Iterability [3] in ECMAScript 6 is one such customization. An object is iterable if it has a method whose key is the symbol (stored in) Symbol.iterator. In the following code, obj is iterable.

symbol-iterator.js

let obj = {
        data: [ 'hello', 'world' ],
        [Symbol.iterator]() {
            const self = this;
            let index = 0;
            return {
                next() {
                    if (index < self.data.length) {
                        return {
                            value: self.data[index++]
                        };
                    } else {
                        return { done: true };
                    }
                }
            };
        }
    };
    
for (let x of obj) {
    console.log(x);
}