javascript - When js array is processed, there is always an extra null value in the header

Question

need:

Two arrays: textArr and valueArr, combine these two arrays to form a string such as: 1^Q1. Separate them with \n. (Preparing to pass it into textarea)

Practical problems:

Every time, an extra blank line is added to the head of the string. The code is very short, but I can't find it, which is frustrating. Please help and take a look. Thanks!

      choicesToString () {
        let textArr = ['Q1', 'Q2', 'Q3'];
        let valueArr = [1, 2, 3];
        let choiceArr = []
        for (let i = 0; i < textArr.length; i++) {
          if (isNull(valueArr[i])) { valueArr[i] = '' }
          if (isNull(choiceArr[i])) { choiceArr[i] = '' }
          if ((valueArr[i] === '') && (choiceArr[i] === '')) { continue }
          choiceArr.push(valueArr[i] + '^' + textArr[i])
          console.log(`${i}: ${choiceArr}`)
        }
        // TODO: BUG! Add an empty cell at the first position. FUCK!!!
        // 难道要我被迫加上这段可耻的代码...
        // choiceArr.splice(0, 1)

        console.log(choiceArr)
        return choiceArr.toString().split(',').join('\n')
      }
      
     isNull (arg) {
         return !arg && arg !== 0 && typeof arg !== 'boolean' ? true : false
     }

Answer 1

You wrote all the judgments of valueArr into choiceArr

Answer 2

forloop When i is equal to 0, look at this sentence

if (isNull(choiceArr[i])) { choiceArr[i] = '' }

At this time, choiceArr is [], choiceArr[i] is choiceArr[0], which is undefined, isNull will return true, so choiceArr[i] = ' ', at this time, the length of the array has become 1, and then after executing the push below, the value of choiceArr at this time is ["", "1^Q1"], so the for loop After one pass, the length of choiceArr is not 1 but 2.

When i is equal to 1, because the value of choiceArr[1] is "1^Q1", so

if (isNull(choiceArr[i])) { choiceArr[i] = '' }

The judgment of

is false, and then execute the push statement below. At this time, the value of choiceArr is ["", "1^Q1", "2^Q2"];
when i is equal to 2 When is the same as when i=1, after the loop ends, choiceArr is ["", "1^Q1", "2^Q2", "3^Q3"].

To sum up, the null value "" is generated when i=0.

Answer 3

As mentioned above, your sentence if (isNull(choiceArr[i])) { choiceArr[i] = '' } is very strange. If you have to write it like this, just add a judgment.

if (choiceArr.length > 0 && isNull(choiceArr[i])) { choiceArr[i] = ''; }