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I'm fairly new in Google Earth Engine and JavaScript. I'm trying to convert Landsat8 DN to TOA reflectance using GEE using a function.

// Load a raw Landsat scene and display it.
var image = ee.Image('LANDSAT/LC08/C01/T1/LC08_116050_20130425');
Map.centerObject(image, 10);
Map.addLayer(image, {bands: ['B4', 'B3', 'B2'], min: 6000, max: 12000},
'raw');

// Compute the toa_radiance using an expression.
var toa = image.expression(
'(ML*QCAL)+AL', {
'ML': ee.Image(ee.Number(image.get('RADIANCE_MULT_BAND_1'))),
'AL': ee.Image(ee.Number(image.get('RADIANCE_ADD_BAND_1'))),
'QCAL': ee.Image(255.0)
});

print('Radiance: ', toa);
Map.centerObject(image, 9);
Map.addLayer(L, {min: -1, max: 255});

After loading running the script, I get a black image. What might probably be wrong?

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May I ask, why are you going about it this way instead of using the built-in function for this purpose: ee.Algorithms.Landsat.TOA(input) ? Have a look at the documentation, but I suspect this might be what you're after.

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