Create and return a new array with the separator interposed between elements; intersperse :: a -> [a] -> [a]

intersperse.js

/**
 * Create and return a new array with the separator interposed between elements.
 *
 * intersperse :: a -> [a] -> [a]
 *
 * Providing the separator as the first argument allows this method to be easily
 * curried and composed with other functions; follows the "data last" approach.
 *
 * @param {*} separator
 * @param {*} array
 */
const intersperse = (separator, list) => {
    const length = list.length;
    const output = [];
    let i = 0;

    // eager return is list cannot be interspersed
    if (length < 2) {
        return Array.from(list);
    }

    // add each elem and separator to new list
    while (i < length - 1) {
        output.push(i, separator);
        i++;
    }

    // add the last index to the new list
    output.push(list[i])

    return output;
}

/**
 * For future-safety (avoid built-ins) we can create the intersperse function as a
 * polyfill with little effort.
 */
export const interspersePolyfill = (function () {
    if (!Array.prototype.intersperse) {
        Object.defineProperty(Array.prototype, "intersperse", {
            writable: false,
            value: intersperse
        })
    }
}
)();

export default intersperse;