Javascript

1. variable

Three ways to declare variables in javascript

Feature	var	let	const
redeclare	Yes	No	No
Update	Yes	Yes	No
Scope	functional	Block	Block

2. module

Links

• export

• import

ES5

If you wants to exports only one variable / object

module.exports = my_object;
exports = my_object; // will not work

If you wants to exports multiple things as object

module.exports.my_variable = my_variable;
module.exports.my_object = my_object;
module.exports.my_function = my_function;

module.exports = {
    my_variable,
    my_object,
    my_function
}

exports.my_variable = my_variable;
exports.my_function = my_function;
exports.my_object = my_object;

// Below will not work
exports = {
    my_variable,
    my_object,
    my_function
}

• We can not use both module.exports and exports at once in our file (import).

ES6

• We can use export default only once in our file.

export default my_variable

• If we want to export more than 1 variable / object then we can use export

export { my_variable, my_function };

• We can use both export default and export at once in our file.

3. closure

A closure is the combination of a function bundled together (enclosed) with references to its surrounding state (the lexical environment). In other words, a closure gives you access to an outer function’s scope from an inner function. In JavaScript, closures are created every time a function is created, at function creation time.

Links for closure

Link 1

uses of closure

• Emulating private methods with closures

4. deep_copy and shallow_copy

Links for deep_copy and shallow_copy

• Link 1

Methods

• spread operator, Object.assign(), JSON.parse(JSON.stringify()) do deep copy on array and object
• spread operator, Object.assign() does not do deep copy of nested object element but JSON.parse(JSON.stringify()) do deep copy of nested objects element on object
• spread operator, Object.assign(), JSON.parse(JSON.stringify()) do deep copy of nested array element on array