Understanding debugging for Javascript when using scope, undefined, and reference error

JavaScript
2

Scope is a simple concept, but there are a lot of wrinkles that make it difficult to explain in a forum post. I suggest you read up on it: You Don’t Know JS: Scope & Closures

Basically, scope defines the context in which a value exists. In JavaScript, scope is most often defined by functions - if you define a variable within a function, you can’t access it from outside of the function. Check out question 7

function func() {
    var a = ‘hello’;
}
consle.log(a);  // a has been defined within the function func(). 
//Since it has not been defined outside of the function, 
//you get a ReferenceError when calling console.log()

This is true so long as you use the var keyword. The new ES6 keywords, let and const, are block scoped. This means they are only defined within the closest curly braces. Often, this is still a function, but it could also be an if clause, a for loop, or just some arbitrarily placed curly braces.

function func() {
    let a = ‘hello’;
    if (true) { let a = ‘goodbye’;} // this definition only exists within the curly braces.
    console.log(a); // 'hello'
}

All of these questions are variations on the same theme. Where is the variable defined?

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