How to check the type of an array?

Question

Is there a way to check what the array "type" is? for example

Array<string> means it is a collection of "string" type variables.

so if i create a function

checkType(myArray:Array<any>){
  if(/*myArray is a collection of strings is true*/){
    console.log("yes it is")
  }else{
    console.log("no it is not")
  }
}

So say your Array has been declared as a collection of strings. Is there a away to check if it is a collection of strings or not and can you do that with a collection object classes? — Newbie2016, Apr 13 '17 at 16:46
Sorry if it is unclear, i'll write a bit more code actually to help what i am trying to find out. — Newbie2016, Apr 13 '17 at 16:51
Well, maybe explain yourself better? Give an example of what you're looking for? — Nitzan Tomer, Apr 13 '17 at 16:51

score 7 · Accepted Answer · answered Apr 13 '17 at 17:19

The type system that typescript offers doesn't exist at runtime.
At runtime you only have javascript, so the only way to know is to iterate over the array and check each item.

In javascript you have two ways of knowing a type of a value, either with typeof or instanceof.

For strings (and other primitives) you need typeof:

typeof VARIABLE === "string"

With object instance you need instanceof:

VARIABLE instanceof CLASS

Here's a generic solution for you:

function is(obj: any, type: NumberConstructor): obj is number;
function is(obj: any, type: StringConstructor): obj is string;
function is<T>(obj: any, type: { prototype: T }): obj is T;
function is(obj: any, type: any): boolean {
    const objType: string = typeof obj;
    const typeString = type.toString();
    const nameRegex: RegExp = /Arguments|Function|String|Number|Date|Array|Boolean|RegExp/;

    let typeName: string;

    if (obj && objType === "object") {
        return obj instanceof type;
    }

    if (typeString.startsWith("class ")) {
        return type.name.toLowerCase() === objType;
    }

    typeName = typeString.match(nameRegex);
    if (typeName) {
        return typeName[0].toLowerCase() === objType;
    }

    return false;
}

function checkType(myArray: any[], type: any): boolean {
    return myArray.every(item => {
        return is(item, type);
    });
}

console.log(checkType([1, 2, 3], Number)); // true
console.log(checkType([1, 2, "string"], Number)); // false


console.log(checkType(["one", "two", "three"], String)); // true

class MyClass { }
console.log(checkType([new MyClass(), new MyClass()], MyClass)); //true

(code in playground)

How to check the type of an array?

1 Answers1

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