JavaScript declarations and their scope limits

Types of JavaScript variable declarations:

1. Var keyword
2. Let keyword
3. Const keyword
4. Undeclared variable (Ex: a =2;)

Scope of declarations:

• Var keyword
• Limited to function level.
• Can be used in strict mode and non-strict mode.
• For example, function scopeOfVar(){ var a; }, here the scope is limited to function scopeOfVar().
• If the variable declared with var inside of a block in a function, then also that variable has scope throughout the function.
• But the variable can be accessed before it’s declaration, if it accessed before declaration will give a ‘undefined’ as the value, which is called as ‘hoisting’.
• Usage var variableName [= value];
• Let keyword
• Should be used only if strict mode is enabled.
• Let will have scope only in the block it is declared.
• Commonly used for  small blocks or scopes.
• Usage let variableName [= value];
• Const keyword
• Should be used only if strict mode is enabled.
• Const will have scope only in the block it is declared.
• Const should be initialized while declaring.
• It can be an object also like var and let.
• Usage: const variableName = constant value;
• Undeclared variable (Ex: a =2;)
• Should be used only in non-strict mode.
• Will throw reference error if used in strict mode.
• If a variable is declared without any keyword in any scope will become a member of global object, i.e., undeclared variable is always in global scope.
• It is not suggested as a best practice to use like this.
• Usage: variableName = value;