interpolate.js

// Adj. Close for Each Month
let adj = [
    30.550734,
    29.040157,
    28.25503,
    32.47147,
    28.410118,
    30.476208,
    30.281532,
    28.305592,
    31.629807,
    31.307159,
    29.93041,
    29.832214,
    27.838818,
    27.912491,
    29.984467,
    30.349529,
    29.816734,
    30.399223,
    29.457609,
    36.048897,
    34.842018,
    41.931904,
    41.759998,
    41.990002
];

console.log("Linearly interpolating...")

// Actually calculates the linear function
function interpolate(point1, point2) {
        let m = doDecimalSafeMath(
            doDecimalSafeMath(point2[1], "-", point1[1]),
            "/",
            doDecimalSafeMath(point2[0], "-", point1[0])
        ),
        b = doDecimalSafeMath(m, "*", point1[0])
    
    return `y = ${m}x + ${b}`
}


function doDecimalSafeMath(a, operation, b, precision) {
    function decimalLength(numStr) {
        var pieces = numStr.toString().split(".");
        if(!pieces[1]) return 0;
        return pieces[1].length;
    }

    // Figure out what we need to multiply by to make everything a whole number
    precision = precision || Math.pow(10, Math.max(decimalLength(a), decimalLength(b)));

    a = a*precision;
    b = b*precision;

    // Figure out which operation to perform.
    var operator;
    switch(operation.toLowerCase()) {
        case '-':
            operator = function(a,b) { return a - b; }
        break;
        case '+':
            operator = function(a,b) { return a + b; }
        break;
        case '*':
        case 'x':
            precision = precision*precision;
            operator = function(a,b) { return a * b; }
        break;
        case '÷':
        case '/':
            precision = 1;
            operator = function(a,b) { return a / b; }
        break;

        // Let us pass in a function to perform other operations.
        default:
            operator = operation;
    }

    var result = operator(a,b);

    // Remove our multiplier to put the decimal back.
    return result/precision;
}

// Creates all the sets of points for each function
let points = [...Array(adj.length - 2)].map((v, i, a) => [adj[i], adj[i + 1]]);

for (var i = 0; i < points.length; i++) {
    var line = points[i];
    console.log(
        interpolate([i, line[0]], [i + 1, line[1]])
    )
    
}

console.log("Done");