javascript - str[index + 1] 在 for 循环中返回 undefined

Question

为什么会出现这种情况？

let str = 'sSAo'
console.log(str[0], str[3]) // all good

for (let i in str) {
    // why str[i+1] is undefined ???
    console.log(i, str[i], str[i+1])
}

标签： javascriptfor-loop

Answer 1

The problem is that for..in loops iterate over the property names of the object. But property names are always strings, not numbers. Hence, for example, on the first iteration:

str[i+1]

evaluates to

str['0'+1]

which is

str['01']

Instead, cast i to a Number first:

let str = 'abcd'

for (let i in str) {
    console.log(i, str[i], str[Number(i)+1])
}