Q:

onclick called method is not defined

I have the code running on jsfiddle.  The code is below.  I can bet a ton of money this is me doing something stupid, but I can't find it.
Clicking on  says that Uncaught ReferenceError: showDetail is not defined 
I have tried varying adding of parameters as well just to try and troubleshoot.  Passing none, one, two doesn't effect the error message.
const lcAppdata = {

    matters: [{
        mid: 100,
        title: "Matter1",
        dateOpened: "01/01/17",
        decedent: "Elvis Presley",
        personalRepresentative: "Alexis Texas",
        attorneyAccountInfo: "John Holmes"
    }, {
        mid: 200,
        title: "Matter2",
        dateOpened: "01/02/16",
        decedent: "Kurt Cobain",
        personalRepresentative: "Kagney Linn Karter",
        attorneyAccountInfo: "Simon Rex"
    }, {
        mid: 300,
        title: "Matter3",
        dateOpened: "01/03/15",
        decedent: "Bob Marley",
        personalRepresentative: "Nikki Benz",
        attorneyAccountInfo: "Ron Jeremy"
    }],
    currentMatter: ''
};

function showDetail(mid){
    console.log(mid);
};

function init(){
    for(let matter of lcAppdata.matters){
    $('#tbodyMatterList').append(`<tr 
            onclick="showDetail(${matter.mid})"> 
            <th scope="row" >${matter.title}</th>
            <td bind>${matter.decedent}</td>
            <td bind>${matter.dateOpened}</td>
            <td bind>${matter.personalRepresentative}</td>
            <td bind>${matter.attorneyAccountInfo}</td></tr>`);

    }

};

init();

A:

You code is fine, it's because of how jsfiddle runs your script. It's set to run the script onLoad i.e. all your code is actually run inside the callback of load event so your function showDetail is not actually defined as a global function, but only local to the load event callback
You can either change your code to attach the showDetail function to the window object to make it global like this
function showDetail() {
  console.log($(this).attr('data-mid'));
};

window.showDetail = showDetail;

or change jsfiddle settings to run your script globally like this

