JavaScript
Scopes
Will Klein
JavaScript Looks Like...
JS looks like C
C has block-level scoping
JS has function-level scoping
JS is not C
Function-level Scoping
var f = function() { var a = 1; if (true) { var a = 2; } return a; // 2};
a is scoped to its parent scope, function f
if, for, while, etc. blocks don't get their own scope
Hoisting
var g = function() {
b = 1;
var b = b + 2;
return b; // 3
};
b is scoped to its parent function g
The above is equivalent to:
var g = function() { var b;
b = 1;
b = b + 2;
return b; // 3
};
Globals
var g1 = 'g1 is a global';g2 = 'g2 is a global';
(function() { var l = 'This is local'; g3 = 'g3 is also a global';})();
Missing a var creates a global!
Don't mind the IIFE:
Reusing the Same Variable Name
var a = 1; // a: 1
(function() {
var a = 2 // a: 2
(function() {
var a = 3; // a: 3
})(); (function(a) { // a: 4 })(4);
})();
// a: 1
a exists uniquely in each scope
Closures
(function() {
var a = 1;
(function() {
a++;
})();
// a: 2
})();
This is a basic closure
Applying Closures
var count = (function() {
var count = 0;
return function() {
count = count + 1;
return count;
};
})();
count(); // 1
count(); // 2
A more useful closure