Hur gör jag detta på rätt sätt? (javascript)

function dropdown() {
	var dropdownTrigger = document.getElementById("dd");
	var dropdownTrigger2 = document.getElementById("dd2");
	var dropdownTrigger3 = document.getElementById("dd3");
	var dropdownTrigger4 = document.getElementById("dd4");

	if (dropdownTrigger.style.display == "none") {

		dropdownTrigger.style.display="block";
		dropdownTrigger2.style.display="none";
		dropdownTrigger3.style.display="none";
		dropdownTrigger4.style.display="none";
	}
	else {
		dropdownTrigger.style.display="none";
	}
}
function dropdown2() {
	var dropdownTrigger = document.getElementById("dd");
	var dropdownTrigger2 = document.getElementById("dd2");
	var dropdownTrigger3 = document.getElementById("dd3");
	var dropdownTrigger4 = document.getElementById("dd4");

	if (dropdownTrigger2.style.display == "none") {

		dropdownTrigger.style.display="none";
		dropdownTrigger2.style.display="block";
		dropdownTrigger3.style.display="none";
		dropdownTrigger4.style.display="none";
	}
	else {
		dropdownTrigger2.style.display="none";
	}
}
function dropdown3() {
	var dropdownTrigger = document.getElementById("dd");
	var dropdownTrigger2 = document.getElementById("dd2");
	var dropdownTrigger3 = document.getElementById("dd3");
	var dropdownTrigger4 = document.getElementById("dd4");

	if (dropdownTrigger3.style.display == "none") {

		dropdownTrigger.style.display="none";
		dropdownTrigger2.style.display="none";
		dropdownTrigger3.style.display="block";
		dropdownTrigger4.style.display="none";
	}
	else {
		dropdownTrigger3.style.display="none";
	}
}
function dropdown4() {
	var dropdownTrigger = document.getElementById("dd");
	var dropdownTrigger2 = document.getElementById("dd2");
	var dropdownTrigger3 = document.getElementById("dd3");
	var dropdownTrigger4 = document.getElementById("dd4");

	if (dropdownTrigger4.style.display == "none") {

		dropdownTrigger.style.display="none";
		dropdownTrigger2.style.display="none";
		dropdownTrigger3.style.display="none";
		dropdownTrigger4.style.display="block";
	}
	else {
		dropdownTrigger4.style.display="none";
	}
}

function dropdownAll() {
	var ddaText = document.getElementById("dda");
	var dropdownTrigger1 = document.getElementById("dd");
	var dropdownTrigger2 = document.getElementById("dd2");
	var dropdownTrigger3 = document.getElementById("dd3");
	var dropdownTrigger4 = document.getElementById("dd4");

	if (ddaText.innerHTML == "Open all"){
		ddaText.innerHTML = "Hide all";
		dropdownTrigger1.style.display="block";
		dropdownTrigger2.style.display="block";
		dropdownTrigger3.style.display="block";
		dropdownTrigger4.style.display="block";
	}
	else {
		ddaText.innerHTML = "Open all";
		dropdownTrigger1.style.display="none";
		dropdownTrigger2.style.display="none";
		dropdownTrigger3.style.display="none";
		dropdownTrigger4.style.display="none";
	}
}

<div class="left-menu">
<p>Menu</p>
<br>
<a href="#" onclick="dropdown()"> +Menu 1</a>
<ul id="dd" style="display:none;">
<li><a href="#">item 1:1</a></li>
<li><a href="#">item 1:2</a></li>
<li><a href="#">item 1:3</a></li>
<li><a href="#">item 1:4</a></li>
</ul>
<a href="#" onclick="dropdown2()"> +Menu 2</a>
<ul id="dd2" class="dropdown" style="display:none;">
<li><a href="#">item 2:1</a></li>
<li><a href="#">item 2:2</a></li>
<li><a href="#">item 2:3</a></li>
<li><a href="#">item 2:4</a></li>
</ul>
<a href="#" onclick="dropdown3()"> +Menu 3</a>
<ul id="dd3" class="dropdown" style="display:none;">
<li><a href="#">item 3:1</a></li>
<li><a href="#">item 3:2</a></li>
<li><a href="#">item 3:3</a></li>
<li><a href="#">item 3:4</a></li>
</ul>
<a href="#" onclick="dropdown4()"> +Menu 4</a>
<ul id="dd4" class="dropdown" style="display:none;">
<li><a href="#">item 4:1</a></li>
<li><a href="#">item 4:2</a></li>
<li><a href="#">item 4:3</a></li>
<li><a href="#">item 4:4</a></li>
</ul>
<a href="#" id="dda" onclick="dropdownAll()">Open all</a>
</div>

        function toggleDropdown(ddName) 
        {
           var drop = document.getElementsByClassName('dropmenu');
           for(i=0; i<drop.length; i++) {
         if(drop[i].style.display ='none'){
            document.getElementById(ddName).style.display='block';

        }
        else {
            
            document.getElementById(ddName).style.display='none';      
        }
    }

        function toggleDropdown(ddName) {
         var drop = document.getElementsByClassName('dropmenu');
         
         for(i=0; i<drop.length; i++) {
             
             if(ddName.style.display='none'){
                document.getElementById(ddName).style.display='block';
            }
            else {
                
                drop[i].style.display = 'none';
            }
        }
    }