notation difference for defining a function in Javascript

Question

I am writing a cellRenderer component for ag-grid.

If I write it like below, it works fine :

function SignalCellRenderer() {}
SignalCellRenderer.prototype.init = function (params) {
    // create the cell
    this.eGui = document.createElement('div');
    this.eGui.innerHTML = '<span class="my-css-class"><button class="btn-simple">Push Me</button><span class="my-value"></span></span>';
};

But if I use the below notation, it fails :

let SignalCellRenderer = () => {}
SignalCellRenderer.prototype.init = function (params) {
    // create the cell
    this.eGui = document.createElement('div');
    this.eGui.innerHTML = '<span class="my-css-class"><button class="btn-simple">Push Me</button><span class="my-value"></span></span>';
};

with this error message

TypeError: Cannot set property 'init' of undefined

I thought those 2 ways of defining a function were equivalent but clearly not.

Could someone please explain the difference ?