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Declaring variable javascript

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    started a topic Declaring variable javascript

    Declaring variable javascript

    Declarating variable javascript

    Programs need to store data, to store data variables are declared and values are assigned
    In javascript there are five keywords for declaring variables:
    1. var
    2. let
    3. const
    4. function
    5. class

    • The Keywords var, let, const declare newly named variables
    • when declared in the global scope variables are available throughout the program
    • for global variables declared with var, their names become properties of the global object
    • generally, declaring variables in the global scope is a bad practice, it can lead to conflicts and confusion it the code
    • when used within a function, they are local variables within the context of the executed function's stack frame
    • a stack frame contains all of the data associated with the invocation of a function, such as context, function name, source code
    • the variable can hold primitive data and object references
    • For the sake of simplicity think of primitives vs objects with respect to variables in the following way:
    • variables containing primitive values store those values on context of the stack frame
    • variables containing objects really only contain a reference to the object which is stored on the heap
    • so primitive values are stored within the variable itself and objects are stored by reference within the variable, with the object itself being stored on the heap

    • For modern javascript coding, only let and const should be used to declare variables
    • the presence of var in source code indicates legacy code that has never been rewritten to upgrade to the latest javascript variable with the initial release of javascript in 1995
    • variables declared with var are read/write but exhibit some unexpected characteristics,especially for developers coming from c++, java and c#.
    • variables declared with var are declared within the global scope or the scope of a function,not a block scope
    • additionallly declarations are hoisted to the top of the global scope or the top of the function scope in which they are declared
    • many developers are unaware of these qualities which can lead to confusion and bugs in javascript code
    • because of this confusion and other reason two new keywords for declaring variables were added in ES 2015: let and const

    • Declaring variables with let and const behave as most developers expect
    • variables are defined within a block scope and there is no hoisting
    • variables defined with const are immutable, while variables defined with let are mutable
    • when declaring variables always prefer const to let unless there is a specific reason for a variable to be mutated

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