# Logical nullish assignment (??=)

## 在此页

The logical nullish assignment ( ``` x ??= y ``` ) operator only assigns if ``` x ``` is nullish ( ``` null ``` or ``` undefined ``` ).

## 句法

```expr1 ??= expr2
```

## 描述

### Short-circuit evaluation

nullish coalescing operator is evaluated left to right, it is tested for possible short-circuit evaluation using the following rule:

``` (some expression that is neither null nor undefined) ?? expr ``` is short-circuit evaluated to the left-hand side expression if the left-hand side proves to be neither ``` null ``` nor ``` undefined ``` .

Short circuit means that the ``` expr ``` part above is not evaluated , hence any side effects of doing so do not take effect (e.g., if ``` expr ``` is a function call, the calling never takes place).

Logical nullish assignment short-circuits as well meaning that ``` x ??= y ``` 相当于：

```x ?? (x = y);
```

And not equivalent to the following which would always perform an assignment:

```x = x ?? y;
```

## 范例

### Using logical nullish assignment

```function config(options) {
options.duration ??= 100;
options.speed ??= 25;
return options;
}
config({ duration: 125 }); // { duration: 125, speed: 25 }
config({}); // { duration: 100, speed: 25 }
```

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