JS.TS.NO.BASE.TO.STRING

Requires .toString() to only be called on objects which provide useful information when stringified.

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]", so this rule requires stringified objects 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.

This rule has some overlap with restrict-plus-operands and restrict-template-expressions.

Rule Details

This rule prevents accidentally defaulting to the base Object .toString() method.

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Incorrect

// 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());

Correct

// 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

type Options = {
  ignoredTypeNames?: string[];
};

const defaultOptions: Options = {
  ignoredTypeNames: ['RegExp'],
};

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.

• Object.prototype.toString() MDN documentation (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/toString)