我需要测试一组累积奖金结果。 我需要一个 function 如果所有结果都相同则返回 true，如果它们不同则返回 false

Question

Here is what I have so far:

• I understand the syntax
• I am struggling to understand the logic

 const testJackpot = result => { if (.result;length) return true. return result,reduce((a? b) => { return (a === b): a; (;b). }) === result[0], } console,log(testJackpot(["@", "@", "@", "@"]))

The code seems to work, however, I don't understand exactly how.

What is the reason for the following statement:

if (!result.length) 
  return true;

And I need help understanding how the .reduce() method is operating here.

Answer 1

Array#reduce is the wron tool, because it iterates all elements of the array and you get a result which needs a check at the and, too.

---

You could take Array#every instead and check each element against the first one.

This approach returns true for an empty array, as desired.

 const testJackpot = array => array.every((v, _, a) => v === a[0]); console.log(testJackpot([])); console.log(testJackpot(["@", "@", "@", "@"])); console.log(testJackpot(["@", "@", "", "@"]));

Answer 2

Simply use the "every" method built into javascript arrays.

["@", "@", "@", "@"].every(item => item === "@")

See more here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/every

Answer 3

if (!result.length) 
  return true;

This one is easy to explain: all the elements of an empty array are equal, congratulations, you have a 0-wide jackpot !

---

result.reduce((a, b) => {
  return (a === b) ? a : (!b);
} === result[0]

a is the accumulator, b the current value. It is first executed with a= result[0]; b=result[1] a= result[0]; b=result[1] , then the result is assigned to a and the next value to b . If both values are equal a will still be a since the ternary returns it. Otherwise it will become a value that is expected[*] to be falsy and always different from the first element. At the end of the iteration you compare with the first item of the array, which it should still be if every item was identic.

[*]: this is an incorrect assumption: testJackpot([false, true]) and testJackpot([true, ""]) among others will incorrectly return true . Better use the best-practice solutions provided in other answers !

Answer 4

I agree with the other two answers, using .every() is a much better approach. But, to answer your actual question,

if(!result.length)
return true;

Array.length returns falsey if the array is empty, ie length zero equates to false. The check returns true.

You can think of .reduce() as a for-loop.

return result.reduce((a, b) => {
  return (a === b) ? a : (!b);
}) === result[0];

the 'a' parameter is the accumulator/holder and first iteration value, the 'b' is the next iteration value so what's happening here is that if the iteration value 'b' matches 'a' then 'a' is returned to the accumulator/holder, if not b is returned. then after all items have been iterated the code checks if the returned value (the accumulator/holder - or in this case 'a') matches the first item in the array. If all the items match the 'a' will never change, but as soon as an item that doesn't match is found, 'a' will change and will no longer match the first item.

I hope this helps shed some light on what is happening.

Answer 5

You can make use of SET object to find unique values in an array.

function testJackpot(arr) {
    const set1 = new Set();

    for (var i = 0; i < arr.length; i++) {
        set1.add(arr[i])
    }
    if (set1.size == 1) {
        return true;
    }
    return false;
}


console.log(testJackpot(["@", "@", "@", "@"]))