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restrict-template-expressions

Enforce template literal expressions to be of string type.

🧠

Extending "plugin:@typescript-eslint/recommended-requiring-type-checking" 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 values used in a template literal string that aren't primitives and don't define a more useful .toString() method.

.eslintrc.cjs
module.exports = {
  "rules": {
    "@typescript-eslint/restrict-template-expressions": "error"
  }
};
Try this rule in the playground ↗

Examples​

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

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

This rule accepts an options object with the following properties:

interface Options {
  /**
   * Whether to allow `number` typed values in template expressions.
   */
  allowNumber?: boolean;
  /**
   * Whether to allow `boolean` typed values in template expressions.
   */
  allowBoolean?: boolean;
  /**
   * Whether to allow `any` typed values in template expressions.
   */
  allowAny?: boolean;
  /**
   * Whether to allow `nullish` typed values in template expressions.
   */
  allowNullish?: boolean;
  /**
   * Whether to allow `regexp` typed values in template expressions.
   */
  allowRegExp?: boolean;
}

const defaultOptions: Options = [{ allowNumber: true }];

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}`;

Resources​