Question

Let’s say you have an object literal:

var d = {
    x: +'35',
    y: '25'
};

And an anonymous function:

function (d) {
    return d.y0 + d.y;
}

When I experiment with this code, I get “undefined” in place of d.y0. But there is plenty of d3.js code that accesses a value by appending 0 to its key. What does this 0 do?

Answer 1

In JavaScript, d.y0 is undefined unless d has a property named y0. I’m not familiar with d3 but if it uses pure JavaScript then the only way they can reference properties with an appended 0 is if the objects from which they’re referencing the properties have a property with that name.