When you try to set a variable inside a forEach callback function in TypeScript, you may encounter issues with the variable's type. Here's a simple example:
let foo: (string | null) = null;
[1,2,3].forEach((i) => {
foo = "bar";
});
if(foo == null) {
throw new Error("not found")
}
# Typescript complains that `length` is not a property of type `never`
console.log(foo.length)
After the forEach loop, you might expect foo to have the type string | null, but it actually ends up being just null. Then, by the time you reach the console.log line, you would anticipate the type to be string, but TypeScript infers it as never, since null has been eliminated as a possibility due to the earlier Error throwing.
Interestingly, the same code without specifying : (string | null) works fine in plain JavaScript, highlighting a discrepancy between my expectations and the behavior of the TypeScript compiler.
So here are my queries:
- I'm looking for an explanation as to why this occurs so I can enhance my understanding of TypeScript.
- What is the best way to resolve this issue? Is there a better approach than using
foo = foo as stringbefore theconsole.logstatement?