I'm a beginner of JavaScript.I learn the course on Udacity. And there is a simple question about Array practice but I can't figure out why. I will be very appreciate if anyone can answer me,thanks!

practice:

• Programming Quiz: Another Type of Loop (6-8)

• QUIZ REQUIREMENTS

• Use the existing test variable and write a forEach loop

• that adds 100 to each number that is divisible by 3.

• Things to note:

• • Inside the loop, you must use an if statement to verify code is divisible by 3
• • Inside the loop, you can update an element ONLY by using the arrayName[index]
• • Outside the loop, you can use console.log to verify the test variable */

  var test = [12, 929, 11, 3, 199, 1000, 7, 1, 24, 37, 4, 19, 300, 3775, 299, 36, 209, 148, 169, 299, 6, 109, 20, 58, 139, 59, 3, 1, 139 ];

Answer:

test.forEach(function (value,index,array){
    if (value % 3 ===0){
    array[index] +=100;
}
    return value;
});
console.log(test);   

My problem is why can't I use value +=100:

test.forEach(function (value,index,array){
    if (value % 3 ===0){
    value +=100;
}
return value;
});

what's the difference?

Thanks!!!!!!!!1