@typescript-eslint/restrict-plus-operands

+ accepts anything. An object or an array is turned into a string first, so [] + 5 is "5" and "User: " + user is "User: [object Object]". A number plus a bigint throws a TypeError when it runs.

By default this rule reports objects, arrays, symbols, unknown, never, and a number mixed with a bigint. Turn the allow options off to make it stricter.

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/restrict-plus-operands" = "error" # or "warn"

Every option, set to its default:

bunfig.toml

[lint.rules]
"@typescript-eslint/restrict-plus-operands" = ["error", {
  allowAny = true,
  allowBoolean = true,
  allowNullish = true,
  allowNumberAndString = true,
  allowRegExp = true,
  skipCompoundAssignments = false,
}]

Examples

Reported:

function describe(user: { name: string }, tags: string[], size: bigint) {
  const label = "User: " + user;
  const list = "Tags: " + tags;
  const next = size + 1;
}

Not reported:

function describe(user: { name: string }, tags: string[], size: bigint) {
  const label = "User: " + user.name;
  const list = "Tags: " + tags.join(", ");
  const next = size + 1n;
}

Options

Option	Default	What it does
`allowAny`	`true`	Allow an operand of type `any`.
`allowBoolean`	`true`	Allow a `boolean` operand.
`allowNullish`	`true`	Allow an operand that can be `null` or `undefined`.
`allowNumberAndString`	`true`	Allow adding a `number` or a `bigint` to a `string`.
`allowRegExp`	`true`	Allow a `RegExp` operand. A `RegExp` plus a `number` is always reported.
`skipCompoundAssignments`	`false`	Don't check `+=`.

With all the allow options off, both sides must have the same type:

function report(count: number, done: boolean, name?: string) {
  const a = "Items: " + count;
  const b = "Done: " + done;
  const c = "Name: " + name;
}

+= is checked like +. This is reported unless skipCompoundAssignments is on:

let log = "";
log += ["started"];