forEach and assign to variable-CodePudding

This is just an example that I want to understand. I know there is a better way to select the last element from the array, but I don't know why it works that way.

const numbers = [1, 2, 3];

let exampleId: number;

numbers.forEach(function(number) {
  exampleId = number;
});

console.log(exampleId);

Why do I get the error "Variable 'exampleId' is used before being assigned." in the line from console.log?

CodePudding user response：

Maybe the interpreter doesn't take into account that the array numbers is not empty in your code.

Because, if it were empty, for sure the variable exampleId would be used before being assigned.

(If the array is empty, the code inside the forEach loop is never executed, and the variable is never assigned)

CodePudding user response：

The assignment is scoped within the forEach loop; If numbers have no values, exampleId would never get defined. TS is essentially warning you about that, as you cannot call a variable that has yet to be defined.

Solution 1:

Initialise your variable with a value

let exampleId = 0;

Solution 2:

Use non-null assertions, although I'd suggest against them

console.log(exampleId!);

With this solution we're essentially telling TS to not worry about it, as there will surely exist a value; That, in other words, remove the type safety, reason why I'd suggest going with option 1.

CodePudding user response：

The way to select the value of the last element from an array is

function lastElementOfArray(arr = []) {
  return arr.length > 0
    ? arr[ arr.length - 1 ]
    : undefined
    ;
}

The reason you get Variable 'exampleId' is used before being assigned. is because there's no guarantee that the Array.forEach() callback that assigns a value to exampleId will ever be invoked (e.g., when the array numbers has a length of 0).

And so, you get that message on the line

console.log(exampleId)