SOURCE

const str = 'eqwe1231abc1qweq'
const tar = 'abc'

// function myIndexOf(big, small) {
//     const reg = new RegExp(small)
//     const res = reg.exec(big)
//     if (res) {
//         return res.index
//     } else {
//         return -1
//     }

// }
// console.log(myIndexOf(str,tar))


function getIndex(big, small) {
    let bigLen = big ? big.length : 0
    let smallLen = small ? small.length : 0
    if (smallLen > bigLen) {
        return -1
    } 
    let res = -1
    for(let i = 0; i <= bigLen - smallLen; i++) {
        const sub = big.substr(i, smallLen)
        if(sub === small) {
            res = i
            break;
        }
    }
    return res

}

console.log(getIndex(str,tar))