Warn user before leaving web page if changes haven't been saved

Question

When searching for a way to warn a user before leaving web page if changes haven't been saved, I found this solution: https://stackoverflow.com/a/48238659/9512437, but the warning pops up even if the user is hitting the save button. I tried adding an event when the user clicks the submit button to set a variable to keep the warning from appearing, but now the warning never appears.

Here is what I tried:

<script>
    "use strict";
    var btn_click = false;

    (() => {
    const modified_inputs = new Set;
    const defaultValue = "defaultValue";
    // store default values
    addEventListener("beforeinput", (evt) => {
        const target = evt.target;
        if (!(defaultValue in target || defaultValue in target.dataset)) {
            target.dataset[defaultValue] = ("" + (target.value || target.textContent)).trim();
        }
    });
    // detect input modifications
    addEventListener("input", (evt) => {
        const target = evt.target;
        let original;
        if (defaultValue in target) {
            original = target[defaultValue];
        } else {
            original = target.dataset[defaultValue];
        }
        if (original !== ("" + (target.value || target.textContent)).trim()) {
            if (!modified_inputs.has(target)) {
                modified_inputs.add(target);
            }
        } else if (modified_inputs.has(target)) {
            modified_inputs.delete(target);
        }
    });
    addEventListener("beforeunload", (evt) => {
        if (modified_inputs.size && !btn_click) {
            const unsaved_changes_warning = "Changes you made may not be saved.";
            evt.returnValue = unsaved_changes_warning;
            return unsaved_changes_warning;
        }
    });
    addEventListener("")
    })();

    document.getElementById("submit").onclick = function save() {
        btn_click = true;
    }
</script>

Any idea what I did wrong?

Answer 1

Turns out I was doing a couple things wrong. My guess is adding the document.getElementById("submit").onclick under "use strict" either caused an error (https://www.w3schools.com/js/js_strict.asp) or caused a problem with the detection since simply adding the function even without the && !btn_click caused it to not work. I also had to change if (modified_inputs.size && !btn_click) { to if (modified_inputs.size >> 0 && !btn_click) {.

In the end, the solution that ended up working for me is as follows:

<script>
    var btn_click = false;

    function save() {
        btn_click = true;
    }

    "use strict";
    (() => {
    const modified_inputs = new Set;
    const defaultValue = "defaultValue";
    // store default values
    addEventListener("beforeinput", (evt) => {
        const target = evt.target;
        if (!(defaultValue in target || defaultValue in target.dataset)) {
            target.dataset[defaultValue] = ("" + (target.value || target.textContent)).trim();
        }
    });
    // detect input modifications
    addEventListener("input", (evt) => {
        const target = evt.target;
        let original;
        if (defaultValue in target) {
            original = target[defaultValue];
        } else {
            original = target.dataset[defaultValue];
        }
        if (original !== ("" + (target.value || target.textContent)).trim()) {
            if (!modified_inputs.has(target)) {
                modified_inputs.add(target);
            }
        } else if (modified_inputs.has(target)) {
            modified_inputs.delete(target);
        }
    });
    addEventListener("beforeunload", (evt) => {
        if (modified_inputs.size >> 0 && !btn_click) {
            const unsaved_changes_warning = "Changes you made may not be saved.";
            evt.returnValue = unsaved_changes_warning;
            return unsaved_changes_warning;
        }
    });
    addEventListener("")
    })();
</script>

Then add your onclick to the element:

<button type="submit" class="w3-button w3-right w3-theme" id="button" onclick="save()">Save</button>