第一种方法(推荐)

function sliceFixLenth(str,length) {
    const result = [];
    for (let i = 0; i <str.length ; i+=length) {
        result.push(str.slice(i,i+length));
    }
    return result;
}

let str = "helloworld"
console.log(sliceFixLenth(str,1)); // ['h', 'e', 'l', 'l','o', 'w', 'o', 'r','l', 'd']
console.log(sliceFixLenth(str,2));// [ 'he', 'll', 'ow', 'or', 'ld' ]
console.log(sliceFixLenth(str,3));// [ 'hel', 'low', 'orl', 'd' ]

第二种方法

function sliceFix(str,length) {
    const count = Math.ceil(str.length / length);
    let start = 0;
    let end = 0;
    const result = [];
    for (let index = 0; index < count; index++) {
        end += length;
        result.push(str.slice(start,end)) 
        start += length;
    }
    return result;
}

console.log("sliceFix",sliceFixLenth(str,3));