JavaScript closures and the misinterpretation of returning values

function generateUniqueCelebrityIDs(celebrities) {
    var i;
    var uniqueID = 100;
    for (i = 0; i < celebrities.length; i++) {
        celebrities[i]["id"] = function () {
            return uniqueID + i;
        };
    };

    return celebrities;
}

var actionCelebs = [
    { name: "Stallone", id: 0 },
    { name: "Cruise", id: 0 },
    { name: "Willis", id: 0 }
];
var updateActionCelebIDs = generateUniqueCelebrityIDs(actionCelebs);
var stalloneID = updateActionCelebIDs[0];
console.log(stalloneID.id()); // 103

In the above example, due to the timing of when the anonymous functions are executed, all celebrity IDs end up as 103 instead of being distinct values starting from 100. This issue arises from how the loop increments and affects the closure around the variable 'i'.

I've researched the concept of callback functions but still struggle to understand why we consistently get ID=103 in this scenario. Seeking clarity and insight on this topic. Thank you.

Answer №1

The solution lies in the concept of closure. You can learn more about it on Stack Overflow and on MDN Web Docs.

Here is a code snippet that can help resolve your issue:

function createUniqueIDs(people) {
    var i;
    var baseID = 100;
    for (i = 0; i < people.length; i++) {
        people[i]["id"] = (function (index) {
            return function () {
                return baseID + index;
            };
        })(i);
    }

    return people;
};

Answer №2

The explanation behind this situation lies in the utilization of a callback function. The purpose of callbacks is to execute once they are called, meaning they will only run when explicitly invoked.

In your particular scenario, the result returned by your callback during each iteration consistently ends up being the final value of i + uniqueID.

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