SOURCE

const _permute = string => {
    if(string.length === 1) {
        return [string]
    }
    const results = []
    for(let s of string){
        const arr = string.split('').filter(str =>str !== s)
        _permute(arr.join('')).forEach(item => {
            results.push(s + item)
        })
    }
    return results
}
console.log(_permute('rap'))