jsFiddle: http://jsfiddle.net/fenivana/xr63gj2x/ ``` js const a = []; Vue.set(a, 10, 'foo'); console.log(a); // ['foo'] alert(a.length); // 1 alert(a[10]); // undefined alert(a[0]); // 'foo' ``` Expected `a` would be `[ undefined x 10, 'foo']`, bug got `['foo']` instead.