js challenge for asserting substrings

substring.js

function substring(str) {
    if (str.length <= 1) {
        return str.length
    }

    let startpoint = 0;
    let max = 0;
    let len = str.length;
    let lookup = new Map();
    let dictionary = new Map();

    for (let i = 0; i < len; i++) {
        let char = str.charAt(i)

        if (lookup.has(char) && lookup.get(char) >= startpoint) {
            const nextPointer = lookup.get(char) + 1
            startpoint = nextPointer
        }
        lookup.set(char, i)
        const endpoint = i
        const difference = (endpoint - startpoint) + 1
        const slicedString = str.slice(startpoint, endpoint + 1)
        dictionary.set(difference, slicedString)
        max = Math.max(max, difference)
    }
    console.log(dictionary.get(max))
    console.log(max)
}

// substring("ghostbusters")