Javascript method populationGrowth(simulationLength) that determines how many amoebas will exist after simulationLength steps

amoeba.js

let fibonacci =  ( n , dict  ) =>{

    if ( dict[n] ){
        return dict[n];
    }

    if ( n <= 1 ){
        return 1;
    }

    let fibN =  fibonacci(n-1 , dict) + fibonacci(n-2 , dict);
    return fibN;
}

let populationGrowth = ( simulationLength ) => {

    if ( simulationLength < 0 ){
        return -1;
    }

    let dict  = {};
    length  = fibonacci(simulationLength , dict );
    return length;
  
};

for ( n = 0 ; n < 10 ; n++ ){
    console.log(populationGrowth(n));
}