Found template literal expression having a non-string type JS-0378

[philipjonsen]

DESCRIPTION
Using only string values in placeholders is recommended as otherwise, the value may be wrongly displayed; for example, if an object value is directly included in a string, it will be shown as [object Object]. Using only string values can also ensure that null or undefined values are not directly displayed.

With template literals, an expression can be embedded in a placeholder. A placeholder is represented by ${}, with anything within the curly brackets treated as JavaScript and anything outside the brackets treated as a string.

Examples
Bad Practice
const arg1 = [1, 2];
const msg1 = arg1 = ${arg1};
const arg2 = { name: 'Cryptosweden' };
const msg2 = arg2 = ${arg2 || null};

Recommended
const arg = 'Cryptosweden';
const msg1 = arg = ${arg};
const msg2 = arg = ${arg || 'default'};

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