I haven't encountered any type warnings in the places where I anticipated them

When I define an object like this:

const x: { str: string, num: number } = {
    str: str,
    num: not_a_num
  };

I am surprised to find that even though 'not_a_num' is a string and not a number, the compiler does not throw an error. Instead, it creates an object with two string properties.

Furthermore, I have a function declared as:

store(array: Array<{ str: string, num: number }>): Promise<any> { //... }

But when I pass an array containing the object 'x' as a parameter, typeof(array[0].num) resolves to "string", which is unexpected given my type annotations.

My question is: Why am I not receiving any compiler warnings or errors in these cases? What's the purpose of using type annotations if they don't catch incorrect data types being passed in?

Thank you for your help!

Answer №1

The issue here arises from the fact that not_a_num is declared as type any. In TypeScript, using any means it can be any other type, leading to potential compatibility issues.

let str = "";
let not_a_num:any = "";

const xWithAny: { str: string, num: any } = {
    str: str,
    num: not_a_num
};
const x: { str: string, num: number }  = xx; // this assignment is valid

Type compatibility in TypeScript is determined by the individual components, hence xWithAny is compatible with x because their str properties have matching types and num:any can be assigned to num:number.

The main issue arises during runtime where not_a_num being of type any can take on any value, potentially causing num to end up as a string instead of a number.

It's generally recommended to avoid using any unless necessary, and if used, one should consciously consider why they are opting for any. Enabling noImplicitAny flag in the TypeScript compiler can provide some assistance in managing this issue.

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