function document.getElementsByTagName does not work

your code is almost correct, but it does not work because you should not put it that way onClick , since this way it will not work, instead you should write it like this onclick , in lowercase, javascript is sensitive to lowercase and capital letters, I'll give you an example:

function iniciar(){
    document.getElementsByTagName('p')[0].onclick = saludar;
}

function saludar(){
    alert('Hola!!');
}

//Ejecutamos 'iniciar()', como si fuera window.onload
iniciar();

<p>ejemplo 1er parrafo</p>
<p>ejemplo 3er parrafo</p>
<p>ejemplo 4to parrafo</p>

You can also use the addEventListener method to 'add' a function to an event, like this:

function iniciar(){
    document.getElementsByTagName('p')[0].addEventListener('click', saludar, false);
}

function saludar(){
    alert('Hola!!');
}

iniciar();

<p>ejemplo 1er parrafo</p>
<p>ejemplo 3er parrafo</p>
<p>ejemplo 4to parrafo</p>

With regard to missing the parentheses, I commented that the parentheses make your function run, that is, if you do:

document.getElementsByTagName('p')[0].onclick = saludar();

You are executing the function saludar and assigning the return of saludar as function onclick , but your function returns nothing, so the assignment would be something like .onclick = undefined; , but you can also return a function so that is assigned to onclick , I leave an example:

function iniciar(){
    document.getElementsByTagName('p')[0].onclick = saludar();
}

function saludar(){
    //Retornamos la función '_saludar'
    return _saludar;
}

function _saludar(){
    alert('Hola!!');
}

iniciar();

<p>ejemplo 1er parrafo</p>
<p>ejemplo 3er parrafo</p>
<p>ejemplo 4to parrafo</p>

I hope and it works, greetings.

My recommendation is as follows. 1- Change onClick to onclick all in lowercase, although it is preferable that you do it with the function addEventListener. 2- Make sure that when you call the getElements function ... the returned value is not null, which is the return value when there are no matches.

Check your code and correct it a bit:

Code :

function ejecuta(){
    document.getElementsByTagName("p")[0].addEventListener("click", saludo);
}

function saludo(){
    alert("Hola");
}

window.onload=ejecuta();

Greetings !!

It's because those elements do not have the 'click' event and a function linked to that event.

Then to solve you can apply to all the elements with tag p by going through and adding to each element so that it listens to the function you want.

your code would be like this:

window.onload = function() {
  ejecuta();
}

function ejecuta() {

  var arrayTags = document.getElementsByTagName("p");

  Object.values(arrayTags).forEach(function(current) {
    current.addEventListener('click', saludo);
  });

}

function saludo() {
  console.log("Hola");
}

<p>ejemplo 2do parrafo</p>
<p>ejemplo 3 parrafo</p>
<p>ejemplo 4 parrafo</p>

Responding to the observation of @ftorres

  

those elements if they have the 'click' property, only that you have it   to assign a function using the syntax 'element.onclick =   function; ', and not' element.onClick = function;

In the following code I will show that the element is not born with the 'click' event code:

var parrafo=document.getElementById('parrafo');

console.log(parrafo.getAttribute('click'));

<p id="parrafo">soy parrafo</p>