What is the reason behind the absence of compile time errors when using 'string' functions on an 'any' field type variable in TypeScript?

Looking at the following typescript code snippet:

 let a; 
 a = "number"; 
 let t = a.endsWith('r'); 
 console.log(t); 

It is worth noting that since variable 'a' is not declared with a specific type, the compiler infers it as an 'any' type. Despite this, when we assign a string value to 'a' and use the 'endsWith' function against it, there is no compile error thrown by the compiler. This is intriguing because 'endsWith' is not a valid function for an 'any' type. Nevertheless, the code still compiles/transpiles into JavaScript successfully and executes without any issues.

The correct way to write this code would be:

 let a : string; 
 a = "number"; 
 let t = a.endsWith('r'); 
 console.log(t); 

However, it raises the question of why the previous code block compiles without errors?

Answer №1

Using the keyword `Any` in TypeScript allows developers to bypass the type system, potentially leading to compilation errors when using methods like a.endsWith() on variables that are not actually of the expected type. More information can be found at https://www.typescriptlang.org/docs/handbook/basic-types.html#any

Answer №2

Every data type functions similarly to the dynamic type in C#. It validates the methods specified after the dot during runtime only. During compilation - you can assign any value to it and it will compile successfully. However, when executed, it verifies if the assigned methods actually exist and throws a run time error if they do not.

Answer №3

When using the any type, the variable's data type is determined at runtime. Whatever data type it initially contains will be the type of the variable.

let someVariable: any;
someVariable = 1000;

If someVariable holds a number, its type will be considered as a number during runtime.

someVariable = "string";

If someVariable now contains a string, its type will be interpreted as a string at runtime.

A type mismatch error will occur at runtime if there is a discrepancy because the type is dynamically assigned with any.

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