TypeScript does not evaluate the boolean left operand when using the && operator

Question

TypeScript does not evaluate the boolean left operand when using the && operator

While working with TypeScript, I encountered a scenario similar to the code snippet below:

const getString = (string: string) => string

// No errors
getString("foo"); 

// Argument of type 'boolean' is not assignable to parameter of type 'string'.ts(2345)
getString(false && "foo"); 

// No errors
getString(1 > 2 && "foo");

I found it interesting that no error was thrown for

getString(1 > 2 && "foo")

. Despite the fact that 1 > 2 evaluates to false, a boolean value, which getString() cannot accept. Why did TypeScript not evaluate 1 > 2 and raise an error in this case?

typescript

Answer 1

Answer №1

If you want to avoid unexpected errors, consider enabling strictNullChecks in your TypeScript configuration - this will help catch errors beforehand.

It's interesting to note that without the strictNullChecks option, TypeScript interprets the argument type as '' | "foo", but with it, the type becomes boolean | "foo".

const aBool = 1 > 2;
const res = aBool && "foo";

When executing the above code, the type of res is automatically inferred as

const res: "" | "foo"

. This behavior may seem like a potential issue, but it's uncertain whether it's a bug or intended functionality.

Answer 2

If you want to avoid unexpected errors, consider enabling strictNullChecks in your TypeScript configuration - this will help catch errors beforehand.

It's interesting to note that without the strictNullChecks option, TypeScript interprets the argument type as '' | "foo", but with it, the type becomes boolean | "foo".

const aBool = 1 > 2;
const res = aBool && "foo";

When executing the above code, the type of res is automatically inferred as

const res: "" | "foo"

. This behavior may seem like a potential issue, but it's uncertain whether it's a bug or intended functionality.

TypeScript does not evaluate the boolean left operand when using the && operator

Answer №1

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