restrict-template-expressions.md

description
Enforce template literal expressions to be of `string` type.

🛑 This file is source code, not the primary documentation location! 🛑

See https://typescript-eslint.io/rules/restrict-template-expressions 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() returns "[object Object]", which is often not what was intended. This rule reports on values used in a template literal string that aren't primitives and don't define a more useful .toString() method.

Examples

❌ Incorrect

const arg1 = [1, 2];
const msg1 = `arg1 = ${arg1}`;

const arg2 = { name: 'Foo' };
const msg2 = `arg2 = ${arg2 || null}`;

✅ Correct

const arg = 'foo';
const msg1 = `arg = ${arg}`;
const msg2 = `arg = ${arg || 'default'}`;

const stringWithKindProp: string & { _kind?: 'MyString' } = 'foo';
const msg3 = `stringWithKindProp = ${stringWithKindProp}`;

Options

allowNumber

Examples of additional correct code for this rule with { allowNumber: true }:

const arg = 123;
const msg1 = `arg = ${arg}`;
const msg2 = `arg = ${arg || 'zero'}`;

allowBoolean

Examples of additional correct code for this rule with { allowBoolean: true }:

const arg = true;
const msg1 = `arg = ${arg}`;
const msg2 = `arg = ${arg || 'not truthy'}`;

allowAny

Examples of additional correct code for this rule with { allowAny: true }:

const user = JSON.parse('{ "name": "foo" }');
const msg1 = `arg = ${user.name}`;
const msg2 = `arg = ${user.name || 'the user with no name'}`;

allowNullish

Examples of additional correct code for this rule with { allowNullish: true }:

const arg = condition ? 'ok' : null;
const msg1 = `arg = ${arg}`;

allowRegExp

Examples of additional correct code for this rule with { allowRegExp: true }:

const arg = new RegExp('foo');
const msg1 = `arg = ${arg}`;

const arg = /foo/;
const msg1 = `arg = ${arg}`;

allowNever

Examples of additional correct code for this rule with { allowNever: true }:

const arg = 'something';
const msg1 = typeof arg === 'string' ? arg : `arg = ${arg}`;

Related To

• no-base-to-string
• restrict-plus-operands