You have to use bracket notation:
var obj = {};
obj[a[i]] = 0;
x.push(obj);
The result will be:
x = [{left: 0}, {top: 0}];
Maybe instead of an array of objects, you just want one object with two properties:
var x = {};
and
x[a[i]] = 0;
This will result in x = {left: 0, top: 0}.