前端js禁止鼠标右键及F12禁止查看源代码究竟为了什么？(源代码)

<script>
    document.onkeydown = function () {
        if (window.event && window.event.keyCode == 123) {
            alert("F12被禁用");
            event.keyCode = 0;
            event.returnValue = false;
        }
        if (window.event && window.event.keyCode == 13) {
            window.event.keyCode = 505;
        }
        if (window.event && window.event.keyCode == 8) {
            alert(str + "\n请使用Del键进行字符的删除操作！");
            window.event.returnValue = false;
        }
    }
</script>

<script>
    document.oncontextmenu = function (event) {
        if (window.event) {
            event = window.event;
        }
        try {
            var the = event.srcElement;
            if (!((the.tagName == "INPUT" && the.type.toLowerCase() == "text") || the.tagName == "TEXTAREA")) {
                return false;
            }
            return true;
        } catch (e) {
            return false;
        }
    }

<script>
    document.onpaste = function (event) {
        if (window.event) {
            event = window.event;
        }
        try {
            var the = event.srcElement;
            if (!((the.tagName == "INPUT" && the.type.toLowerCase() == "text") || the.tagName == "TEXTAREA")) {
                return false;
            }
            return true;
        } catch (e) {
            return false;
        }
    }
</script>

<script>
    document.oncopy = function (event) {
        if (window.event) {
            event = window.event;
        }
        try {
            var the = event.srcElement;
            if (!((the.tagName == "INPUT" && the.type.toLowerCase() == "text") || the.tagName == "TEXTAREA")) {
                return false;
            }
            return true;
        } catch (e) {
            return false;
        }
    }
</script>

<script>
    document.oncut = function (event) {
        if (window.event) {
            event = window.event;
        }
        try {
            var the = event.srcElement;
            if (!((the.tagName == "INPUT" && the.type.toLowerCase() == "text") || the.tagName == "TEXTAREA")) {
                return false;
            }
            return true;
        } catch (e) {
            return false;
        }
    }

<script>
    document.onselectstart = function (event) {
        if (window.event) {
            event = window.event;
        }
        try {
            var the = event.srcElement;
            if (!((the.tagName == "INPUT" && the.type.toLowerCase() == "text") || the.tagName == "TEXTAREA")) {
                return false;
            }
            return true;
        } catch (e) {
            return false;
        }
    }
</script>