Sample Code

// Use these independent variables in the harmonic regression.

var harmonicIndependents = ee.List(['constant', 't', 'cos', 'sin']);

//print(harmonicIndependents, 'harmonicIndependents');

// Add harmonic terms as new image bands.

var harmonicLandsat = landsatCombined.map(function(image) {

    var timeRadians = image.select('t').multiply(2 * Math.PI);

    return image

        .addBands(timeRadians.cos().rename('cos'))

        .addBands(timeRadians.sin().rename('sin'))

        .clip(roi);

});

print(harmonicLandsat, 'harmonicLandsat');

// // Fit the model.

var harmonicTrend = harmonicLandsat

    .select(harmonicIndependents.add(dependent))

    // The output of this reducer is a 4x1 array image.

    .reduce(ee.Reducer.linearRegression(harmonicIndependents.length(),

        1));

print(harmonicTrend, 'harmonicTrend');