Javascript Variable Scope

Basic block scope

Javascript looks out, but not in.

var message = "Hello";

function doSomething(){
  console.log( message );
}

doSomething();

message is defined "globally" and can be seen everywhere.

function doSomething(){
  var message = "Hello";
}

console.log( message );

Variables defined inside of functions can only be seen inside the function and from any functions called inside the function.

Still basic block scope + async timing

People commonly make this error:

for (var i=0; i<5; i++) {

  var link = document.createElement("a");

  link.innerHTML = "Link " + i;

  link.addEventListener('click', function() {
    console.log(i);
  });

  document.body.appendChild(link);
}

The value of i is not held inside the event listener function.

solutions

Use foreach

var links = [];

for (var i=0; i<5; i++) {

  links.push( document.createElement("a") );

}

links.forEach(function(link, i){

  link.innerHTML = "Link " + i;

  link.addEventListener('click', function () {
    console.log(i);
  });

  document.body.appendChild(link);
});

Local function scope

var setLink = function(i){

  var link = document.createElement("a");

  link.innerHTML = "Link " + i;

  link.addEventListener('click', function () {
    console.log(i);
  });

  document.body.appendChild(link);
};

for (var i=0; i<5; i++) {
  setLink(i);
}

Closure Scope 2:

Immediately Invoking Function Expression

for (var i=0; i<5; i++) {

  (function(i){

    var link = document.createElement("a");

    link.innerHTML = "Link " + i;

    link.addEventListener('click', function () {
      console.log(i);
    });

    document.body.appendChild(link);
  })(i);
}

Pass the parameter you want to "hold onto" into the function expression

Pairing Exercise

• After each line write a console.log("what is happening at this line", variable-to-console.log )
• Callbacks and scope become more clear when you understand the values of each thing at each place in the program.
• Try each example out.