Looking at an online source code I came across this at the top of several source files.  var FOO = FOO || {}; FOO.Bar = …;   But I have no idea what || {} does.  I know {} is equal to new Object() and I think the || is for something like "if...
Snippet 1:    var a; // undefined variable named 'a' function a(foo) { // a function named 'a'   var foo = "Hello World";   console.log(foo); } console.log(a); // output is: [Function: a], but why not undefined?     Snippet 2:...
The title says it all, but I will provide more clarification:  After seeing many samples of javascript where all variables are declared as type var, and seeing support for other datatypes, why aren't variables of a specific datatype declared as...
See this code:    <script> let {foo} = null; // TypeError </script> <script> // Here I want to assign some some value to foo </script>     The first script attempts to let-declare foo via a destructuring assignment. However, n...
What is the difference between these?  var a = 13;   this.b = 21;   document.write(a);   document.write(b);...
I was trying to understand the scope in JavaScript. If I declare a variable outside of a function, it is GLOBAL. Hence I tested the following code to understand sequence of execution. In the following code, I expected the "demo1" to take the...
Option1 : multiple var without assignment  function MyFunction() {    var a = null;   var b = null;   ....   var z = null;    a = SomeValue;   b = SomeValue2;   .... }   Option 2: one var statement, no assignment  function MyFunction() {    var a, b...
In Stefanov's JS Design Patterns book, he writes "you use one var statement and declare multiple variables delimited by commas", and then gives an example of the "single var" pattern as follows:  function func() {     var a = 1,...
I have noticed in an other question the performance difference in loops while using let and var variables declaration.  The initial  question is correctly answered that using let in the for loop is slower since let creates a new scope for each iterat...

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