How to Flatten an Array in JavaScript

This is a brief run through on how to flatten an array in JavaScript.

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[jkwon-]

when you used the concat method, why did you pass in flatten(element)? It seems to work just passing in (element). Can you explain? Thank you!

[deepsarkar]

flatArray = flatArray.concat(element) this code will also work, we don't need to call the function recursively

[charlesloehle]

My code is logging [1, 2, 3, undefined]. Can't figure out why.
here's my code:
const data = [[1, 2], 3, 4[5, [6, 7]]];

function flatten(arr) {
let flatArray = []

arr.forEach(element => {
if (Array.isArray(element)) {
flatArray =
}
else {
flatArray.push(element);
}
});

return flatArray;
}

const newArray = flatten(data);

console.log(newArray);