javascript instance method
Map.prototype.getOrInsert()
The getOrInsert() method of Map instances returns the value corresponding to the specified key in this Map. If the key is not present, it inserts a new entry with the key and a given default value, and returns the inserted value.
If the computation of the default value is expensive, consider using getOrInsertComputed() instead, which takes a callback to compute the default value only if it’s actually needed.
const map = new Map([["bar", "foo"]]);
console.log(map.getOrInsert("bar", "default"));
// Expected output: "foo"
console.log(map.getOrInsert("baz", "default"));
// Expected output: "default"
Syntax
getOrInsert(key, defaultValue)
Parameters
key- : The key of the value to return from the
Mapobject. Object keys are compared by reference, not by value.
- : The key of the value to return from the
defaultValue- : The value to insert and return if the key is not already present in the
Mapobject.
- : The value to insert and return if the key is not already present in the
Return value
The value associated with the specified key in the Map object. If the key can’t be found, defaultValue is inserted and returned.
Description
The getOrInsert() method is equivalent to the following:
if (map.has(key)) {
return map.get(key);
}
map.set(key, defaultValue);
return defaultValue;
It is also similar to the following pattern (which is slightly less reliable if null or undefined are valid values in your map):
map.set(key, map.get(key) ?? defaultValue);
Examples
Multimap
In a map where each key is mapped to an array of values, you can use getOrInsert() to ensure that the array exists for a given key before attempting to push a new value to the array.
map.getOrInsert(key, []).push(value);
Applying default values
You can use getOrInsert() to ensure that a key exists in a map, even if you currently don’t need its value. This is usually to normalize user input.
Imagine you have a map of user preferences, and you want to ensure that a certain preference is always set to a default value if the user hasn’t specified it:
const options = readConfig();
options.getOrInsert("theme", "light");
options.getOrInsert("fontSize", 14);
// Later in your code, you can safely assume these options exist
document.body.dataset.theme = options.get("theme");