javascript assign function to variable - How do JavaScript closures work?

javascript function expression / javascript / function / variables / scope / closures

How would you explain JavaScript closures to someone with a knowledge of the concepts they consist of (for example functions, variables and the like), but does not understand closures themselves?

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Answer #1

Closures are often used for creating functions with some hidden private data (but it's not always the case).

var db = (function() {
    // Create a hidden object, which will hold the data
    // it's inaccessible from the outside.
    var data = {};

    // Make a function, which will provide some access to the data.
    return function(key, val) {
        if (val === undefined) { return data[key] } // Get
        else { return data[key] = val } // Set
    }
    // We are calling the anonymous surrounding function,
    // returning the above inner function, which is a closure.
})();

db('x')    // -> undefined
db('x', 1) // Set x to 1
db('x')    // -> 1
// It's impossible to access the data object itself.
// We are able to get or set individual it.