Implicit type conversion for method arguments in TypeScript

It's important to remember that Typescript will never actually change a data type depending on the type information. This understanding comes from the fundamental nature of types: They are there to help you to avoid making mistakes, they don't take an active role in the code.

That being said, Typescript can be as loose or as strict as you like. The stricter it is, the more "helpful" it is to you as a coder (and conversely, the more annoying it can be if you have lazy habits :P)

I would recommend setting a few options in your tsconfig.json, which would have caught your error and you'd have been able to easily see the issue:

"strict": true,
"noImplicitReturns": true,
"noImplicitAny": true,
"noImplicitThis": true,

The result would have been that TypeScript would throw a compile error alerting you to the problem (the problem being that you're passing in the index, not the item):

error TS2345: Argument of type 'number' is not assignable to parameter of type 'string'.

Incidentally, try to avoid using that old foreach syntax - it's very expensive (calling a function for every item) and can lead to messy code flow.

Try this instead:

for (const item of arr) {
    testArgs(item);
}

you are getting type of index (i) that is a number you need to get typeof rec

const arr:string[] = ['apple', 'banana', 'grapes'];

arr.forEach((rec, i) => {
  this.testArgs(rec); // you need to call this
  console.log(typeof rec); //this is the one that will get the type of array element
  console.log(typeof i); // This is getting type of index not the array
});

If you call testArgs method with a number it won't convert it into a string Typescript is meant for casting objects in javascript into a type, or in other words giving objects a type. It is not meant for type converting, basically converting your object's type into other

Data types in TypeScript are just for making sure that we are passing the correct type of data. After compilation it's just JavaScript, so when you use typeof on an argument, JavaScript will evaluate the types based on the actual value passed, no matter what you have used as the datatype in the TypeScript function.

testArgs(1) - "number"
testArgs("1") - "string"