Replacing array value with objects

Question

I have this JavaScript object:

 const DayOfWeek = [
 { key: 'MON', value: 'M' },
 { key: 'TUE', value: 'T' },
 { key: 'WED', value: 'W' },
 { key: 'THU', value: 'T' },
 { key: 'FRI', value: 'F' },
 { key: 'SAT', value: 'S' },
 { key: 'SUN', value: 'S' },
 ];

Note: If frequencyDayOfWeek is WED replace with object { key: 'WED', value: 'W' }

let frequencyDayOfWeek = [
 "WED",
 "THU",
 "FRI",
 "SAT"
];
frequencyDayOfWeek.map((day, index) => { //not working
 day.find((data, i) => {
 if (DayOfWeek[i].key == data) {
 //return match arrays of object;
 // [{ key: 'MON', value: 'M' }]
 // [{ key: 'WED', value: 'W' }]
 }
 })
});

So here is the result I want to get:

 "frequencyDayOfWeek": [
 { key: 'WED', value: 'W' },
 { key: 'THU', value: 'T' },
 { key: 'FRI', value: 'F' },
 { key: 'SAT', value: 'S' },
 ]

How can I implement it using JavaScript?

• Heretic Monkey: Does this answer your question? How to filter array when object key value is in array

Answer 1

You need to return a value from the mapping function.

For example:

const DayOfWeek = [
 { key: 'MON', value: 'M' },
 { key: 'TUE', value: 'T' },
 { key: 'WED', value: 'W' },
 { key: 'THU', value: 'T' },
 { key: 'FRI', value: 'F' },
 { key: 'SAT', value: 'S' },
 { key: 'SUN', value: 'S' },
];
let frequencyDayOfWeek = [
 "WED",
 "THU",
 "FRI",
 "SAT"
];
const out = frequencyDayOfWeek.map(k => {
 // actually return a value
 return DayOfWeek.find(x => x.key === k);
});
// or short:
// const out = frequencyDayOfWeek.map(k => DayOfWeek.find(x => x.key === k));
console.log(out);

• Yoshi: @HereticMonkey Well, link a duplicate. The one you linked concerns filtering, which is not what this question is about. It's about mapping one structure using data from another. Obviously the same could be achieved with filtering (in this specific context). But ultimately that's not the problem here. The problem is that they need to return a value from map/find or filter (if they so choose).
• Heretic Monkey: A distinction without a difference. There is no difference between the output of your map and the output of filter. Both arrays. Both point to the original objects.
• Heretic Monkey: And, I'm guessing you didn't even look for a duplicate before answering.
• Yoshi: @HereticMonkey You're right for this specific case, though we don't know if frequencyDayOfWeek always is a subset of DayOfWeek it might be a longer stream. In which case filtering DayOfWeek would yield the wrong result.
• Heretic Monkey: There are infinite ways in which both filter and map could fail. Your code will only return the first object with the key that matches; if there are two, then the second one will never get selected. So what? We can only answer the question as stated by the OP. And, as you've just admitted, this specific question is a duplicate of the mentioned question (which has answers that use map).

Answer 2

frequencyDayOfWeek.map(f => DayOfWeek.find(d => d.key === f));