no-base-to-string
Require
.toString()to only be called on objects which provide useful information when stringified.
Extending "plugin:@typescript-eslint/strict" in an ESLint configuration enables this rule.
This rule requires type information to run.
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() returns "[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
Functionprovides its own.toString()that returns the function's code. Functions are not flagged by this rule.
module.exports = {
"rules": {
"@typescript-eslint/no-base-to-string": "warn"
}
};
Examples
- ❌ Incorrect
- ✅ Correct
// 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());
// 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: ${literalWithToString}`;
Options
This rule accepts an options object with the following properties:
interface Options {
ignoredTypeNames?: string[];
}
const defaultOptions: Options = [
{ ignoredTypeNames: ["Error", "RegExp", "URL", "URLSearchParams"] },
];
ignoredTypeNames
A string array of type names to ignore, this is useful for types missing toString() (but actually has toString()).
There are some types missing toString() in old version TypeScript, like RegExp, URL, URLSearchParams etc.
The following patterns are considered correct with the default options { ignoredTypeNames: ["RegExp"] }:
`${/regex/}`;
'' + /regex/;
/regex/.toString();
let value = /regex/;
value.toString();
let text = `${value}`;
When Not To Use It
If you don't mind "[object Object]" in your strings, then you will not need this rule.