How Can I Create Functions That Execute Only Once in JavaScript?

An In-Depth Look at Single-Execution Functions in JavaScript

A common requirement in programming involves creating functions that can only be executed once. While using static variables in languages like C and Java might come to mind, JavaScript offers an elegant alternative: closures.

Understanding Closures

Closures in JavaScript are functions that have access to the scope of their parent functions, even after the parent functions have returned. By utilizing closures, we can create a function that keeps track of whether it has been executed before.

Consider the following example:

<code class="javascript">var something = (function() {
    var executed = false;
    return function() {
        if (!executed) {
            executed = true;
            // Do something
        }
    };
})();

something(); // "Do something" happens
something(); // Nothing happens</code>

In this example, the outer function creates a closure around the inner function. The inner function checks if the executed flag is set to false, indicating that it has not been run before. If false, the flag is set to true, and the desired action is executed. On subsequent calls, the executed flag remains true, causing the function to do nothing.

Third-Party Library Functions

Various third-party JavaScript libraries, such as Underscore and Ramda, provide utility functions like once(). These functions accept another function as an argument and return a function that executes the supplied function exactly once. The returned function also caches the value from the first execution and returns that on subsequent calls.

One notable implementation is provided by David Walsh:

<code class="javascript">function once(fn, context) {
    var result;
    return function() {
        if (fn) {
            result = fn.apply(context || this, arguments);
            fn = null;
        }
        return result;
    };
}</code>

This function creates a closure around the supplied function (fn) and a context (context). On the first execution, it sets result to the return value of fn and sets fn to null, effectively preventing future executions. On subsequent calls, it simply returns the cached result.

Usage

Using these single-execution functions is straightforward. Simply pass the function you want to execute once to the utility function, and the returned function can be called as needed without any concerns about re-execution.

For instance, with David Walsh's implementation:

<code class="javascript">function something() { /* Do something */ }
var one_something = once(something);

one_something(); // "Do something" happens
one_something(); // Nothing happens</code>

Conclusion

Closures and utility functions provide elegant ways to create single-execution functions in JavaScript. By understanding and utilizing these techniques, you can efficiently handle situations where a specific functionality should be executed only once.

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