95 lines
2.6 KiB
Plaintext
95 lines
2.6 KiB
Plaintext
---
|
|
description: 'Require `.toString()` to only be called on objects which provide useful information when stringified.'
|
|
---
|
|
|
|
import Tabs from '@theme/Tabs';
|
|
import TabItem from '@theme/TabItem';
|
|
|
|
> 🛑 This file is source code, not the primary documentation location! 🛑
|
|
>
|
|
> See **https://typescript-eslint.io/rules/no-base-to-string** for documentation.
|
|
|
|
JavaScript will call `toString()` on an object when it is converted to a string, such as when `+` adding to a string or in `${}` template literals.
|
|
The default Object `.toString()` uses the format `"[object Object]"`, which is often not what was intended.
|
|
This rule reports on stringified values that aren't primitives and don't define a more useful `.toString()` method.
|
|
|
|
> Note that `Function` provides its own `.toString()` that returns the function's code.
|
|
> Functions are not flagged by this rule.
|
|
|
|
## Examples
|
|
|
|
<Tabs>
|
|
<TabItem value="❌ Incorrect">
|
|
|
|
```ts
|
|
// Passing an object or class instance to string concatenation:
|
|
'' + {};
|
|
|
|
class MyClass {}
|
|
const value = new MyClass();
|
|
value + '';
|
|
|
|
// Interpolation and manual .toString() calls too:
|
|
`Value: ${value}`;
|
|
({}).toString();
|
|
```
|
|
|
|
</TabItem>
|
|
<TabItem value="✅ Correct">
|
|
|
|
```ts
|
|
// These types all have useful .toString()s
|
|
'Text' + true;
|
|
`Value: ${123}`;
|
|
`Arrays too: ${[1, 2, 3]}`;
|
|
(() => {}).toString();
|
|
|
|
// Defining a custom .toString class is considered acceptable
|
|
class CustomToString {
|
|
toString() {
|
|
return 'Hello, world!';
|
|
}
|
|
}
|
|
`Value: ${new CustomToString()}`;
|
|
|
|
const literalWithToString = {
|
|
toString: () => 'Hello, world!',
|
|
};
|
|
|
|
`Value: ${literalWithToString}`;
|
|
```
|
|
|
|
</TabItem>
|
|
</Tabs>
|
|
|
|
## Options
|
|
|
|
### `ignoredTypeNames`
|
|
|
|
A string array of type names to ignore, this is useful for types missing `toString()` (but actually has `toString()`).
|
|
There are some types missing `toString()` in old version TypeScript, like `RegExp`, `URL`, `URLSearchParams` etc.
|
|
|
|
The following patterns are considered correct with the default options `{ ignoredTypeNames: ["RegExp"] }`:
|
|
|
|
```ts option='{ "ignoredTypeNames": ["RegExp"] }' showPlaygroundButton
|
|
`${/regex/}`;
|
|
'' + /regex/;
|
|
/regex/.toString();
|
|
let value = /regex/;
|
|
value.toString();
|
|
let text = `${value}`;
|
|
```
|
|
|
|
## When Not To Use It
|
|
|
|
If you don't mind a risk of `"[object Object]"` or incorrect type coercions in your values, then you will not need this rule.
|
|
|
|
## Related To
|
|
|
|
- [`restrict-plus-operands`](./restrict-plus-operands.mdx)
|
|
- [`restrict-template-expressions`](./restrict-template-expressions.mdx)
|
|
|
|
## Further Reading
|
|
|
|
- [`Object.prototype.toString()` MDN documentation](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/toString)
|