How to assign variable value to an object property in javascript?

Question

Consider this example: var v = 4; canvas.freeDrawingBrush.width = v;. I then change the value of v, and run canvas.freeDrawingBrush.width = v; again. The brush width changes as intended, however, it causes lots of bugs where some drawn paths are shifted or are partially erased. What am I doing wrong? Is it even allowed to initialize an objects property through a variable? (I am using fabric.js library).

EDIT: My original function looks like this:

switch (thickness) {
    case "small-thickness":
        v = smallThickness;
        break;
    case "medium-thickness":
        v = mediumThickness;
        break;
    case "large-thickness":
        v = largeThickness;
        break;
    default:
        v = smallThickness;
}

pencilThickness = thickness;
canvas.freeDrawingBrush.width = v;

I change the value of smallThickness, and call this block of code again.