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• reduce initial value

reduce initial value

QUESTION: What is the problem with both snippets below?

[].reduce((previous, current) => {
  return previous + current;
});

''.split('').reduce((previous, current) => {
  return `${previous}-${current}`;
});

ANSWER: Correct:

The problem is that with empty arrays and no default initial values for reduce, the JS engine doesn't have anything to return and throws an exception.

TIP: Always provide an initial, default value for your reduces. 0 or 1 for reduces that sum or subtract, 1 for addition and subtraction, empty string for strings. For other cases, consider each one carefully.

[].reduce((previous, current) => {
  return previous + current;
}, 0);

''.split('').reduce((previous, current) => {
  return `${previous}-${current}`;
}, '');