JavaScript interview question

What are global variables?


Global variables are declared outside of a function or declared with a window object for accessibility throughout the program (unless shadowed by locals). If you declare a variable without using var, even if it's inside a function, it will still be seen as global.

The var statement declares a function-scoped or globally-scoped variable, optionally initializing it to a value.


var x = 10;

if (x === 10) {
  var x = 20;

  // expected output: 20

// expected output: 20

Example: Declaring global variable within function

window.value = 90;

// Declaring global variable by window object
function setValue() {
  window.value = 100;

// Accessing global variable from other function
function getValue() {
  return window.value;

console.log(getValue()); // 100

Using Undeclared Variables:

  • In strict mode, if you attempt to use an undeclared variable, you'll get a reference error when you run your code.
  • Outside of strict mode, however, if you assign a value to a name that has not been declared with let, const, or var, you'll end up creating a new global variable. It will be global no matter how deeply nested within functions and blocks your code is, which is almost certainly not what you want, is bug-prone, and is one of the best reasons for using strict mode!
  • Global variables created in this accidental way are like global variables declared with var: they define properties of the global object. But unlike the properties defined by proper var declarations, these properties can be deleted with the delete operator.

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