CODEWITHSUNDEEP
javascriptarraysreduceshorthand
Arcaster
Arcaster

Reputation: 119

Javascript .reduce Boolean Unexpected Return

Trying to create a function to check for a tall number (i.e., every digit is less than or equal to the digit to its right, such as 123, 059, etc.).

Following code is the issue:

const n = parseInt(readline());
if (n.toString().length === 1)
    console.log('true');
else
{
    var z = n.toString().split('');
    console.log(z.reduce((a, b) => b > a));
}

This test fails: 01223047 (returns true when it should return false).

It worked on numerous other tests, but not this value. Shouldn't reduce be testing each pair and returning true/false? If there's one false, it should return false, correct? Am I using reduce incorrectly? Is there another function I could use to shorthand this test instead of writing a loop?

Upvotes: 1

Views: 2139

Answers (1)

T.J. Crowder
T.J. Crowder

Reputation: 1075059

Remember that with reduce, the first argument supplied to your callback is the return value of the previous call to the callback. (In the first call, if you haven't provided a seed value — and you haven't — the first value will be the first entry in the array.)

So no, that reduce doesn't test each pair. It tests the first pair, and then it tests the rest of the values (individually) against the previous callback's returned flag.

For what you're doing, you probably want a simple loop, or some, or every (probably every). With some and every you'd use the index supplied as the second argument to the callback to get the next (or previous) character. Using every would also eliminate the special case for a single-character string:

function test(str, expect) {
    const z = str.split(""); // I'd probably use `= Array.from(str);` on ES2015+
    const result = z.every((ch, i) => i === 0 || ch > z[i - 1]);
    console.log(str, result, !result === !expect ? "Good" : "ERRROR");
}


test("123", true);
test("1", true);
test("21", false);

(There's probably an argument for using localCompare in that: ch.localeCompare(z[i - 1]) > 0 instead of ch > z[i - 1].)

Upvotes: 5

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