JavaScript - add value to Set and return true on success or false on fail

In this article, we would like to show you how to add value to Set and return true on success or false on fail in JavaScript.

Quick solution:

const set = new Set();

if (set.size !== (set.add('Some value ...'), set.size)) {
    console.log('Some value added!');
}

Practical examples

1. Using size accessor

In this example, we check the set size before and after adding a value to it. If the sizes are the same, it means the element hasn't been added and the function returns false, otherwise returns true.

// ONLINE-RUNNER:browser;

const addValue = (set, value) => set.size !== (set.add(value), set.size);


// Usage example:

const set = new Set();

console.log(addValue(set, 'a'));  // true
console.log(addValue(set, 'a'));  // false
console.log(addValue(set, 'b'));  // true
console.log(addValue(set, 'c'));  // true

2. Using has() method

In this example, we use has() method to check whether an element with the specified value exists in a Set object. If the value exist, it means the element can't be added and the function returns false, otherwise returns true.

// ONLINE-RUNNER:browser;

const addValue = (set, value) => set.has(value) ? false : (set.add(value), true);
// or: const addValue = (set, value) => !set.has(value) && (set.add(value), true);


// Usage example:

const set = new Set();

console.log(addValue(set, 'a'));  // false
console.log(addValue(set, 'a'));  // true
console.log(addValue(set, 'b'));  // false
console.log(addValue(set, 'c'));  // false

References

1. Set.prototype.size - JavaScript | MDN
2. Set.prototype.has() - JavaScript | MDN