Generator Causing TypeScript Trouble

Encountering issues when experimenting with this TypeScript code in VSCode:

enum Title {
    Manager,
    Developer
};

interface Staff {
    Name: string;
    Position: Title;
}

let staffNames: string[] = ['Alice', 'Bob'];
//Error message for missing Name property
function* getStaff(): Staff {
    for (let name in staffNames) {
        if (name === 0) //Issue raised here for name being a string
            yield { Name: staffNames[name], Position: Title.Manager }
        else
            yield { Name: staffNames[name], Position: Title.Developer }
    }
}

for (let employee of getStaff()) {
    console.log(`${employee.Name} holds the title of ${employee.Position}`);
}   

Seeking clarification on this matter...

Answer №1

When you use a for...in loop, it goes through the properties (keys) of an object that are enumerable, and in the case of an array, these keys are converted to strings. For example, if you iterate over an array, the keys will be the array indices represented as strings.

To see this in action, try running the following code:

for (let n in names) {
    console.log(n);
    console.log(typeof n);
}

The output will show:

0
string
1
string

This is why TypeScript struggles with comparing the string value of n to the number 0.

It's interesting to note that if you added a key-value pair like so:

names["key"] = "value";

And ran the same iteration code, you would now also see:

0
string
1
string
key
string 

This illustrates why using for...in to loop over array indices isn't recommended. For more insights, check out the link shared by @Mike McCaughan in the comments.

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