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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.

window.onload = function(){
    var item = document.querySelector(".items .item");
    item onClick = activeItem;

    function activeItem() {
        console.log(item);
    }
}

In this example inner function activeItem() have access to global variable item. This called closure – when inner function have access to all variables, defined in global function.

To show variable type use typeof:

let value = true;
console.log(typeof value); // boolean

Let’s convert our variable to string:

value = String(value); // now value is a string "true"
console.log(typeof value); // string

Numeric Conversion

Numeric conversion happens in mathematical functions and expressions automatically.

For example, when division / is applied to non-numbers:

console.log( "6" / "2" ); // 3, strings are converted to numbers

We can use the Number(value) function to explicitly convert a value to a number:

let str = "123";
console.log(typeof str); // string

let num = Number(str); // becomes a number 123
console.log(typeof num); // number

Explicit conversion is usually required when we read a value from a string-based source like a text form but expect a number to be entered.

If the string is not a valid number, the result of such a conversion is NaN. For instance:

let age = Number("an arbitrary string instead of a number");
console.log(age); // NaN, conversion failed

Examples of numeric conversion:

console.log( Number(" 123 ") ); // 123
console.log( Number("123z") );  // NaN (error reading a number at "z")
console.log( Number(true) );    // 1
console.log( Number(false) );   // 0

Please note that null and undefined behave differently here: null becomes zero while undefined becomes NaN.

Boolean Conversion

The conversion rule:

Values that are intuitively “empty”, like 0, an empty string, null, undefined, and NaN, become false.
Other values become true.

For instance:

console.log( Boolean(1) ); // true
console.log( Boolean(0) ); // false

console.log( Boolean("hello") ); // true
console.log( Boolean("") ); // false

Summary

The three most widely used type conversions are to string, to number, and to boolean.

String Conversion – Occurs when we output something. Can be performed with String(value). The conversion to string is usually obvious for primitive values.

Numeric Conversion– Occurs in math operations. Can be performed with Number(value).

Boolean Conversion – Occurs in logical operations. Can be performed with Boolean(value).