+= vs push | which one is better to add data to an array

Hi guys,
Long story short, I read a file with a file reading API then sliced readable data into chunks and send them to reassemble by another function to avoid reading large amount of data.
Part of the code would look like this:

var data = '';
request.on('data', (chunk) => {
    data += chunk;
}).on('end', () => {
    // do something with the data
});

Code (markup):

Then I found that someone does it like this:

var data = [];
request.on('data', (chunk) => {
    data.push(chunk);...

Code (markup):

+= vs push | which one is better to add data to an array

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