Optional parameters in the for loop

    const arr = [1, 2, 3]
    for (let i = 0; i < arr.length; i++) {
      console.log(i);
    } 

In daily development , Many times you will use for loop , Do you know when for When the first parameter or the third parameter in the loop is not filled , What will it do ？

Situation 1   The first parameter is not filled in  

var longestCommonPrefix = function(strs) {
if (strs.length === 0) return "";
let one = strs[0];
for (let i = 1; i < strs.length; i++) {
    let j = 0;
    // j  To be less than strs The length of the string in the array starting from the second element 
    //  And less than one, That is to say strs The length of the first element string 
    for (; j < strs[i].length && j < one.length; j++) {
        if (strs[i][j] !== one[j]) break;
    }
    //  the second for The first parameter of the loop is not filled , For the second for It can also be used outside the loop j Parameters 
    one = strs[i].substr(0, j);
    if (one === "") return ""
}
return one;
};

for The first parameter in the loop is not filled , In order to in for Outside of the loop , You can also use the first parameter   

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  Situation two   The third parameter is not filled in  

var romanToInt = function(s) {
const obj = {
    I: 1,
    IV: 4,
    V: 5,
    IX: 9,
    X: 10,
    XL: 40,
    L: 50,
    XC: 90,
    C: 100,
    CD: 400,
    D: 500,
    CM: 900,
    M: 1000,
};
let result = 0;
for (let i = 0; i < s.length;) {
    if (i + 1 < s.length && obj[s.substring(i, i + 2)]) {
        result += obj[s.substring(i, i + 2)];
        i += 2;
    } else {
        result += obj[s.substring(i, i + 1)];
        i++;
    }
}
return result;
};

 for The third parameter in the cycle is not filled , It's for different conditions , by for Cycle through different steps .

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expand  
substr and substring The difference between  

    const str = "123456789";
    console.log(str.substr(2,5)); // 34567
    console.log(str.substring(2,5)); // 345 
    console.log(str.slice(2, 5)); // 345
    console.log(str); // 123456789

substr The second parameter is length
substring The second parameter is the index value
slice Be similar to substring, The second parameter is also the index value