SOURCE

let context = {a: 1, b:2, c:3, d:4};
console.log(JSON.stringify(getDependencies(context, (context) => {
    if (context.a) {
        return context.b;
    } else {
        return context.c;
    }
})));
/*
    希望返回 {
        value: 2
        dependencies: ['a', 'b']
    }
*/
function getDependencies(context, callback) {
    //实现
    let dependencies = [];
    let value;
    function observe(data, key, value) {
        let check = {};
        Object.defineProperty(data, key, {
            get() {
                if (!check[key]) {
                    dependencies.push(key);
                    check[key] = true;
                }
                return value;
            },
            set(newVal) {
                if (newVal !== value) {
                    value = newVal;
                }
            }
        })
    }
    Object.keys(context).forEach((key) => {
        observe(context, key, context[key]);
    });
    value = callback(context);
    return {value, dependencies}
}