case for `??` is to provide a default value for a potentially undefined variable.
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The common use case for `??` is to provide a default value.
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For example, here we show `user` if defined, otherwise `Anonymous`:
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For example, here we show `user` if its value isn't `null/undefined`, otherwise `Anonymous`:
We can also use a sequence of `??` to select the first value from a list that isn't `null/undefined`.
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Let's say we have a user's data in variables `firstName`, `lastName` or `nickName`. All of them may be not defined, if the user decided not to enter a value.
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Let's say we have a user's data in variables `firstName`, `lastName` or `nickName`. All of them may be not defined, if the user decided not to fill in the corresponding values.
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We'd like to display the user name using one of these variables, or show "Anonymous" if all of them aren't defined.
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We'd like to display the user name using one of these variables, or show "Anonymous" if all of them are `null/undefined`.
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Let's use the `??` operator for that:
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That means that, just like `||`, the nullish coalescing operator `??` is evaluated before `=` and `?`, but after most other operations, such as `+`, `*`.
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So if we'd like to choose a value with `??`in an expression with other operators, consider adding parentheses:
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So we may need to add parentheses in expressions like this:
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```js run
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let height =null;
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// without parentheses
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let area = height ??100* width ??50;
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// ...works the same as this (probably not what we want):
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