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Variables in standard JavaScript have no type attached, so any value (each value has a type) can be stored in any variable. Starting with ES6, the 6th version of the language, variables could be declared with var for function scoped variables, and let or const which are for block level variables.
Nowadays JavaScript has three different keywords to declare a variable — var, let and, const. Scope in JavaScript refers to context (or portion) of the code which determines the accessibility ...
Variables without value are automatically // set to undefined. // var is generally considered bad practice and let and const are usually preferred. var x; // Variables can be manually set to `undefined` like so let x2 = undefined; // Declares a block-scoped variable named `y`, and implicitly sets it to // `undefined`.
Variables may have only forward declaration and lack definition. During compilation time these are initialized by language specific rules (to undefined values, 0, NULL pointers, ...). Variables that are defined in other source/object files must have a forward declaration specified with a keyword extern:
Perl autovivification allows a programmer to refer to a structured variable, and arbitrary sub-elements of that structured variable, without expressly declaring the existence of the variable and its complete structure beforehand. [1] In contrast, other programming languages either:
Immediately invoked function expressions can be used to avoid variable hoisting from within blocks, protecting against polluting the global environment and simultaneously allowing public access to methods while retaining privacy for variables defined within the function. In other words, it wraps functions and variables, keeping them out of the ...
In other languages, variables are often initialized to known values when created. Examples include: VHDL initializes all standard variables into special 'U' value. It is used in simulation, for debugging, to let the user to know when the don't care initial values, through the multi-valued logic, affect the output.
While a variable or function may be declared many times, it is typically defined once (in C++, this is known as the One Definition Rule or ODR). Dynamic languages such as JavaScript or Python generally allow functions to be redefined, that is, re-bound; a function is a variable much like any other, with a name and a value (the definition).