TypeScript: Tagged Template Literals shows Error

Question

I am trying to use tagged template literal of ES5 with typescript but it seems like type script does not have full support for it. I have following piece of code.

class TemplateLiterals { 
    age: number = 24;
    name: 'Luke Skywalker'
    private tag(strings: string[], personExp, ageExp) :string{
        var str0 = strings[0]; // "that "
        var str1 = strings[1]; // " is a "
        var ageStr;
        if (ageExp > 99) {
            ageStr = 'centenarian';
        } else {
            ageStr = 'youngster';
        }
        return str0 + personExp + str1 + ageStr;
    }
    toString() {
        return this.tag `that ${ this.name } is a ${ this.age }`;
    }
}

In toString method, typescript is showing me following error.

Argument of type 'TemplateStringsArray' is not assignable to parameter of type 'string[]'.
Property 'push' is missing in type 'TemplateStringsArray'.

I don't know why it's showing me this error. As per the mozilla's doc "The first argument of a tag function contains an array of string values." So it should accept the array of string. But the actual expectation is TemplateStringsArray. not sure how and when the interface TemplateSringsArray is being defined. Currently I am using TemplateSringsArray type to avoid this error. Can anyone please explain whats happening. Thanks. Here's the playground.

Answer 1

After exploring more, I finally found explanations in the change log. Hope it will help someone. It says

ES2015 tagged templates always pass their tag an immutable array-like object that has a property called raw (which is also immutable). TypeScript names this object the TemplateStringsArray.

Conveniently, TemplateStringsArray was assignable to an Array, so it's possible users took advantage of this to use a shorter type for their tag parameters:

function myTemplateTag(strs: string[]) {
    // ... 
} 

However, in TypeScript 2.0, the language now supports the readonly modifier and can express that these objects are immutable. As a result, TemplateStringsArray has also been made immutable, and is no longer assignable to string[].

Recommendation:

Use TemplateStringsArray explicitly (or use ReadonlyArray<string>).