Question 251866 in Javascript Question posted in
A very good W3school tutorial can be found here.

Javascript – Merge data from an object into existing Map()?

How do we merge data from an object to an existing Map?

To create a Map from an object we can use this:

const myMap = new Map(Object.entries(myObject));

Is there a way to do the same to an existing Map() without using for loop?

const myObject = {
  test: "blah",
  ok: 123
}

const myMap = new Map(Object.entries(myObject));

const mySecondObject = {
  test: "new value",
  foo: "bar"
}

for(let i in mySecondObject)
{
  myMap.set(i, mySecondObject[i]);
}

console.log({size: myMap.size, test: myMap.get("test"), myMap});
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2

Answers


  1. 0

    Note: the console in stackoverflow prevents Maps from being printed properly. It will work if you paste the code elsewhere.

    If the objective is concise code, you can do this:

    const myObject = {
  test: "blah",
  ok: 123
}

const myMap = new Map(Object.entries(myObject));

const mySecondObject = {
  test: "new value",
  foo: "bar"
}

Object.entries(mySecondObject).forEach(x => myMap.set(...x))

console.log({size: myMap.size, test: myMap.get("test"), myMap});

    If you don’t mind creating a new merged map, you can do so like this:

    const myObject = {
  test: "blah",
  ok: 123
}

const myMap = new Map(Object.entries(myObject));

const mySecondObject = {
  test: "new value",
  foo: "bar"
}

const mergedMap = new Map([...myMap, ...Object.entries(mySecondObject)])

console.log({size: mergedMap.size, test: mergedMap.get("test"), mergedMap});
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  2. 0

    You’re looking for the Map.prototype.merge method from the collection methods proposal, which is still in stage 1. With a polyfill, you could write

    myMap.merge(Object.entries(mySecondObject));

    But currently no such method exists. There are many ways to have a helper method do the enumeration of the object keys and/or iterate over them, which would allow you to avoid for syntax, but they’re not as straightforward:

    for (const p in obj) map.set(p, obj[p]);
Object.keys(obj).forEach(p => map.set(p, obj[p]));
Object.entries(obj).forEach(([k, v]) => map.set(k, v));
Object.entries(obj).forEach(map.set.apply.bind(map.set, map))
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