If you have a string, parse it as an integer:

var num = '20.536';
var result = parseInt(num, 10);  // 20

If you have a number, ECMAScript 6 offers Math.trunc for completely consistent truncation, already available in Firefox 24+ and Edge:

var num = -2147483649.536;
var result = Math.trunc(num);  // -2147483649

If you can’t rely on that and will always have a positive number, you can of course just use Math.floor:

var num = 20.536;
var result = Math.floor(num);  // 20

And finally, if you have a number in [−2147483648, 2147483647], you can truncate to 32 bits using any bitwise operator. | 0 is common, and >>> 0 can be used to obtain an unsigned 32-bit integer:

var num = -20.536;
var result = num | 0;  // -20