For the code below, I tried to get the position of Time Square in the line "N", which is 0, however when I run my code, it shows: startLine.indexof is not a function.

const mta = {
    subwayLines: {
        N: ["Times Square", "34th", "28th", "23rd", "Union Square", "8th"],
        L: ["8th", "6th", "Union Square", "3rd", "1st"],
        SIX: ["Grand Central", "33rd", "28th", "23rd", "Union Square", "Astor Place"]
    },
    findStartIndex: function(station, line) {
        if (line in this.subwayLines) {
            let startLine = this.subwayLines[line];
            let indexOfStart = startLine.indexof(station);
            console.log(indexOfStart);
        }
    }
}

How do I fix this code? Thank you.