void in javascript

void.js

// void turns any JavaScript expression to the primitive undefined
// Try the following in your browser's console or node REPL

// Outputs: undefined
console.log(void 0);

// Outputs: undefined
console.log(void 4);

// Outputs: undefined
console.log(void true);

// undefined is not a JavaScript keyword, but a property of the global object
// though it can't be changed

// Outputs: undefined
console.log(this.undefined);

// Oututs: undefined
this.undefined = "foo";
console.log(this.undefined);

// undefined can be shadowed inside a function
(function() {
  var undefined = "shadowed";
  // Outputs: shadowed
  console.log(undefined);
})();

// void can be used to compare with the
// undefined primitive always
(function() {
  var undefined = "shadowed";
  var notdefined;
  
  // Outputs: false
  console.log(undefined === notdefined);
  
  // Outputs: true
  console.log(void 0 === notdefined);
})();