use JSDoc annotations
/**
* @type {{a: boolean, b: boolean, c: number}}
*/
var x = {a: true};
x.b = false;
x. // <- "x" is shown as having properties a, b, and c of the types specified
/**
* @param {string} param1 - The first argument to this function
*/
function Foo(param1) {
this.prop = param1; // "param1" (and thus "this.prop") are now of type "string".
}