How to create a one-off event handler in JavaScript

When you register an event handler to an event of an element by using the addEventListener() method, the event handler executes every time the event occurs.

To create a one-off event handler that executes only once when the event occurs for the first time, you can use the third parameter of the addEventListener() method:

elem.addEventListener(type, handler, {
    once: true
});

The third parameter of the addEventListener() method is an optional object that defines the properties of the event listener. One of its properties is once; a boolean value indicating that the event listener should be invoked at most once after being added. If it is set to true, the event handler will be automatically removed after the first execution.

Suppose that you have the following <button> element:

<button id="register">Register Now</button>

The following example demonstrates how you can register a one-off event handler to the click event of the button:

const btn = document.querySelector('#register');

// Attach a one-off event handler
btn.addEventListener('click', (e) => {
    console.log(`Button is clicked!`);
}, {
    once: true
});

The third parameter of addEventListener() method only works in modern browsers. For old browsers like Internet Explorer, you have to manually remove the event handler by using removeEventListener() after the first execution:

// Create an Event Handler
const handler = (e) => {
    console.log(`Button is clicked!`);

    // Remove event handler after first execution
    btn.removeEventListener('click', handler);
};

// Attach event handler to button
const btn = document.querySelector('#register');

btn.addEventListener('click', handler);

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