Terse way to intersperse element between all elements in JavaScript array?

Question

Say I have an array var arr = [1, 2, 3], and I want to separate each element by an element eg. var sep = \"&\", so the output is [1, \"&a

Answer 1

A few years later, here's a recursive generator solution. Enjoy!

const intersperse = function *([first, ...rest], delim){
    yield first;
    if(!rest.length){
      return;
    }
    yield delim;
    yield * intersperse(rest, delim);
};
console.log([...intersperse(array, '&')]);

Answer 2

ONE-LINER and FAST

const intersperse = (ar,s)=>[...Array(2*ar.length-1)].map((_,i)=>i%2?s:ar[i/2]);

console.log(intersperse([1, 2, 3], '&'));

Answer 3

In ES6, you'd write a generator function that can produce an iterator which yields the input with the interspersed elements:

function* intersperse(iterable, separator) {
    const iterator = iterable[Symbol.iterator]();
    const first = iterator.next();
    if (first.done) return;
    else yield first.value;
    for (const value of iterator) {
        yield separator;
        yield value;
    }
}

console.log(Array.from(intersperse([1, 2, 3], "&")));

Answer 4

If you have Ramda in your dependencies or if willing to add it, there is intersperse method there.

From the docs:

Creates a new list with the separator interposed between elements.

Dispatches to the intersperse method of the second argument, if present.

R.intersperse('n', ['ba', 'a', 'a']); //=> ['ba', 'n', 'a', 'n', 'a']

Or you can check out the source for one of the ways to do it in your codebase. https://github.com/ramda/ramda/blob/v0.24.1/src/intersperse.js