What are the parameters that are affected by the noImplicitAny compiler flag?

Question

What are the parameters that are affected by the noImplicitAny compiler flag?

The TypeScript documentation details the significance of the noImplicitAny compiler flag, which is designed to:

Flag errors on expressions and declarations that have an implicit any type.

Consider the code snippet below:

let x;            // x is implicitly of type `any`, but no error

function foo(y) { // error: parameter 'y' implicitly has an 'any' type. 
    let z;        // z is implicitly of type `any`, but no error
}

Given this scenario, would it not be logical for x and z to also be identified as implicitly typed to any?

typescript

Answer 1

Answer №1

This particular behavior is a result of a correction implemented in version 2.1. Had it not been for this update, your code would have produced errors.

The release notes shed some light on this:

Upon the introduction of TypeScript 2.1, instead of defaulting to 'any,' TypeScript now infers types based on the subsequent assignments made in the code.

For instance:

let x;

// 'x' remains flexible and can be assigned any value.
x = () => 42;

// Following the last assignment, TypeScript 2.1 deduces that 'x' is of type '() => number'.
let y = x();

// Consequently, it will raise an error if you try to add a number to a function.
console.log(x + y);
//          ~~~~~
// Error! Operator '+' cannot be applied to types '() => number' and 'number'.

// You are still allowed to assign any value to 'x'.
x = "Hello world!";

// Yet, TypeScript now recognizes that 'x' is a 'string'!
x.toLowerCase();

Hence, TypeScript will dynamically infer types based on your assignments:

function foo(y) { 
    let z;
    z = "somestring";
    z.toUpperCase(); // 'z' is a string now. No error;
    z = 10;
    z.toUpperCase(); // 'z' is a number now; Error
}

Answer 2

This particular behavior is a result of a correction implemented in version 2.1. Had it not been for this update, your code would have produced errors.

The release notes shed some light on this:

Upon the introduction of TypeScript 2.1, instead of defaulting to 'any,' TypeScript now infers types based on the subsequent assignments made in the code.

For instance:

let x;

// 'x' remains flexible and can be assigned any value.
x = () => 42;

// Following the last assignment, TypeScript 2.1 deduces that 'x' is of type '() => number'.
let y = x();

// Consequently, it will raise an error if you try to add a number to a function.
console.log(x + y);
//          ~~~~~
// Error! Operator '+' cannot be applied to types '() => number' and 'number'.

// You are still allowed to assign any value to 'x'.
x = "Hello world!";

// Yet, TypeScript now recognizes that 'x' is a 'string'!
x.toLowerCase();

Hence, TypeScript will dynamically infer types based on your assignments:

function foo(y) { 
    let z;
    z = "somestring";
    z.toUpperCase(); // 'z' is a string now. No error;
    z = 10;
    z.toUpperCase(); // 'z' is a number now; Error
}

What are the parameters that are affected by the noImplicitAny compiler flag?

Answer №1

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