Enforce template literal expressions to be of string type (restrict-template-expressions)

Examples of correct code:

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

Examples of incorrect code:

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

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

Options

The rule accepts an options object with the following properties:

type Options = {
  // if true, also allow number type in template expressions
  allowNumber?: boolean;
  // if true, also allow boolean type in template expressions
  allowBoolean?: boolean;
  // if true, also allow null and undefined in template expressions
  allowNullable?: boolean;
};

const defaults = {
  allowNumber: false,
  allowBoolean: false,
  allowNullable: false,
};

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'}`;

allowNullable

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

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