A function for simulating key event in JavaScript. You just have to choose what key and keyboard event you want to simulate.

KeyEventSimulator.js

/**
 * Simulate a key event.
 * @param {Number} keyCode The keyCode of the key to simulate
 * @param {String} type (optional) The type of event : down, up or press. The default is down
 * @param {Object} modifiers (optional) An object which contains modifiers keys { ctrlKey: true, altKey: false, ...}
 */
function simulateKey (keyCode, type, modifiers) {
	var evtName = (typeof(type) === "string") ? "key" + type : "keydown";	
	var modifier = (typeof(modifiers) === "object") ? modifier : {};

	var event = document.createEvent("HTMLEvents");
	event.initEvent(evtName, true, false);
	event.keyCode = keyCode;
	
	for (var i in modifiers) {
		event[i] = modifiers[i];
	}

	document.dispatchEvent(event);
}

// Setup some tests

var onKeyEvent = function (event) {
	var state = "pressed";
	
	if (event.type !== "keypress") {
		state = event.type.replace("key", "");
	}
	
	console.log("Key with keyCode " + event.keyCode + " is " + state);
};

document.addEventListener("keypress", onKeyEvent, false);
document.addEventListener("keydown", onKeyEvent, false);
document.addEventListener("keyup", onKeyEvent, false);

// Using the function
simulateKey(38);
simulateKey(38, "up");
simulateKey(45, "press");

That was super helpful! Thank you!

Thanks @GlauberF

Pretty nice! Thx!

can i use this to make alt key forever down, and if so how?

Will it work on Safari?

how can I send the simulated keys into an input element?

Thank you. This is amazing. Works in Chrome and CEF (Max/MSP jweb)

This doesn't work for me, on Chrome 109. What could be the reason? Any help with diagnosing the issue would be appreciated. Thanks!

Also not working for me on Chrome Version 109.0.5414.120

some improvements and applied the function to typescript.

simulateKey(keyCode: number, type: string, modifiers: {[key: string]: any} = {'isTrusted': true}): void {
        const evtName = (typeof type === 'string') ? 'key' + type : 'keydown';
        const modifiersObj = (modifiers instanceof Object) ? modifiers : {};

        const event = new KeyboardEvent(evtName, { keyCode });

        Object.assign(event, modifiersObj);

        document.dispatchEvent(event);
    }

what to call

simulateKey(38);
simulateKey(38, "up");
simulateKey(45, "press");

Tested on Chrome ( Version 110.0.5481.77 (Official Version) 64-bit)