@typescript-eslint/no-base-to-string

An object that doesn't define its own toString() turns into the text "[object Object]". That text ends up in a log line, a URL or a message on the page, and the data you wanted to show is lost.

The rule checks .toString(), .toLocaleString(), String(value), "text" + value, ${value} in a template, and .join() on an array of such objects. The report says "will" if the type always gives [object Object], and "may" if only some members of a union do.

bun lint --type-aware turns this rule on in TypeScript files.

Configure

Off by default. This rule needs types, so turning it on makes bun lint type check your code first.

bunfig.toml

[lint.rules]
"@typescript-eslint/no-base-to-string" = "error" # or "warn"

Every option, set to its default:

bunfig.toml

[lint.rules]
"@typescript-eslint/no-base-to-string" = ["error", {
  ignoredTypeNames = ["Error", "RegExp", "URL", "URLSearchParams"],
  checkUnknown = false,
}]

Examples

Reported:

const user = { name: "Ada" };

console.log(`Hello, ${user}`);
console.log("Hello, " + user);
console.log([user].join(", "));

Not reported:

const user = { name: "Ada" };

console.log(`Hello, ${user.name}`);
console.log(JSON.stringify(user));

class Point {
  constructor(private x: number, private y: number) {}

  toString(): string {
    return `${this.x},${this.y}`;
  }
}

console.log(`At ${new Point(1, 2)}`);

Options

Option	Default	What it does
`ignoredTypeNames`	`["Error", "RegExp", "URL", "URLSearchParams"]`	Names of types that are never reported. Types that extend them aren't reported either. Your list replaces the default.
`checkUnknown`	`false`	Also report values of type `unknown`, which may be plain objects.

interface Money {
  amount: number;
}

declare const price: Money;

console.log(`Total: ${price}`);

function describe(value: unknown): string {
  return `Got ${value}`;
}