检查数值是否在 object 内

Question

I'm trying to map each number in the array as a key to the empty object, and set the value as the index. However, the first index, 0, gets skipped...?

function solve(nums) {

    let obj = {};
    for (let i=0; i<nums.length; i++) {
        if (!obj[nums[i]]) {  // if key does not exist, set key/value pair 
            obj[nums[i]] = i
        } else if (obj[nums[i]]) {
            obj[nums[i]] = 0  // if key exists, set to 0 
        }

    }

    console.log(obj)
}
solve([50, 30, 50, 90, 10])

Answer 1

You could use Map instead. Something like this:

function solve(nums) {
    let obj = new Map();
    for (let i=0; i<nums.length; i++) {
        if (obj.get(nums[i]) == undefined) {  // if key does not exist, set key/value pair
            obj.set(nums[i]) = i;
        } else {
            obj.set(nums[i], 0);  // if key exists, set to 0 
        }
    }
    console.log(obj);
}
solve([50, 30, 50, 90, 10]);

UPDATE

It seems what you really need is something different. I will try to explain it a bit better since you have 2 numbers having the same value 50 and the only thing that's unique is the index, then you could have the index as the key, but also, you could need just one value and having both indexes as a value.

For the first option you could use this:

function solve(nums) {
    let obj = new Map();
    for (let i=0; i<nums.length; i++) {
        obj.set(i,nums[i]);
    }
    console.log(obj);
}
solve([50, 30, 50, 90, 10]);

For the second option this would be the code:

function solve(nums) {
    let obj = new Map();
    for (let i=0; i<nums.length; i++) {
        if (obj.get(nums[i]) == undefined) {  // if key does not exist, set key/value pair 
            obj.set(nums[i]) = i;
        } else {
            obj.set(nums[i], obj.get(nums[i]) + ',' + i);  // if key exists, append the second index 
        }
    }
    console.log(obj);
}
solve([50, 30, 50, 90, 10]);

Answer 2

Everything is ok. It does not get skipped. You pass 50 two time but Object can not have two same properties. So second 50 is overriding the first value .

When you call solve([500, 30, 50, 90, 10])

You will get {10: 4 30: 1 50: 2 90: 3 500: 0} Which is expected result.

Answer 3

First of all, I am confused a bit with else if block, why you set it to 0 , if you find same number again.

But it is skipped, because you have 50 two times in your array. And when you loop through nums, when you get to index i = 2 , you will have in object { 50: 0, 30: 1} .

And because of that, !0 returns true , so you will get again in if block, and you will override your value for key 50 with 2 .

Try with an array with 5 different values and it will work as you expect it