Briefly
The has method checks whether a value exists in a Set. If the value is in the collection, the method returns true; otherwise, it returns false.
Example
const watchList = new Set(['The Shining', 'Interstellar', 'Casino'])console.log(watchList.has('Casino'))// trueconsole.log(watchList.has('Black Panther'))// falseconst watchList = new Set(['The Shining', 'Interstellar', 'Casino']) console.log(watchList.has('Casino')) // true console.log(watchList.has('Black Panther')) // false
How to Write
The method takes one argument - the value to check.
Returns a boolean value:
trueif the collection contains the sought value;falseif the value is not in the collection.
In practice
Advice 1
🛠 You need to be careful when working with objects. Objects are stored and compared by reference, so the result of such code may surprise you:
const users = new Set([ {name: 'Ivan', lastName: 'Petrov'}, {name: 'Petr', lastName: 'Ivanov'}])console.log(users.has({name: 'Ivan', lastName: 'Petrov'}))// false 🤷♂️const users = new Set([ {name: 'Ivan', lastName: 'Petrov'}, {name: 'Petr', lastName: 'Ivanov'} ]) console.log(users.has({name: 'Ivan', lastName: 'Petrov'})) // false 🤷♂️
The thing is, two Ivans Petrov are actually different Ivans Petrov. This is easy to notice if you pay attention to the curly braces. Each pair of curly braces creates a new independent object, even though they look the same.
If you compare them with each other, the comparison result will also be false:
console.log({name: 'Ivan', lastName: 'Petrov'} === {name: 'Ivan', lastName: 'Petrov'})// falseconsole.log({name: 'Ivan', lastName: 'Petrov'} === {name: 'Ivan', lastName: 'Petrov'}) // false
The check will work correctly if we look for the same object:
const petrov = {name: 'Ivan', lastName: 'Petrov'}const ivanov = {name: 'Petr', lastName: 'Ivanov'}const users = new Set([petrov, ivanov])console.log(users.has(petrov))// trueconst petrov = {name: 'Ivan', lastName: 'Petrov'} const ivanov = {name: 'Petr', lastName: 'Ivanov'} const users = new Set([petrov, ivanov]) console.log(users.has(petrov)) // true
🛠 If you need to add a value to a Set, you don't need to check if the value is already in the collection. Call add. If the element is already in the collection, the addition will be ignored. Similarly with delete.