JavaScript nested inner for loop not working

You had a typo in the inner loop: you mistyped length as lenght!

const bobsFollowers = ['vinit','vidyesh','bipin','shobhana'];
const tinasFollowers = ['vinit','vidyesh','manish'];
const mutualFollowers = [];
for (let i = 0; i < bobsFollowers.length; i++){
  for (let j = 0; j < tinasFollowers.length; j++){
    if (bobsFollowers[i] === tinasFollowers[j]){
      mutualFollowers.push(tinasFollowers[j]);
    }     
  }
};
console.log(mutualFollowers);

you made a mistake in your code : in the second "for" you wrote "tinasFollowers.lenght" instead of "tinasFollowers.length" ;)
Here is the correct code :

const bobsFollowers = ['vinit','vidyesh','bipin','shobhana'];
const tinasFollowers = ['vinit','vidyesh','manish'];
const mutualFollowers = [];
for (let i = 0; i < bobsFollowers.length; i++){
    for (let j = 0; j < tinasFollowers.length; j++){
        if (bobsFollowers[i] === tinasFollowers[j]){
            mutualFollowers.push(tinasFollowers[j]);
        }
    }
};
console.log(mutualFollowers);

Much easier with filter

const bobsFollowers = ['vinit','vidyesh','bipin','shobhana'];

const tinasFollowers = ['vinit','vidyesh','manish'];


const mutualFollowers  = bobsFollowers.filter(element => tinasFollowers.includes(element));

console.log(mutualFollowers);

Seems like you are misspelling length as lenght on the 5th line. and also there is an extra semicolon on the 10th line, which is not causing the error. Here is the solution.

const bobsFollowers = ['vinit','vidyesh','bipin','shobhana'];
const tinasFollowers = ['vinit','vidyesh','manish'];
const mutualFollowers  = [];
for(let i = 0; i < bobsFollowers.length; i++){
    for(let j = 0; j < tinasFollowers.length; j++){
        if(bobsFollowers[i] === tinasFollowers[j]){
            mutualFollowers.push(bobsFollowers[i]);
        }
    }
}
console.log(mutualFollowers );