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JavaScript JavaScript Loops, Arrays and Objects Tracking Data Using Objects The Build an Object Challenge, Part 1

ian izaguirre
ian izaguirre
3,220 Points
Posted by ian izaguirre
ian izaguirre
3,220 Points

Question on Scope in For loops and Array literals ?

First Question on Scope:

// Student records 
var students = [
    {
        name: 'Ian',
        track: 'iOS',
        achievements: 5,
        points: 120
    },
    {
        name: 'Juan',
        track: 'Design',
        achievements: 32,
        points: 450
    }
];

function print(message) {
  document.write(message);
}

var name;
var j = students.length;


for ( var i = 0; i < j; i+=1 ) {
  name = students[i].name;
    print( name );
}

The Above Code works ... BUT when I change declaring the variable name outside of the For Loop :

// Student records 
var students = [
    {
        name: 'Ian',
        track: 'iOS',
        achievements: 5,
        points: 120
    },
    {
        name: 'Juan',
        track: 'Design',
        achievements: 32,
        points: 450
    }
];

function print(message) {
  document.write(message);
}

var name = students[i].name;
var j = students.length;


for ( var i = 0; i < j; i+=1 ) {
    print( name );
  }

I get undefined? I do not understand how this gives an error if only functions have local scope?

2 Answers

Joshua Kaufman
Joshua Kaufman
19,193 Points

You might need to play around with it a bit to get a stronger feel for it.
"i" MUST be an integer to pull values out of an array, since arrays have index numbers to be identified (starting at 0).

When I gave you that code snippet, we set i = 0. Since i = 0 before running students[i].name, it looks for students[0].name, which is the name VALUE inside of the FIRST object. If you did students[1].name, it would pull "Juan" instead because the second object (at index 1) has a name VALUE of "Juan".

When you fail to initial "i", or assign it to a string, it CAN'T find an index inside of the students array to go off of, and won't give you anything useful: students[''].name doesn't make sense to a computer in JavaScript. 

//name = students[''].name doesn't work because '' is not an integer

Likewise, when you don't initialize the value for "i", it stays undefined. students[undefined].name means nothing to the computer, and can't be found at all.

///name = students[undefined].name doesn't work because it doesn't make logical sense, and is also not an integer

We use the for loop to get all the values because it automatically sets AND increments "i". When "i" is the loop counter, it is not only a number but also the number you NEED.

This is the first iteration

for(var i = 0; i < students.length; i++) //IMPORTANT! students.length = 2
{
//runs at i = 0 because 0 < 2;
 name = students[i].name; // gives "name" the value of students[0].name, which is "Ian"
    print( name ); //runs the print function
}

This is the second interation

for(var i = 0 i < students.length; i++) //WHEN THIS RUNS AGAIN, i = 1
{
name  = students[i].name //gives "name" the value of students[1].name, which is "Juan"
print(name); //runs the print function
}

Basically, for loops are sick for writing stuff out of arrays AND objects, you need integers to do it in JavaScript, and anything else won't work.

ian izaguirre
ian izaguirre
3,220 Points
ian izaguirre
ian izaguirre
3,220 Points

Awesome, thank you for taking the time to give such an in depth answer.

Joshua Kaufman
Joshua Kaufman
19,193 Points
Joshua Kaufman
Joshua Kaufman
19,193 Points

The "i" variable gets thrown out of memory right after the loop ends. Outside of it, "i" isn't recognized as a variable. Since code reads top to bottom, it will try to find the index "i" for students.name, but "i" doesn't exist! "i" ONLY has meaning within the loop.

You COULD assign var i = 0 right above the var name = students[i].name; line...

//like this
var i = 0
name = students[i].name;
//name equal "Ian" when you do this, but we use for loops to go through ALL of the entries.

I wouldn't recommend it though, since you won't get all of the values from your array of objects.

Does that make more sense? If not, I'll try to clarify more.

ian izaguirre
ian izaguirre
3,220 Points
ian izaguirre
ian izaguirre
3,220 Points

Thank you, I understand now my mistake. Also by doing it in a function it works:

var i;
function startProgram() {
    var  name = students[i].name;
var  track = students[i].track;

print( name + track );
}



var j = students.length;

for ( i = 0; i < j; i+=1 ) {
    startProgram();

}